Fault Zones: Structure, Geomechanics and Fluid Flow

Transcription

Fault Zones: Structure,
Geomechanics and
Fluid Flow
16-18 September 2008
The Petroleum Group would like to thank Badley
Geoscience, BP and Shell for their support of
this event:
September 2008
Page 1
Tuesday 16 September
08:30
Registration + coffee
09:00
Welcome and opening
09:10
Caine (US Geological Survey)
KEYNOTE: New Insight on Structural Inheritance and Fault-Vein Permeability Structures in the
Colorado Mineral Belt, USA
Session 1
Structural Properties of Fault Zones
09:40
Gudmundsson (University of London)
Local stresses, fracture apertures, and fluid transport in fault zones
10:00
Reeves (BGS)
Repository Excavations and the Self Sealing of the Excavation Damaged Zone (EDZ) in
Mudrocks: An Overview
10:20
Shackleton (Midland Valley)
Can strain maps be used as an indicator for the extent of fault zone damage?
10:40
Yonkee (Weber State University)
Geometry, Kinematics, and Fracture Network Characteristics with Fault Segment Boundaries,
Wasatch Fault Zone, Utah, USA
11:00
Tea / Coffee
11:30
Schueller (University of Bergen)
Characterization of fault damage zone and deformation band populations based on outcrop
data
11:50
Wibberley (Total)
Mechanics of fault-zone localisation in high-porosity sandstones and impact on flow efficiency
12:10
McLellan (James Cook University)
Strain accumulation and fluid flow in and around basin bounding fault zones of the Leichhardt
River Fault Trough, Qld. Australia
12:30
Agosta (Universita Di Camerino)
Structural and statistical analyses of fault-controlled hydrocarbon migration and
accumulation
12:50
Lunch
September 2008
Page 2
14:00
Schlische (Rutgers University)
Experimental modeling of extensional fault domains and fault-domain boundaries (transfer
zones / accommodation zones)
14:20
Thornton (Rockfield Software Limited)
Predictive Modelling of the Evolution of Fault Zone Structure: 3-D Sandbox and Field Scale
Modelling
14:40
Henza (Rutgers University)
Influence of pre-existing fabric on normal-fault development: An experimental study
15:00
Granger (Haley & Aldrich, Inc)
Fault-surface corrugations: Insights from scaled experimental models of extension
15:20
Nottveit (University of Bergen)
Fault Facies modeling; possibilities and difficulties
15:40
Freeman (Badley Geoscience Ltd)
Using empirical geological rules to reduce structural uncertainty in seismic
interpretation of faults
16:00
Tea / Coffee
16:30
Tueckmantel (University of Leeds)
Fault seal prediction of seismic-scale normal faults in porous sandstone: A case study from the
eastern Gulf of Suez rift, Egypt
16:50
Frost (University of Southern California)
Structural analysis of the exhumed SEMP fault zone, Austria: Towards an understanding of
fault zone architecture and mechanics throughout the seismogenic crust
17:10
Braathen (University of Bergen)
Fault Facies methodology for systematizing analogue outcrop data to 3D fault grids in
reservoir models
17:30
Agar (ExxonMobil)
What are the Potential Impacts of Low-offset Faults on Carbonate Reservoir Performance?
17:50
Childs (University College Dublin)
KEYNOTE: A geometric model for the development of fault zone and fault rock thickness
variations
18:35
Wine Reception
September 2008
Page 3
Wednesday 17 September
08:40
09:00
Rice (Harvard University)
KEYNOTE: How granulated/cracked fault border zones, and their pore fluids, interact with
earthquake rupture dynamics
09:30
Session 2
Fault/fracture mechanisms and mechanics
Haimson (University of Wisconsin)
The effect of the intermediate principal stress on shear band strike and dip in the siltstone
straddling the active Chelungpu Fault, Taiwan
09:50
Greenhough (University of Edinburgh)
Geomechanical sensitivity of reservoirs from statistical correlations of flow rates
10:10
Van Marcke (EIG Euridice)
Excavation induced fractures in a plastic clay formation: observations at the HADES URF
10:30
Tea / Coffee
11:00
Aydin (Stanford University)
Fault growth and the related fundamental physical processes
11:20
Moir (University of Strathclyde)
Modelling development of a simple fault zone in the Sierra Nevada
11:40
Mitchell (University of Hawaii)
Mechanics of sheeting joints
12:00
Ishii (Japan Atomic Energy Agency)
Relationship between growth mechanism of faults and permeability variations with depth of
siliceous mudstone in northern Hokkaido, Japan
12:20
Welch (University of Leeds)
Fault growth in mechanically layered sequences: A modelling approach
12:40
Lunch
13:30
Jostad (Norwegian Geotechnical Institute)
Geomechanical integrity of a sealing fault during late life depletion of a petroleum reservoir
13:50
Zhang (GRS)
Experimental study on self-sealing of indurated clay
September 2008
Page 4
14:10
Muhuri (Chevron)
Kinetics of Time-dependent Processes in Fault Zones: Implications for Fault Seal Analysis
14 :30
Niemeijer (Pennsylvania State University)
Strong velocity weakening in fault gouges: results from rock analogue experiments
14 :50
Zhang (Chinese Academy of Sciences)
Characterisation of fault sealing for hydrocarbon migration and entrapment
15:10
Tea / Coffee
15 :40
Session 3
Fault Structure and Earthquakes
Bennington (University of Wisconsin)
Constrained Inversions of Geophysical Data in the Parkfield Region of California
16 :00
Cooke (University of Massachusetts)
The role of slip-weakening friction in damage zone geometry
16 :20
De Paola (University of Durham)
The Nucleation of Large Earthquakes Within Overpressured Fault Zones in Evaporitic
Sequences
16 :40
Evans (Utah State University)
The nature of the San Andreas Fault at seismogenic depths: Insight from direct access via the
SAFOD boreholes
17 :00
Wojtal (Oberlin College)
Displacement field in the borderlands of the San Andreas Fault, Durmid Hill, CA and the origin
of late sinistral faults
17 :20
Nicol (GNS Science, New Zealand)
Fault Interactions and the Growth of Faults on Earthquake and Geological Timescales
17 :40
Cowie (University of Edinburgh)
KEYNOTE: Quantifying Fault Slip rates and Earthquake Clustering along Active Normal Faults
in Central Italy: Insights from Cosmogenic Exposure Dating and Numerical Modelling
19 :00
Conference Dinner
September 2008
Page 5
Thursday 18 September
08:40
09:00
Talwani (University of South Carolina)
KEYNOTE: Seismogenic Permeability
09:30
Session 3 contd
Fault Structure and Earthquakes
Pitarello (Universita degli Studi di Padova)
Energy partitioning during seismic slip in pseudotachylyte-bearing faults (Gole Larghe Fault,
Adamello, Italy)
09:50
Balsamo (Universita Roma Tre)
Particle size distribution analysis in pristine and faulted quartz-rich, poorly cohesive
sandstones: influence of analytical procedures in laser diffraction analysers
10:10
Spivak (Institute of Geospheres Dynamics of Russian Academy of Sciences)
Rigidity of tectonic faults and their temporal variation
10:30
El Hariri (University of Boston)
The role of fluids in triggering earthquakes: Observations from reservoir induced earthquakes
10:50
Tea / Coffee
Session 4
Faults and fluids
11:15
Medeiros (UFRN, Natal)
Results from field pumping experiments testing connectivity across deformation bands in
Tucano Basin, NE Brazil
11:35
Guillemot (Andra)
Different scales of fracturing in the Callovo-Oxfordian argillite of the Meuse /Haute-Marne Andra
URL area, France
11:55
Liberty (Boise State University)
Fault imaging in the western US using high resolution seismic reflection methods
12:15
Brinton (University of Idaho)
The influence of regional stress on geostatistical patterns of fault permeability at Smith
Creek Hot Springs, Neveda, USA
12:35
Masset (Swiss Federal Institute of Technology)
Large scale Hydraulic Properties of Faults and Fault Zones of the Central Aar and Gotthard
Massifs (Switzerland)
September 2008
Page 6
12:55
Lunch
Session 4 contd
Faults and fluids
13:45
Woods (BP Institute)
Buoyancy driven gas dispersion along an inclined low permeability boundary
14:05
Amano (Japan Atomic Energy Agency)
3D Structures of Permeable and Impermeable Faults in Granite: A Case Study in the Mizunami
Underground Research Laboratory, Japan
14:25
Tveranger (University of Bergen)
Volumetric fault zone modelling using fault facies
14 :45
Wilson (Stanford University)
Using outcrop observations, 3D discrete feature network (DFN) fluid flow simulations, and
subsurface data to constrain the impact of normal faults and opening mode fractures on the
migration and concentration of hydrocarbons in an active asphalt mine
15:05
Rocher (IRSN, France)
Differential fracturing pattern in clay/limestone alternations at Tournemire (Aveyron, France)
and in the Maltese Islands
15 :25
Caine (US Geological Survey)
Contrasting Styles of Faults and Fault Rocks in the Rio Grande Rift of Central New Mexico,
USA: Their Relationships to Rift Architecture and Groundwater Resources
15:45
Tea / Coffee
16:10
Lunn (University of Strathclyde)
Assessing temporal changes in fault permeability for radioactive waste disposal
16:30
Simms (John Hopkins University)
Fault zone control of fluid flow in extensional basins
16:50
Peacock (Fugro Robertson Ltd)
Pull-aparts, scaling and fluid flow
17:10
Cuisiat (Norwegian Geotechnical Institute)
Fault formation in uncemented sediments. Insight from laboratory experiments
17:30
Younger (University of Newcastle)
KEYNOTE: Extraordinary permeability associated with major W-E rock-mass discontinuities
cutting Carboniferous strata in northern England and central Scotland - some cautionary tales
18 :00
Conference End
September 2008
Page 7
Posters
Bastesen
Extensional fault cores in carbonates; thickness-displacement relationships
Novakova (tbc)
Reactivation of brittle tectonic structures in the Sudetic Marginal Fault vicinity (in north east of Bohemian
Massif)
Cunningham (SRK Consulting)
The role of faulting in the concentration of Fe and Zn-Pb ores within the Paleoproterozoic Earaheedy Basin,
Western Australia
Bell (National Oceanography Centre)
Fault development and control on rift basin evolution in the Gulf of Corinth, Greece
Müller (University of Vienna)
Fault zone characteristics of a low-angle normal fault on northern Kea (Western Cyclades, Greece)
Alessandroni (Universita di Camerino)
Statistical analysis of stylolites and sheared stylolites in layered carbonate rocks: an attempt for a new
methodological approach
Kanjanapayont (University of Vienna)
Kinematics of the Klong Marui continental wrench fault, southern Thailand
Taylor (University of Manchester)
A three-dimensional approach to the interpretation of major fault zone properties
Ikari (tbc)
Pore pressure generation in sheared marine sediments
Smith (Durham)
Laboratory measurements of the frictional strength of a natural low-angle normal fault: the Zuccale fault,
Elba Island, Italy
Storti (Universita Roma Tre)
Influence of analytical methods on fault core rock particle size distributions obtained from laser-aided
analysers
September 2008
Page 8
Mittempergher (Museo Tridentino di Scienze Naturali)
Effects of fault orientation on fault rock assemblages of exhumed seismogenic sources
Haimson (University of Wisconsin)
The effect of the intermediate principal stress on shear band strike and dip in the siltstone straddling the
active Chelungpu Fault, Taiwan
Sehhati (Washington State University)
Porosity and particle shape changes leading to shear localization in small-displacement faults
Lawther (University of Glasgow)
Fluid-fault-rock interactions in faults exhumed from seismogenic depths
Kirkpatrick (University of Glasgow)
Fault structure, slip and fluid flow interactions; insights from small seismogenic faults
Fachri (University of Bergen)
Sensitivity of fluid flow to faulted siliciclastic reservoir configurations
Pittarrello (Universita degli Studi di Padova)
Deep-seated pseudotachylytes from the Ivrea Zone metagabbros (Southern Alps, Italy)
Mittempergher (Museo Tridentino di Scienze Naturali)
Hydrogen isotopes in natural and experimental pseudotachylyte-bearing faults: the origin of fluids at
seismogenic depth
September 2008
Page 9
September 2008
Page 10
KEYNOTE:
New Insight on Structural Inheritance and Fault-Vein Permeability Structures in the
Colorado Mineral Belt, USA
Jonathan Saul Caine
U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, CO, 80225, USA
[email protected]
A long history of mining and geologic mapping in the Front Range of the central Colorado
Rocky Mountains has resulted in an exceptionally rich dataset on the geologic structure of
epithermal ore deposits. These regional-scale data were among the first to lead geologists to
ponder the role of Precambrian structural inheritance in the localization of Tertiary mineral
deposits. Of particular significance was the idea that localization of epithermal, polymetallic
fault-veins in this region was controlled by a pre-existing crustal “weakness”, the Proterozoic
Idaho Springs-Ralston ductile shear zone (ISRZ). However, recent compilation of structural
and mineral deposit data from existing 1:24,000 geologic maps, reports, argon geochronology
on fault and hydrothermally altered rocks, and new structural data from outcrop in the Front
Range results in five major observations: 1) There is little correlation between the locations of
inferred mineral deposit-related plutons and the ISRZ or major brittle fault zones. 2) Mapped
features suggest that myriad directions of potential permeability structures existed during the
Tertiary and that metalliferous hydrothermal fluids may have flowed in many directions at any
given time during evolution of the Colorado Mineral Belt. 3) Small displacement fault-veins with
striated and cataclasized margins that carried ore bearing fluids show steep dips and either
preferential ENE trends well correlated with model paleostress directions for the Laramide
orogeny or radial trends around Late Cretaceous to Tertiary igneous intrusions. These
relationships hold regardless of co-planarity with preexisting foliations in metasediments or in
massive unfoliated metaigneous plutons. 4) The total gas 40Ar/39Ar age of alteration is older
than that of the brittle faults and none are Proterozoic. 5) There are only minor differences in
orientation and intensity of potential structures that may have controlled permeability from
within the ISRZ compared with similar structures outside the ISRZ. These observations
suggest that Proterozoic inheritance in the Front Range is not the primary control of mineral
deposit permeability structure, location, or orientation. Rather, responsible processes likely
include a) proximity to shallowly emplaced plutons, b) self-generated, hydro-fracturing-like
permeability due to thermally driven pore fluid pressure changes associated with pluton
emplacement; and c) competition between varying magnitudes and orientations of shallow
regional horizontal principal stresses, overburden load, and local stress perturbations related to
pluton emplacement.
September 2008
Page 11
NOTES
September 2008
Page 12
Local stresses, fracture apertures, and fluid transport in fault zones
Agust Gudmundsson1, Shigekazu Kusumoto2, Silje S. Berg3, Trine S. Simmenes3, Belinda
Larsen3, Sonja L. Philipp4
1
Department of Earth Sciences, Royal Holloway University of London, UK
School of Marine Science and Technology, Tokai University, Shizuoka, Japan
3
Department of Earth Science, University of Bergen, Norway
4
Geoscience Centre, University of Gottingen, Germany
2
Many fault zones are mechanically very heterogeneous and develop heterogeneous local
stresses. At depth, much of the fluid transport in active fault zones is through fractures that
subsequently become mineral veins. Measurements of many veins, mostly 2-6 m long (strike
dimension), with a maximum thickness of 10-25 mm, show that the aperture (thickness)
normally varies irregularly along the vein length; commonly by 20-40%, but occasionally by 5070%, of the maximum vein thickness. Such aperture variations may lead to flow channelling
and significantly affect fluid transport in fault zones. Most veins are extension fractures, the
stress acting perpendicular to them being the minimum compressive (maximum tensile)
principal stress, S3. For such fractures, we define overpressure as the total fluid pressure in the
fracture minus S3. In a fault zone where the local stress is heterogeneous, fracture
overpressure may vary irregularly. Here we use Fourier cosine series to provide analytical
solutions for the displacement and stress fields around a fracture opened by an irregular
overpressure. The solutions can be used to estimate the aperture variation of essentially any
fluid-driven extension-fracture. The results should improve our understanding of fluid transport
and flow channelling, as well as that of local stresses and displacements, in fault zones.
September 2008
Page 13
NOTES
September 2008
Page 14
Repository Excavations and the Self Sealing of the Excavation Damaged Zone (EDZ) in
Mudrocks: an Overview
H.J. Reeves, R.J. Cuss & J.F. Harrington
When a repository opening (tunnel, shaft, gallery or disposal vault is excavated, the stresses
acting in the rock are altered by the tunneling activities and by the removal of the rock from the
cross-section of newly-formed excavation. A zone of stress concentration is formed around all
the underground excavations in rock. Close to the walls of an excavation, the radial stress falls
and the tangential stress rises. The maximum shear stress is determined by the difference
between these two principle stresses. Depending on the stress field prior to excavation, the
shear stress close to the excavation can be sufficiently large for the stress path to enter the
domain of dilatants her deformation. Rapid radial de-stressing of the rock in the vicinity of an
excavation may also lead to localized extensile failure. Fractures formed in this way are
sometimes referred to as “unloading cracks”. Regardless of the precise rupture mechanism,
open fractures may be formed around excavations, leading to a region of enhanced
permeability known as the Engineering Damage Zone (EDZ).
The presence of an EDZ is acknowledged to be a particularly important issue in the
performance assessment for the disposal of radioactive waste. Interconnection of fractures in
the EDZ could lead to the development of a preferential flow path extending along the
emplacement holes, access tunnels and shafts of a repository towards overlying aquifers and
the biosphere.
The size and the properties of the EDZ depend on the excavation method, the state of stress,
the pore water pressure and the hydro-mechanical properties of the rock. Bedding plane
anisotropy can be an important factor. In clays and argillaceous rocks, the most pervasive
forms of damage are caused by stress redistribution and unloading. Three basic forms of
fracturing may be defined: (a) shear fractures, (b) tensile fractures, and (c) extensile fractures.
Recent experience during development operations in several Underground Research
Laboratories clearly demonstrates that the dominant mode of fracturing can be quite different
from one mudrock to another. Examples from tunneling operations in the Boom Clay and the
Opalinus Clay will be compared to show this variation.
September 2008
Page 15
NOTES
September 2008
Page 16
Can strain maps be used as an indicator for the extent of fault zone damage?
Shackleton, R., Bond C.E., Munro, L., Shipton, Z.K., and Seed, G.
Areas of damage around faults are of interest for their potential role as a barrier or conduit for
fluids and gases; thus, fault damage zones influence groundwater resources, hydrocarbon
extraction and mineralisation, sub-surface waste disposal, and greenhouse gas storage.
Consequently, predicting the geomechanical and hydrological properties of ‘damage’ around
faults and the spatial distribution of these zones is a key question for applied geoscience.
Previously, prediction of fault damage zone width has focused on fault length/displacement
profiles, which can be sub-grouped lithologically as a proxy for the geomechanical properties of
a given rock type. These studies give a wide spread in the observed scaling relationships
between fault length and displacement. Here, we use strain maps produced by fully threedimensional (non-plane strain) geomechanical restorations as a proxy for fault damage. The
geomechanical algorithm restores displacement on faults while minimizing strain in the
surrounding surface using a mass-spring solver. Prescribed mechanical properties govern the
behaviour of the surface and therefore, the distribution of strain around faults. To evaluate the
efficacy of the restoration in predicting fault damage, we compare the spatial distribution of
modelled strain to observed fault damage zones in well documented field examples of natural
reservoir analogues.
September 2008
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NOTES
September 2008
Page 18
Geometry, Kinematics, and Fracture Network Characteristics with Fault Segment
Boundaries, Wasatch Fault Zone, Utah, U.S.A.
Yonkee, W.A.1, Evans, J.P.2, Bruhn, R.L.3
1
Department of Geosciences, Weber State University, Ogden, UT 84408-2507, U.S.A.
2
Department of Geology, Utah State University, Logan, UT 84322, U.S.A.
3
Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112, U.S.A.
Structural boundaries divide most major fault zones into segments that have different
geometries and often different rupture histories. Multiple directions of faulting are generally
required to transfer displacement across boundaries, resulting in development of complex
minor fault networks and concentrated alteration. Such boundaries may act as sites of rupture
nucleation or rupture termination, and be associated with microsesimicity between major
faulting events. Here we describe characteristics of two structural boundaries in the active
Wasatch fault zone of northern Utah: 1- the Pleasant View salient that separates the Brigham
City and Weber segments; and 2- the Traverse Mountains area that separates the Salt Lake
City and Provo segments.
The Pleasant View salient is marked by a major bend from northerly strike to ~315 within the
boundary, an ~3 km left step in the main fault, and a structurally elevated, complexly faulted
block that continues SW in the subsurface. The footwall contains a damage zone of fractured
and altered quartzo-feldspathic gneiss. Fault-related rocks show a progression from older
chlorite breccia and minor phyllonite that likely formed at deeper levels, to microbreccia zones,
to younger highly polished surfaces with well developed slip lineations. North of the boundary,
the footwall damage zone is < 30 m thick and has relatively simple kinematics with mostly westdipping normal faults. Within the structural boundary, the damage zone is >200 m thick and
kinematically complex, with SW-dipping, SE-dipping, and NW-dipping faults that have mostly
normal slip (indicating σ2~σ3).
The Traverse Mountains area is also marked by a major bend from a typical northerly strike to
~ 270 within the boundary, and a complexly faulted hanging wall block (Traverse Mountains)
that continues to the WSW. The footwall contains a damage zone of fractured and altered
granite that increases in thickness from ~ 20 m to the north to >200 m in the boundary. Fault
rocks show a progression from chlorite phyllonite with plastic deformation of quartz that formed
near the base of the seismogenic zone, to cataclastite zones with zeolite veins. Within the
structural boundary, the damage zone is >200 m thick and kinematically complex, with gently to
moderately SW-dipping, steep SE-dipping, and steep NW-dipping faults with normal to oblique
slip (indicating σ2~σ3 along with temporal changes in stress).
Interaction of complex minor fault networks in boundaries may result in geometric hardening if
faults meet at high angles, or in geometric softening if faults meet at low angles with slip
lineations parallel to their intersections. Preferred orientations of minor faults and fracture
networks produce enhanced, anisotropic permeability, modulated by variations in mean and
deviatoric stress, and in reduced, anisotropic elastic moduli. Sealing of minor faults and
fractures may strengthen the damage zone, whereas alteration of feldspar to mica and
hydrolytic weakening of quartz weaken the zone.
September 2008
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NOTES
September 2008
Page 20
Characterization of fault damage zone and deformation band populations based on
outcrop data
S. Schueller1, A. Braathen1,2 and H. Fossen2
1
entre for Integrated Petroleum Research, University of Bergen, Allégaten 41, 5007 Bergen,
Norway
2
University Centre in Svalbard, 9171 Longyearbyen, Norway
Fault damage zones in porous sandstones contain small-scale structures, notably deformation
bands, which may influence fluid flow in reservoirs. This study aims to characterize the
geometry of fault damage zone and especially the distribution of deformation bands using an
outcrop-based database. The bulk of these analogue data was gathered mainly in Utah and
Egypt. Processing of 106 damage zone scanlines reveals a non-linear relationship between the
damage zone width and the fault throw. The results also indicate a logarithmic decrease in
deformation band frequency away from the fault core as well as a fractal spatial distribution
responsible for the clustering of the deformation bands. Parameters such as the footwall and
hanging-wall positions or the folding of the damage zone are also analyzed with regard to the
damage zone width and the deformation band density in the media. This database reveals
several statistical trends that help to characterize damage zones of extensional faults in
siliciclastic sedimentary rocks.
The trends derived from this analysis can be used to simulate statistically the growth of the
damage zone, and the evolution of deformation band populations. These probabilistic models
can then be implemented in reservoir models in order to evaluate reservoir performance of fault
damage zones.
September 2008
Page 21
NOTES
September 2008
Page 22
Mechanics of fault-zone localisation in high-porosity sandstones and impact on flow
efficiency
Christopher Wibberley1 and Elodie Saillet2
1
Total, CSTJF, Av. Larribau, 64018 Pau, France. e-mail: [email protected]
éosciences Azur CNRS, Université de Nice – Sophia Antipolis, 250 rue A. Einstein, 06560
Valbonne, France e-mail: [email protected]
2
Excellent exposures of Cretaceous high-porosity sands and sandstones from the Bassin du
Sud-Est, France, allowed us to examine: (i) the role of tectonic loading path on cataclastic
deformation band network development; (ii) the development of larger ultracataclastic faults
during deformation, and (iii) the likely impact of deformation bands and faults on flow efficiency
in high-porosity sandstone reservoirs. For a study area which had been subjected mainly to late
Cretaceous shortening, a 250 m long outcrop recorded a persistent high density of reversesense conjugate deformation bands which did not appear to cluster around any mapped faults.
For two study areas which had experienced significant Oligocene-Miocene extension, a
moderate, undulating background density of normal-sense deformation bands was recorded,
which became focussed into clusters in places. Thus tectonic loading path and the nature of the
stress changes causing deformation may strongly influence strain distribution. Larger
ultracataclastic faults and discrete slip planes are found localised within or at the edges of some
of the deformation band clusters, demonstrably post-dating the deformation band cluster in one
case, but other clusters are present without larger faults within them. Hence these structures
formed by progressive localisation of deformation through deformation band clustering to form
the larger ultracataclastic faults, rather than in a damage zone which spreads with displacement
increase after fault initiation. Permeability measurements of these ultracataclastic faults suggest
that they may severely impact on flow efficiency during production of hydrocarbon reservoirs,
and sub-seismic prediction of such zones is therefore critical to production management. Lowdisplacement deformation bands however, have a variable effect on flow efficiency but impact
most when produced by tectonic shortening.
September 2008
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NOTES
September 2008
Page 24
Strain accumulation and fluid flow in and around basin bounding fault zones of the
Leichhardt River Fault Trough, Qld. Australia
J. G. McLellan, Predictive Mineral Discovery Co-operative Research Centre, Economic
Geology Research Unit, James Cook University, Townsville, Queensland, 4811, Australia
The Leichhardt River Fault Trough (LFRT) in the western Mount Isa Inlier, northwest
Queensland, provides a good example of a relatively well preserved rifted basinal architecture,
which allows a solid framework for rigorous testing of numerical scenarios in such a setting.
The Pb-Zn-Ag and Cu mineral endowment of the Mount Isa Inlier is world-class, and this
provides a strong foundation for current and future exploration in the region. To increase our
predictive capacity we must try to better understand the early deformational influence (basin
development) over fluid pathways and fluid driving mechanisms. The LRFT has undergone a
protracted deformational history and here the deformation, fluid flow and mineralization
processes are addressed by several simulations in the numerical code FLAC3D. During
extensional rifting, deformation is partitioned with major basin bounding structures
accommodating the majority of the strain, areas of high shear strain, dilation and fluid flow are
focused in basin bounding structures, particularly in and around the western basin margin. This
focussing mechanism on the western basin margin is the result of a self-organised behaviour
related to the asymmetry of the basin geometry. A thickening wedge to the west and a
basement detachment zone which influences the distribution of strain within the upper crustal
components of the system. Extension and topography play an important role in facilitating
downward migration of fluids deep into the system. Deformation induced dilatancy and
topography provide the required conditions suitable for brine reflux within the superbasins,
which is an important process for mineralising systems. Later basin inversion facilitates
potential mixing of shallow basinal and deep seated basement derived fluids before migration to
depositional sites primarily in the hanging-wall sediments of the Isa Superbasin. The hangingwall sediments and intersections of N-S trending basin bounding structures and E-W trending
structures are key areas for focusing shear strain, dilation, high cumulative fluid flux and
potential mineralization in the Leichhardt River Fault Trough, western Mount Isa Inlier.
September 2008
Page 25
NOTES
September 2008
Page 26
Structural and statistical analyses of fault-controlled hydrocarbon migration and
accumulation
*Agosta F. ([email protected]), *Alessandroni M., **Antonellini M., *Tondi E.
*Dipartimento di Scienze della Terra, Università di Camerino, Via Gentile III da Varano, 62032
Camerino (MC), ITALY
**CIRSA, Centro Interdipartimentale di Ricerca per le Scienze Ambientali, Università di
Bologna, Via Tombesi dall'Ova 55, 48100 Ravenna, ITALY
Excellent outcrops located within a hydrocarbon-rich quarry, cropping out along the northern
side of the Majella anticline (Central Italy), allowed us to study both deformation mechanisms
and hydraulic properties of normal-oblique faults. By combining large-scale geological mapping
with detailed structural and statistical analyses of their internal deformation, we were able to
assess: (i) the mechanisms of fault initiation and fault growth within a carbonate grainstones
protolith, (ii) the timing of faulting with respect to large-scale folding of the anticline, and (iii) the
role played by faults (distinguished in small, medium, and large, respectively) and fractures in
the migrations and accumulation of hydrocarbons.
At a large scale, the oil show (the studied quarry) is located within an extensional relay ramp
bounded by two oblique normal faults. These large faults developed to a few km in length, and
solved up to 10’s meters offset. Their internal architecture is comprised of inner fault cores
made up of brecciated and comminuted fault rocks and major slip surfaces surrounded by
thicker damage zones. The latter zones are characterized by intense fracturing, dilation of
favourably oriented pre-existing stylolites and sheared stylolites, and minor faults. Within the
quarry, hydrocarbons in form of tar are largely present in the faults damage zones, and in the
less deformed portions of the fault cores (breccia), as well as along some of the major slip
surfaces bounding these cores.
The whole extensional relay ramp is crosscut by several normal, oblique, and strike-slip faults
that are classified as medium (1m < offset < 10m) and small faults (offset < 1m). The
architecture of medium faults is made up of inner fault cores, comprised of fragmented
carbonates and discontinuous slip surfaces that bound isolated blocks of fault breccias and
comminuted fault rocks, and outer damage zones that include stylolites, sheared stylolites,
subvertical cracks and veins, and small faults. These small faults, conversely, rarely show
presence of inner fault cores. Their architecture generally consists of discontinuous bedbounded slip surfaces, stylolites, sheared stylolites, and rare subvertical cracks and veins. Tar
distribution shows that extensional jogs bounded by adjacent normal and oblique small and
medium faults represent the favoured sites for hydrocarbon migration.
The relations among the individual fracture characteristics (orientation, spacing, length, and
opening) and tar distribution were statistically analyzed in the damage zones of the two large
faults. The results are consistent with the following conclsuions: (i) Hydrocarbon migration was
not influenced neither by fracture density nor by fracture length. (ii) Field evidences suggest
that connectivity to, and distance from, nearby larger hydrocarbon conduits (e.g., slip surfaces)
played the most important role. These evidences are more pronounced in the hanging wall
damage zones, probably due to the pronounced cracking that occurred in these zones. (iii)
Fracture infilling, as well as fracture opening, were also affected by the current hmax acting in
central Italy. These conclusions will be tested soon by the results of well logs (acoustic,
resistivity, and gamma ray data), core, and hydraulic analyses of the largest oblique normal
fault (offset > 40m).
September 2008
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NOTES
September 2008
Page 28
Experimental modeling of extensional fault domains and fault-domain boundaries
(transfer zones / accommodation zones)
Roy W. Schlische and Martha Oliver Withjack, Department of Earth & Planetary Sciences,
Rutgers
University,
610
Taylor
Road,
Piscataway,
NJ
08854-8066,
USA
([email protected]; [email protected])
Fault domains, in which all or most normal faults dip in the same direction, are common in
many extensional provinces. Fault-domain boundaries are zones that separate adjacent fault
domains, and are variously referred to as transfer zones or accommodation zones. We have
used experimental (analog) models of uniform extension to study the origin, geometry, and
evolution of fault domains and their boundaries. Our models show that fault domains and their
boundaries develop with both orthogonal and oblique extension and with both dry sand and wet
clay as the modeling material. The size and shape of the fault domains and the number and
orientation of their boundaries is highly variable, even for identical models. Generally, faultdomain boundaries are broad zones of deformation, consisting of overlapping tips of normal
faults from adjacent fault domains, fault-displacement folds, and numerous small-scale normal
faults. The fault-domain boundaries in our models differ significantly from those in published
conceptual models of transfer zones / accommodation zones. Specifically, the fault-domain
boundaries in our models are broad zones of deformation, not discrete strike-slip or oblique-slip
faults; their orientations are not systematically related to the extension direction; and they can
form spontaneously without any prescribed pre-existing zones of weakness.
We infer that the fault domains in our models result from the self-organized growth of fault
populations in which the stress-reduction zones of large, parallel faults are less likely to overlap
and inhibit fault growth. The spatial arrangement of fault domains and their boundaries is
governed by the spatial distribution and dip direction of the earliest formed large normal faults,
the locations of which are, at least in part, controlled by a random distribution of flaws
(nucleation points). Our models show that the presence of multiple fault domains affects the
size of normal faults because the length of an individual fault cannot exceed the fault-parallel
width of its fault domain. Consequently, fault lengths are more likely to be constrained as strain
increases and fault domains interact. Additionally, although the fault population as a whole will
show a positive relationship between fault length and displacement, the displacement-length
scaling relationship may change with increasing strain. The presence of fault domains may
contribute, in part, to the large scatter in length-displacement data observed for natural fault
populations.
September 2008
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NOTES
September 2008
Page 30
Predictive Modelling of the Evolution of Fault Zone Structure: 3-D Sandbox and Field
Scale Modelling
D.A. Thornton, Rockfield Software Limited, Technium, Prince of Wales Dock, Swansea, UK
A.J.L. Crook, Rockfield Software Limited, Technium, Prince of Wales Dock, Swansea, UK
J.G. Yu, Rockfield Software Limited, Technium, Prince of Wales Dock, Swansea, UK
Predictive modeling of fault zone structure requires reconstruction of the stress and
deformation history using an integrated modelling framework that accounts for the
simultaneous evolution of the internal state of the rock formation due to the imposed
boundary conditions. This necessitates the concurrent computation of displacement, fluid
pressure and temperature history, together with the additional variables dependent upon the
specific physics included in the model.
This paper describes ongoing research on some of the key elements required for this class of
simulation methodology and, in particular, presents predictive 3-D simulations of fault zone
growth and discusses issues relating to the application of fully-coupled geomechanical and
fluid flow models to field scale applications. Issues addressed include:
1
2
3
4
The strongly coupled nature of the mechanical deformation and the flow fields.
Algorithms for prediction of the onset and evolution of faults.
Scale up from laboratory-scale sandbox tests to field scale models.
Appropriate constitutive models for the evolution of the material state boundary surface.
This work is an extension of a previously published study (Crook et al., 2006a, 2006b) that
focused on predictive modelling of structure evolution in sandbox experiments. The
computational approach adopts the Lagrangian finite element method, complemented by
robust and efficient automated adaptive meshing techniques, a constitutive model based on
critical state concepts, and global energy dissipation regularized by inclusion of fracture
energy in the equations governing state variable evolution. The modelling approach has been
benchmarked by forward simulation of two extensional sandbox experiments that exhibit
complex fault development. It is emphasized that no initial perturbations or fault seeding is
imposed so that structure evolves solely from the prescribed movement on the basal
detachment.
In this study, simulations for compression and inversion tectonic regimes are briefly presented
based on sandbox experiments investigating the evolution of doubly vergent thrust systems
(McClay et al, 2004) and the evolution of inverted listric systems (McClay and Buchanan,
1991). Simulations of 3-D extensional sandbox experiments performed by (Yamada and
McClay, 2003) will then be presented. These results, in conjunction with the previously
presented extensional tectonic simulations Crook et al. (2006a), show that the model is able
to reproduce the experimentally observed faulting style in all three deformational regimes; i.e.
the model is truly predictive.
The extension from laboratory-scale to field-scale necessitates coupling of displacement and
pore pressure evolution together with an appropriate treatment of the complex constitutive
response. For example: (i) overpressure development; (ii) porosity reduction induced by
mechanical and/or chemical compaction; and (iii) strengthening due to cementation, all alter
the position of the stress state relative to the state boundary surface, thereby either increasing
or decreasing the likelihood of fault formation. It is shown that in order to capture these
mechanisms the constitutive model must trace the evolution of a state boundary surface that
is defined in terms of the complete stress tensor rather than being only dependent on
porosity. While this class of model, formulated by extending critical state concepts, has
previously been adopted by several researchers (e.g. Luo et al., 1998; Pouya et al., 1998;
Duedé et al., 2004), generally only mechanical compaction has been considered.
Furthermore, most previous studies have focused on relatively simple sedimentation
problems which do not require the additional complex computational framework necessary to
represent evolving faults with large relative displacements.
September 2008
Page 31
A field scale reconstruction with evolving fault architecture driven by tectonically induced
stress will be presented to illustrate the impact of differing assumptions for pore pressure
evolution on the predicted fault architecture, and also highlight several issues related to
practical field scale coupled geomechanical/flow modelling.
September 2008
Page 32
September 2008
Page 33
References
Buchanan, P.G. and McClay, K.R. [1991] Sandbox experiments of inverted listric and planar
fault systems. Tectonophysics 188, 97-115.
Crook, A.J.L., Willson, S.M., Yu, J.G., Owen, D.R.J. [2006a] Predictive modelling of structure
evolution in sandbox experiments. J. Struct. Geol. 28, 729-744.
Crook, A.J.L., Owen, D.R.J., Willson, S.M., Yu, J.G. [2006b] Benchmarks for the evolution of
shear localisations with large relative sliding in frictional materials. Comp. Meth. Appl. Mech.
Engng. 195, 4991-5010.
Deudé, V., Dormieux, L., Maghous, S., Bathélémy, J.F., Bernaud, D. [2004] Compaction
process in sedimentary basins: role of stiffness increase and hardening induced by large
plastic strains. Int. J. Num. Anal. Meth. Geomech. 28, 1279-1303.
McClay, K.R. [1990] Extensional fault systems in sedimentary basins: a review of analogue
model studies. Marine and Petroleum Geology 7, 206-233.
McClay, K.R., Whitehouse, P.S., Dooley, T., Richards. M. [2004] 3D evolution of fold and
thrust belts formed by oblique convergence. Marine and Petrol. Geology 21, 857-877.
Luo, X., Vasseur, G., Pouya, A., Lamoureux-Var, V., Poliakov, A. [1998] Elastoplastic
deformation of porous media applied to the modelling of compaction at basin scale. Marine
and Petroleum Geology 15, 145-162.
Pouya, A., Djeran-Maigre, I., Lamoureux-Var, V., Grunberger, D. [1998] Mechanical
behaviour of fine grained sediments: experimental compaction and three-dimensional
constitutive model. Marine and Petroleum Geology 15, 129-143.
Schneider, F., Hay, S. [2001] Compaction model for quartzose sandstones application to the
Garn Formation, Haltenbanken, Mid-Norwegian Continental Shelf. Marine and Petroleum
Geology 18, 833-848.
Wangen, M. [2001] A quantitative comparison of some mechanisms generating overpressure
in sedimentary basins. Tectonophysics 334, 211-234.
Yamada, Y. , McClay, K. [2003] Application of geometric models to inverted listric fault
systems in sandbox experiments. Paper 1: 2D hanging wall deformation and section
restoration. J. Struct. Geol., 25, 1551-1560.
Yamada, Y., McClay, K.
3-D Analog Modelling of Inversion Thrust Structures,
in K. R. McClay, ed., Thrust tectonics and hydrocarbon systems: AAPG Memoir 82, pp. 276301
September 2008
Page 34
NOTES
September 2008
Page 35
Influence of pre-existing fabric on normal-fault development: an experimental study
Alissa A. Henza1, Martha O. Withjack1, Roy W. Schlische1, Iain K. Sinclair2
1
Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Rd,
Piscataway, NJ 08854 USA
2
Husky Energy, Suite 810 Scotia Centre, 235 Water St., St. John’s, NL, Canada
Many rift basins have undergone multiple episodes of extension with differing extension
directions. Do the normal faults that form during an early episode influence the development
of normal faults that form during subsequent episodes? Does this influence depend on the
characteristics of the early-formed faults (i.e., their number, density, length, displacement)?
To address these questions, we have conducted a series of scaled experimental (analog)
models with wet clay. Each model had two phases of distributed extension, and the
extension directions during the first and second phases differed by 45°. Because the
characteristics of the fault populations at the end of the first phase depended on the total
magnitude of extension, we incrementally varied the total magnitude of the first-phase
extension from 18 to 35%. As the magnitude of extension increased, the number, density,
length, and displacement of the normal faults that formed during the first phase also
increased. In all models, the total magnitude of extension was 35% during the second phase
of extension.
The experimental models show that the characteristics of the fault populations that formed
during the first phase of extension profoundly affected the fault patterns that developed during
the second phase of extension. When the total magnitude of the first-phase extension was
small (~18%), only a few short normal faults developed during the first phase. This poorly
developed fabric associated with these first-phase faults had little influence on the
subsequent deformation. Specifically, the normal faults that formed during the second phase
of extension had orientations, lengths, and displacements similar to those in models without a
first phase of extension. When the total magnitude of the first-phase extension was greater
than ~20%, numerous large normal faults developed during the first phase, and they
significantly affected the subsequent deformation. Many of the first-phase normal faults were
reactivated as oblique-slip faults during the second phase of extension. Additionally,
numerous new normal faults developed during the second phase of extension. The secondphase normal faults were most likely to cut the first-phase normal faults when the magnitude
of the first-phase extension was small. Otherwise, most of the second-phase normal faults
nucleated at the first-phase faults or terminated against them. Generally, the second-phase
normal faults had anomalously short lengths compared to the first-phase faults, indicating that
the presence of the first-phase faults had inhibited the propagation and growth of the secondphase faults. Interestingly, the orientations of the second-phase normal faults were both
orthogonal and oblique to the direction of the second-phase extension. This suggests that the
formation of the second-phase normal faults was influenced by local perturbations of the
stress state associated with first-phase faults.
The model fault patterns resemble those observed on 3D seismic data from the Grand Banks
(e.g., Jeanne d’Arc basin), an area hypothesized to have undergone two non-coaxial
extensional phases. Thus, the models may provide templates for interpreting the fault
patterns and interactions in the Grand Banks as well as other regions with multiple phases of
extension.
September 2008
Page 36
NOTES
September 2008
Page 37
Fault-surface corrugations: Insights from scaled experimental models of extension
Amber B. Granger, Haley & Aldrich, Inc, 299 Cherry Hill Rd, Suite 105, Parsippany, New
Jersey 07054-1124, USA ([email protected])
Martha Oliver Withjack and Roy W. Schlische, Department of Earth & Planetary Sciences,
Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854-8066, USA
([email protected]; [email protected])
Many fault surfaces, observed in outcrop and 3D seismic data, have complex morphologies
with numerous corrugations that trend parallel to the slip direction. We have used scaled
experimental (analog) models with wet clay to study these features. Our models have
simulated extensional deformation (i.e., normal faulting) using three common basal boundary
conditions: two diverging, overlapping plates; a stretching, basal rubber sheet; and a
stretching, basal layer of silicone polymer. During the experiments, we photographed the top
surface of the models at regular time increments. After the experiments, we constructed
structure-contour maps for several normal-fault surfaces using closely spaced (1-mm apart)
serial sections. The surface photographs, showing exposed fault scarps, and the structurecontour maps, showing subsurface features, clearly demonstrate that the normal-fault
surfaces in all models are corrugated at various scales. The surface photographs indicate
that many of the large-scale corrugations formed during the linkage of originally separate fault
segments. The origin of small-scale corrugations, however, remains enigmatic. These
corrugations are subparallel to the slip direction, and are present along the entire extent of the
fault surfaces. These observations suggest that the original small-scale corrugations are not
tool-and-groove features because their lengths exceed the net slip. Furthermore, small,
relatively isolated normal faults exhibit the same small-scale corrugations as larger normal
faults.
Experimental models with two non-coaxial phases of extension provide insight into the origin
of the small-scale corrugations. During the second phase of extension, many of the firstphase normal faults reactivate as oblique-slip faults. New small-scale corrugations develop
on the exposed fault scarps of these reactivated faults. These new small-scale corrugations
overprint the original corrugations, are less well defined than the original corrugations, and
are subparallel to the new slip direction. They are not related to fault propagation and linkage
because they develop on pre-existing, through-going fault surfaces. Does the same process
produce the small-scale corrugations during the first and second phases of extension? We
hypothesize that the small-scale corrugations are related to incremental differential slip along
fault-segment surfaces, both during initial fault development and fault reactivation.
September 2008
Page 38
NOTES
September 2008
Page 39
Fault Facies modeling; possibilities and difficulties
Nøttveit, H., Espedal, M.S. & Tveranger, J., Centre for Integrated Petroleum Research,
University of Bergen, Norway
Depending on their internal structure and distribution of petrophysical properties, fault zones
may act as barriers and/or conduits in subsurface reservoirs. However, our means for
implementing, and thus also quantifying the impact of 3D fault zone architecture on reservoir
flow are limited by technical constraints of conventional modeling software.
The Fault Facies modeling concept offers a means for more realistic description of faults in
reservoir models. By providing volumetric fault zone grids, all standard facies and
petrophysical modeling tools developed for sedimentary facies modeling, can be employed for
fault zone modeling, facilitating explicit implementation of fault zone features as well as multiphase flow properties. Uncertainty assessment also benefits from this approach, as the
sedimentary and structural heterogeneities are treated equal.
The present work focuses on the property modeling when using the Fault Facies modeling
concept, emphasizing the representation of multi-scale multi-continuum fault properties in
Fault Facies models.
Fault facies modeling of complex fault architectures is demonstrated for two fundamentally
different faults zones (carbonates and sandstones).
Fluid flow simulations performed on any scale require estimation of effective properties
(upscaling). Upscaling is, however, complicated by the complex scaling relationships within
faults. A nested local upscaling method is presented, giving improved realism to the estimated
effective properties.
Faults in highly brittle rocks often involve high-permeable fracture networks. Simulating fluid
flow in these faults require a dual-porosity approach. A dual-porosity model-setup is
demonstrated, and implications for upscaling are discussed.
September 2008
Page 40
NOTES
September 2008
Page 41
Using empirical geological rules to reduce structural uncertainty in seismic
interpretation of faults
Freeman, B.1, Boult, P.2, and Yielding G. 1
1
Badley Geoscience Ltd, Hundleby, Spilsby, United Kingdom.
2
Consultant, Adelaide, Australia.
Good seismic interpretation of faults should include a workflow that checks the interpretation
against known structural properties of fault systems (a knowledge-based rule set). Estimates
of wall-rock strains provide one objective means for discriminating between correct and
incorrect structural interpretations of 2D and 3D seismic data - implied wall-rock strain should
be below a geologically plausible maximum. We call this the strain minimisation approach.
Fault population statistics from several dozen publications show that fault strike lengths and
maximum throws have a log-log distribution, their geometries are scale-invariant, and that
maximum displacement on faults rarely exceeds 1/10 of their strike length. Interpreters can
use this knowledge base as a check for geologically plausible seismic interpretations. By
assuming that the maximum dip-dimension of faults is ½ the maximum strike dimension, an
upper limit of 0.1 can be placed on plausible wall-rock shear strain, and 0.2 for maximum wallrock longitudinal strain when measured in the displacement direction. Small-scale variation of
fault wall-rock strain also adheres to this rule, except in specific areas of strain localisation
such as relay zones.
We present a case study where these simple rules provided a quantitative check on the
plausibility of an interpretation. We reviewed an original structural model (interpretation of 2D
seismic surveys completed by a third party), and by mapping shear and extensional strain on
their fault planes showed that the computed wall-rock strains for these parameters were
commonly above 0.1 and 0.2 respectively. Thus this third party structural model was very
suspect. We then reinterpreted the area in an iterative manner using the strain minimisation
approach. By using regions of implied high wall-rock strain as an indicator of high uncertainty
in the interpretation, we were able to break out two self-consistent faults sets, which had
geologically plausible wall-rock strains, where previously there had only been one fault set
with highly implausible wall-rock strains. The new structural interpretation based on the 2D
seismic data was later found to be consistent with an interpretation of a nearby 3D seismic
volume that only became publicly available after the original work.
September 2008
Page 42
NOTES
September 2008
Page 43
Fault seal prediction of seismic-scale normal faults in porous sandstone: A case study
from the eastern Gulf of Suez rift, Egypt
Christian Tueckmantel1,2, Quentin Fisher1, Rob Knipe1,2, Henry Lickorish3 and Samir Khalil4
1
Centre for Integrated Petroleum Engineering and Geoscience, School of Earth and
Environment, University of Leeds, Leeds, LS2 9JT
2
Rock Deformation Research Limited, University of Leeds, Leeds, LS2 9JT
3
22 140 Point Drive NW, Calgary, T3D 4W3, Canada
4
Geology Department, Faculty of Science, Suez Canal University, Ismailia, 41522, Egypt
A study of normal faults in the Nubian Sandstone Sequence, from the eastern Gulf of Suez
rift, has been conducted to investigate the relationship between the microstructure and
petrophysical properties of cataclasites developed along seismic-scale faults and smaller
offset faults (deformation bands) found in their damage zones. This was to quantify the
uncertainty associated with predicting the fluid flow behaviour of seismic-scale faults by
analysing small faults in core, a common procedure in the petroleum industry. The
microstructure of the cataclasites was analysed as well as their single-phase permeability,
threshold pressure and grain-size distribution. Faulting occurred at a maximum burial depth of
~1 km. Cataclasites delineate major slip surfaces and build up damage-zone deformation
bands. Our results show that the lowest measured deformation-band permeabilities provide a
good estimate for the permeability of the major slip cataclasites. This suggests that cataclastic
permeability reduction is mostly established early in the deformation history. Stress at the
time of faulting rather than final strain seems to be the critical factor. For viable predictions it
is important that the slip cataclasites and deformation bands originate from the same host. On
the other hand, a higher uncertainty is associated with threshold pressure prediction, as the
lowest slip-cataclasite threshold pressure exceeds the highest deformation-band threshold
pressure by a factor of ~3. This may be due to microfractures introduced during exhumation
or sampling, which bypass thin deformation bands but do not affect thick slip cataclasites.
September 2008
Page 44
NOTES
September 2008
Page 45
Structural analysis of the exhumed SEMP fault zone, Austria: Towards an
understanding of fault zone architecture and mechanics throughout the seismogenic
crust
Frost, E., Dolan, J. F., Sammis, C.G., University of Southern California
Hacker, B., Cole, J., University of California at Santa Barbara
Ratschbacher, L., University of Freiberg.
One of the most exciting frontiers in earthquake science is the linkage between the internal
structure and mechanical behavior of fault zones. Little is known about how fault-zone
structure varies as a function of depth, yet such understanding is vital if we are to understand
the mechanical instabilities that control the nucleation and propagation of seismic ruptures.
This has led us to the Salzach-Ennstal-Mariazell-Puchberg [SEMP] fault system in Austria, a
major left-lateral strike-slip fault that has accommodated ~ 60 km of displacement during
Oligo-Miocene time. Differential exhumation of the SEMP has resulted in a fault zone that
reveals a continuum of structural levels along strike. This provides us with a unique
opportunity to directly observe how fault-zone properties change with depth, from nearsurface levels, down through the seismogenic crust, across the brittle-ductile transition, and
into the uppermost part of the lower crust in western Austria. Here we present results from
four key outcrops and discuss the mechanical implications of these new data.
Our brittle outcrop at Gstatterboden has been exhumed from at least 4 km depth. Here the
SEMP juxtaposes limestone of the Wettersteinkalk on the south against dolomite of the
Ramsaudolomit on the north. Faulting has produced extremely asymmetric damage,
extensively shattering and shearing the dolomite while leaving the limestone largely intact.
We interpret this brittle damage using both mesoscopic calculations of damage intensity and
microscopic grain-size-distribution analysis, and propose that strain has progressively
localized to a zone ~ 10 m wide. These findings are compared to those from two outcrops
(Kitzlochklamm and Liechtensteinklamm) that bracket the brittle-ductile transition, exhumed
from depths of ≥ 10 km. Here, the SEMP juxtaposes Greywacke Zone rocks on the north
against carbonate mylonites of the Klammkalk to the south. We calculate the strain gradient
in the ductile Klammkalk rocks by analyzing the lattice preferred orientation (LPO) of calcite
grains throughout the outcrop. Deformation in the Greywacke Zone, however, contains a
significant component of solution mass transfer, and we therefore estimate the strain in these
rocks by calculating the change in bulk volume. These analyses do not find significant levels
of strain distributed within the Klammkalk or Greywacke Zone, again revealing a highly
localized fault zone.
Our investigation of the downward continuation of the SEMP into the Tauern Window
indicates that the fault remains discrete at mid-crustal levels, with the majority of strain
occurring in a 100-m-wide ductile shear zone (Cole et al., 2007). Combined with the recent
work of Rosenberg et al. (2007), who have studied the deepest exposures of the SEMP in the
western Tauern Window, these data allow us to present a three-dimensional picture of fault
zone architecture and mechanics from the top of the seismogenic zone all the way into the
ductile lower crust.
September 2008
Page 46
NOTES
September 2008
Page 47
Fault Facies methodology for systematizing analogue outcrop data to 3D fault grids in
reservoir models
Braathen1,2, A., Tveranger1, J., Fossen1, H., Schueller1, S., Espedal1
1
Centre for Integrated Petroleum Research, University of Bergen, Norway
2
University Centre in Svalbard, 9171 Longyearbyen, Norway
Fault facies methodology aims on systematic description and representation of faults
observed in nature. The approach has three steps; (i) establishing empirical relationships for
fault zoning, (ii) applying facies classification schemes on structural elements in the zones,
and (iii) assessing the systematic fault element characteristics by statistical analysis.
Together, these steps define datasets that can be used to condition volumetric fault reservoir
grids.
The concept of fault facies encompasses the deformational products of any rock volume
affected by faults. The presented facies database describes extensional faults in sand-shale
sequences, with datasets from Sinai, Utah, Corsica, and Norway. The analogue database is
organized from the fault envelope downwards into core and surrounding damage zones, and
further into Facies Associations that consist of one or more Fault Facies. For example, the
Core Architectural Element is commonly made up of various fault rock membranes, lenses,
and fracture and deformation band sets. By considering for example lenses of host sandstone
as one Facies Association, several facies can be identified, based on the occurrence of
deformation band sets within the lenses. Statistical analysis of the fault facies database
establishes dimensions, geometries and scales of various structural elements. Critical
assessments of length and width relations of core and damage zone reveal complementary
empirical trends that can be used in fault scaling considerations.
In total, fault facies modeling represents a powerful reservoir assessment tool. It opens for
evaluation of fault-parallel flow, capillarity effects and communication between non-juxtaposed
cells.
September 2008
Page 48
NOTES
September 2008
Page 49
What are the Potential Impacts of Low-offset Faults on Carbonate Reservoir Performance?
Susan M. Agar1, Stephan Matthai2, Ravi Shekhar1, Isha Sahni1
1. ExxonMobil Upstream Research Company, Houston, TX 77007
2. Imperial College, London SW7 2AZ
Some of the world's largest hydrocarbon reservoirs are found in weakly deformed carbonate rocks at
shallow crustal levels (< 5 km). The assemblages of fractures, stylolites and low-offset faults that are
typically found in these reservoirs can have substantial impacts on the flow behavior, even though the
bulk strain is very low. Observations of outcrop analogs for these reservoirs in the Middle East and N.
Africa, as well as seismic interpretations, indicate that many low-offset faults in carbonate rocks can
develop substantial vertical and lateral continuity (100 m - 1 km) even though their normal and strikeslip offsets are on the order of 0.5 m - 25 m. Other common characteristics of these fault zones
include: a segmented character with numerous small relay zones, clearly-defined, discrete fault slip
planes between segments, very limited or no fault gouge development, very limited or no damage
zone development, incomplete cementation, alteration haloes and vuggy, karst / fault breccia-type
porosity.
The scale of continuity of these low-offset faults means that they can have significant impacts on flow
performance, acting either as conduits or baffles. If a low-offset fault acts primarily as a conduit, it can
provide pressure and fluid communication between different reservoir units that would otherwise
remain isolated by the lower-permeability beds between them. In this and the case of dominantly
baffling behavior, there may be substantial reductions in sweep efficiency. Consequently, these very
subtle faults are likely to have substantial economic impacts on hydrocarbon recovery.
Many of these faults are at or below the threshold for seismic resolution and core samples from
subsurface fault zones are commonly not available. As a result, many assumptions for the specific
architectures and mineralization of these faults are required for flow simulations. In an attempt to
understand the sensitivity of flow behavior to these assumptions, preliminary, generic flow
experiments have been undertaken to determine how much difference changes to the low-offset fault
architecture make to a flow prediction. Our initial results indicate that even with homogeneous matrix
properties the assumptions for the number of fault segments, the degree of overlap between the
segments and the extent of damage zone development can introduce substantial differences in flow
predictions for sub-km volumes of rock.
Recognizing the limitations of the modeling approaches used in these experiments, our preliminary
results for water injection, suggest that subtle changes in the fault zone characteristics can make
large differences in predicted times to water breakthrough. The results also reinforce the fact that
while well data provide information for a volume-averaged flow response, other approaches are
needed to gain insights to the impacts of specific geologic features on flow paths and velocities on
production timescales. In this work, our aim is to develop a better understanding of the specific lowoffset fault characteristics that have the greatest impact on the distribution of flow velocities in the
reservoir. Through this approach, we aim to improve strategies for hydrocarbon recovery and history
matching.
September 2008
Page 50
NOTES
September 2008
Page 51
KEYNOTE:
A geometric model for the development of fault zone and fault rock thickness
variations
Conrad Childs1, Tom Manzocchi1, John J. Walsh1, Christopher G. Bonson2, Andrew Nicol3,
Martin P.J. Schöpfer1
1
Fault Analysis Group, University College Dublin, Dublin, Ireland
2
SRK Consulting (UK) Limited, Cardiff, UK, CF10 2HH
3
GNS Science, Lower Hutt, New Zealand.
The thicknesses of fault rock and fault zones and the fault normal separations for intact and
breached relay zones each show a positive correlation with fault displacement. The
displacement to thickness ratio for these different structures increases from intact relay zones
(median value = 0.28) to fault rocks (median value = 50). The frequently recorded positive
correlation between fault displacement and fault rock thickness is often interpreted as a
growth trend controlled primarily by fault rock rheology. However recognition of similar
correlations for the other fault components suggests a geometrical model may be appropriate.
In this model a fault initiates as a segmented array of irregular fault surfaces. As displacement
increases, relay zones separating fault segments are breached and fault surface irregularities
are sheared off, to form fault zones containing lenses of fault bounded rock. With further
displacement these lenses are progressively comminuted, and ultimately converted to zones
of thickened fault rock. The final fault rock thickness is therefore influenced strongly by fault
structure inherited from the geometry of the initial fault array. The model is one of progressive
strain concentration within a zone within which the active fault surface progressively
approaches, albeit along a potentially complex path, a more planar geometry. The large scale
range on which fault segmentation and irregularities occur provides the basis for application
of this model over a scale range of 8 orders of magnitude. The model is consistent with
outcrop observations of the internal structure of fault zones, the large variations in fault rock
thickness observed for a given displacement and with recently developed discrete element
models of fault zone evolution.
September 2008
Page 52
NOTES
September 2008
Page 53
September 2008
Page 54
KEYNOTE:
How granulated/cracked fault border zones, and their pore fluids, interact with
earthquake rupture dynamics
James R. Rice, Department of Earth and Planetary Sciences, and Division of Engineering and
Applied Sciences, Harvard University, Cambridge, MA 02138
Recent contributions on fault zones include insightful field characterizations of their fine
structure, new laboratory experiments that reveal response in rapid and/or large slip, and new
theoretical concepts for modeling. The purpose of this talk is to review those new
perspectives, particularly those relating to damaged fault border zones and the fluids which
they host, and their impact on how we think about earthquake rupture dynamics.
Maturely slipped faults show a generally broad zone of damage by cracking and granulation,
but nevertheless suggest that shear in individual earthquakes takes place with extreme
localization to a long-persistent slip zone, < 1-5 mm wide, within a finely granulated,
ultracataclastic fault core. Relevant fault weakening processes during large crustal events are
therefore likely to be thermal and, given the damage zones and geologic evidence of waterrock interactions within them, it seems reasonable to assume pore fluid presence.
It is suggested that there are two primary dynamic weakening mechanisms during seismic
slip, both of which are expected to be active in at least the early phases of nearly all crustal
events. Those are (1) Flash heating at highly stressed frictional micro-contacts, and (2)
Thermal pressurization of fault-zone pore fluid. Both have characteristics which promote
extreme localization of shear. At sufficiently large slip, macroscopic melting will occur in cases
for which those processes have not efficiently enough reduced heat generation, and thus
limited temperature rise. Thermally driven decompositions may instead occur in lithologies
such as carbonates and, in silica-rich lithologies, formation of a thixotropic gel-like layer may
contribute to weakening at large slip.
Theoretical modeling based on mechanisms (1) and (2), as constrained with lab-determined
hydrologic and poroelastic properties of fault core material and high-speed friction studies,
suggests that earthquakes on mature faults might be plausibly described by those
mechanisms. Results suggest that faults may be statically strong but dynamically weak under
typical seismic conditions. Such allows major faults to operate under low overall driving
stress, with realistic seismic stress drops, a self-healing rupture mode, low heat outflow, and
an absence of shallow fault melting.
Another source of dynamic weakening, at least in mode II slip, comes from contrast across
the fault of far-field elastic stiffness and density of the bordering crustal rock. Recent work has
shown that contrast across the fault of permeability and poroelastic properties within fluidsaturated damage fringes along the fault walls has an analogous effect. Both allow for
reductions of effective normal stress during suitably directed non-uniform slip, like at a rupture
front, although the "preferred" rupture direction based on one effect may either align with, or
may oppose, that based on the other.
Other new perspectives in recent work involve understanding the interaction of rupture with
off-fault damage (branches, damage zones) and the induction of off-fault plasticity, together
with their interaction back onto rupture dynamics. As examples, in some cases the transition
to supershear may be suppressed, or at least ling delayed, by plasticity and, for dissimilar
materials, the inclusion of elasticity can reverse an elastically preferred direction.
September 2008
Page 55
NOTES
September 2008
Page 56
The effect of the intermediate principal stress on shear band strike and dip in the
siltstone straddling the active Chelungpu Fault, Taiwan
Bezalel Haimson1 and John Rudnicki2
1
University of Wisconsin, USA
2
Northwestern University, USA
The Taiwan Chelungpu-fault Drilling Project (TCDP) was initiated in order to investigate the
rupture mechanism of the 1999 disastrous Chi-Chi earthquake (Mw 7.6). Two adjacent (40 m
apart) scientific boreholes were drilled, which intersected the fault at about 1120 m and
reached depths of 2000 m (hole A) and 1400 m (hole B). We conducted true triaxial
compression tests in the Pliocene Chinshui siltstone, which hosts the Chelungpu fault.
Rectangular prismatic specimens were prepared from three cores, one from the hanging wall
(depth of 891 m) in hole A, and two from the footwall (1251 m in hole A and 1285 m in hole
B). Specimens were subjected to constant least (σ3) and intermediate (σ2) principal stresses
and an increasing maximum principal stress (σ1) until brittle failure occurred (at σ1,peak) in
the form of a shear band or fault. Several sets of experiments were conducted, each for a
fixed σ3, and a σ2 that was kept constant during testing but was varied from test to test
between σ2 = σ3 and σ2 σ1,peak. Minor differences were observed between the two cores
from hole A, and more substantial ones between the two footwall cores in holes A and B
(Haimson et al, 2008). However, all tests showed a consistent pattern of significant increase
in σ1,peak as σ2 was raised above the fixed σ3, in contrast to predictions based on the MohrCoulomb condition that neglects the intermediate principal stress effect. Similar increases in
elastic modulus and onset of dilatancy were also discerned. Some of the more important
observations were related to the induced faults attitude.
Upon reaching σ1,peak, specimens invariably develop a through-going shear band or fault
that strikes subparallel to σ2 direction and dips steeply in the σ3 direction. Measurements
revealed that fault dip angle (θ) decreases monotonically with increasing σ3 for a constant σ2,
and increases monotonically with σ2 for fixed σ3. This variation of θ with intermediate
principal stress is inconsistent with Mohr Coulomb theory, which asserts that the angle should
be independent of σ2. The observations do indicate that for constant σ3 fault dip angle
increases as the deviatoric stress state parameter (N) varies from
for axisymmetric
for axisymmetric extension (σ2 = σ1). The increase of θ with
compression (σ2 = σ3) to
decreasing N is consistent with the Rudnicki and Rice (1975) prediction based on shear
localization theory using a Drucker-Prager (two invariant) type material relation. Same
experimental data show a decrease in θ with increasing mean stress (σ = (σ1  + σ2 + σ3)/3).
In the plot of observed fault angles as a function of mean stress (σ) the line fit for pure shear
(N = 0), yields predictions for θ that are lower than observed; the line fit for axisymmetric
) yields predicted fault angles that are higher than observed. Despite
compression (N =
the discrepancy between the two predictions, the results are consistent with the observed
dependence of fault dip angle on σ2, which is not inherent in Mohr-Coulomb theory.
September 2008
Page 57
NOTES
September 2008
Page 58
Geomechanical sensitivity of reservoirs from statistical correlations of flow rates
John Greenhough1, Kes Heffer2, Ian Main1, Xing Zhang3, Nick Koutsabeloulis3
1
University of Edinburgh
2
Reservoir Dynamics Ltd.
3
Schlumberger Reservoir GeoMechanics Center of Excellence
While conventional reservoir modelling neglects geomechanical effects, there exists growing
evidence that they play a key role in fluid flow. Coupled modelling of geomechanics and flow
supports the possibility of fault reactivation via changes in fluid pressure and temperature,
and
such faults are likely to have considerable influence on flow paths. Furthermore, recent
developments in statistical techniques highlight flow correlations that are not only long-range
but related to faults and stresses; knowledge of all these characteristics is therefore of great
potential value in reservoir management. Using various North Sea fields as examples, we
present a novel, parsimonious model that identifies only well-pair correlations of the highest
statistical significance, combined with a geomechanical model, and suggest ways in which
these tools might be integrated with other management processes.
September 2008
Page 59
NOTES
September 2008
Page 60
Excavation induced fractures in a plastic clay formation: observations at the HADES URF
Philippe Van Marcke, Wim Bastiaens, EIG Euridice, Boeretang 200, 2400 Mol, Belgium
The geological disposal of radioactive waste has been studied in Belgium since the early
seventies by the Belgian Nuclear Research Center (SCK•CEN). The research is focused on
the Boom Clay layer: a poorly-indurated clay that is found from a depth of 190 metres under
the site in Mol where it has a thickness of about 100 metres. It displays a plastic behaviour
which results in self-sealing properties and a relatively high convergence when excavating
galleries at depth. The hydraulic conductivity is in the order of 10−12 m/s.
In 1980 SCK•CEN started the construction of an underground research facility HADES. Its
purpose was to examine the feasibility to construct a repository and to provide SCK•CEN with
an underground infrastructure for experimental research on the geological disposal of
radioactive waste. Not much knowledge and experience on excavating galleries in a deep
plastic clay formation was available at that time. The evolution of excavation techniques and
geomechanical understanding throughout time is reflected in the successive excavation
phases of HADES. By the later construction of a second shaft and new galleries by industrial
techniques (1997-2007) the feasibility to build an underground repository in the Boom Clay
has been demonstrated.
In 2002 the second shaft was linked to the existing underground infrastructure by the
connecting gallery. Several measurement and research programmes were carried out before,
during and after the construction works. The fracture pattern in the clay massif was
systematically observed. The focus was on shear planes, recognisable by their slickensided
surface. The fracture pattern consisted of two conjugated fracture planes: one in the upper
part dipping towards the excavation direction, the other in the lower part dipping towards the
opposite direction. The distance between fractures is a few decimetres and they originate at
about 6 metres ahead of the front. Borings performed shortly after the construction of the
gallery revealed the presence of fractures up to a radial extent of 1 metre into the clay. The
orientation of the observed fracture planes could be explained by the stress state around the
gallery.
In addition laboratory measurements and numerical modelling were performed to characterise
the geomechanical behaviour of the clay and to assess the impact of the excavation on the
clay massif. Several European Commission projects were dedicated to this subject:
SELFRAC, TIMODAZ and CLIPEX. The impact is probably limited by the sealing
mechanisms that have been evidenced by laboratory measurements. Furthermore it has been
evidenced that the behaviour of the Boom Clay is characterised by a strong hydromechanical
coupling, already noticeable at an unexpectedly large distance from the excavation, and by a
clear time dependency. Also the impact of the excavation on the hydraulic conductivity in the
surrounding clay formation was examined by measurements at different distances from the
gallery.
In 2007 the Praclay gallery was constructed perpendicular to the connecting gallery. The
fracture pattern was described and several parameters were measured to characterise the
geomechanical behaviour of the clay. Also the impact of the excavation on the hydraulic
conductivity in the surrounding clay formation was again examined.
September 2008
Page 61
NOTES
September 2008
Page 62
Fault growth and the related fundamental physical processes
Atilla Aydin, Rock Fracture Project, Stanford University, Stanford, California, USA
Ghislain de Joussineau, Rock Fracture Project, Stanford University, Stanford, California, USA
(Now at Beicip-Franlab, Paris, France)
James Berryman, Lawrence Livermore National Laboratory, Livermore, California, USA
[email protected]
Increases in the length, height, and width including the thickness of fault rock and the
surrounding damage zone collectively are quantitative measures of fault growth. Indeed,
much data shows that fault dimensions increase by some fashion as the slip across the fault
zone increases. However, the details of the physical processes responsible for the
incremental growth of fault zones remain to be poorly understood. This paper aims to
contribute to the current understanding of the issues related to fault zone lengthening and
widening through the potential physical process involved.
The increase in fault length and height has been attributed to the linkage of isolated faults,
fault segments in a system or nearby fault strands. In this regard, one of the most revealing
information about the process of fault growth is the variation in size, frequency, and effective
petrophysical properties of fault steps as a function of fault slip. We will present an extensive
data set compiled from literature survey and complemented by our own recent work to show
that mean step lengths and mean step widths correlate with maximum fault slip through a
positive power law indicating that fault steps are created and destroyed in a systematic way
during fault growth. The former is controlled primarily by fault interaction and the later by
linkage of the neighboring faults which results in an increase of the mean segment length.
The effective modules of the fault steps as well as that of the entire volume of the faulted rock
may provide a reference frame for the obliteration of fault steps and the linkage of the
neighboring faults or fault segments.
Finally, we address the widening of fault zones and present a simple model based on the
concept of a highly fractured inner damage zone and its critical effective modulus describing
the progressive generation of additional cataclastic zone and its annexation into the fault rock.
September 2008
Page 63
NOTES
September 2008
Page 64
Modelling Fault Zone Development within Brittle Rocks, at Scales Ranging from Meters
to Several Kilometres.
Moir H.1, Lunn R.J.1and Shipton Z.K.2
1
Department of Civil Engineering, University of Strathclyde, Glasgow, Scotland
Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, Scotland
2
Within crystalline basement rocks, permeable faults are a dominant feature of subsurface flow
systems. For example, research at the EU’s Soultz-sous-Forệt Hot Dry Rock test site (Evans
et al., 2005) show that 95% of flow at the test site occurs within a single fault zone at nearly 4
km depth. Consequently, predicting the permeability of faults is of major interest to many
industries including hydrocarbon exploitation, nuclear waste disposal, sequestering of carbon
dioxide and mining.
Current predictions of fault zone permeability are highly error prone, producing great
uncertainties in flow and contaminant transport simulations, both in terms of large scale flow
behaviour and in the detailed structure of the fault zone. To improve estimates of fault zone
permeability, it is important to understand the underlying hydro-mechanical processes of fault
zone formation. In this research, we explore the spatial and temporal evolution of fault zones
in brittle rock through development and application of a 2D hydro-mechanical finite element
model. We simulate the evolution of fault zones from pre-existing joints and explore controls
on the growth rate and locations of multiple splay fractures which link-up to form complex
damage zones. The simulations are carried out on different scales ranging from meters to
several kilometres and at all scales the natural heterogeneity of the host rock is considered.
September 2008
Page 65
NOTES
September 2008
Page 66
Mechanics of sheeting joints
Kelly J. Mitchell and Stephen J. Martel, Department of Geology and Geophysics, University of
Hawaii at Manoa
Sheeting joints, long known as “exfoliation” in granites, have an important impact on society.
Although these fractures have been studied for centuries, their cause has remained
enigmatic. They are commonly attributed to removal of overburden. However, this is not a
mechanically viable cause because it merely decreases confining pressure perpendicular to
the surface; it does not provide a mechanism for the tension required to open these surfaceparallel fractures. We propose that sheeting joints form as a result of tensile stresses induced
by high compressive stresses acting parallel to a curved surface topography. Based on our
hypothesis, we use an expression derived from the differential equations of equilibrium to
predict the presence or absence of sheeting joints based on topography and surface stresses.
Numerous methods are being utilized to test this hypothesis. We are using aerial LIDAR data
collected over a 77sq km area of Yosemite National Park, USA, to analyze topography at high
resolution. Field observations, photographs, and surveyed field maps are also being utilized
in our analysis. Our initial findings support the above hypothesis and imply that the long-term
strength of rock may be orders of magnitude less than indicated in laboratory strength tests
over short time scales.
September 2008
Page 67
NOTES
September 2008
Page 68
Relationship between growth mechanism of faults and permeability variations with
depth of siliceous mudstone in northern Hokkaido, Japan
E. Ishii, H. Funaki, T. Tokiwa, K. Ota
Abstract: The relationship between the growth mechanism of faults of the folded Neogene
siliceous mudstone containing Opal-CT in northern Hokkaido, Japan and the permeability
variations with depth is presented here. Hydraulic tests performed in vertical boreholes
(drilling depth: ≤ 1 km) in high permeable sections (>10-7 m/s) show that they are restricted to
a depth of less than 400-500 m. Based on outcrop observation and borehole investigations, a
large number of faults crossing bedding planes are observed in the rock. The faults strikes
are oblique-normal to a folding axis and the majority of displacement senses are strikeoblique slip. Propagation of splay cracks from the fault, especially at the fault tip, is observed
in outcrop. The rock matrix between overstepping faults is generally heavily fractured with
most showing tensile features. In borehole cores, the tensile fracture density variation with
depth is greater above 400-500 m than below. No such variation is observed in the fault
density. The faults with the above-mentioned orientation and displacement are generally
formed in response to the residual stress (shear stress) accumulated during folding and
stress release (normal stress decreasing) by the thermal-elastic contraction accompanying
uplift and erosion. Furthermore, tensile fractures propagated from a fault can be formed by
concentrations of tensile stress generated when a slip nucleates and propagates in a fault.
However, assuming that the principal stresses are horizontal and vertical and the vertical
stress is the overburden, and that pore pressure is hydrostatic, a tensile fracture is rarely
formed below a depth of several hundreds metres according to a combined Griffith-Coulomb
criterion. Even if the tensile stress occurs, a shear fracture propagated from the fault tip in the
extended direction is easily formed under the stress state. A previous study on burial
diagenesis indicates that the siliceous mudstone was buried below a depth of 1 km. It is
inferred that, during uplift and erosion the faults grew above 400-500 m by propagation of the
tensile fractures and linking with the other adjacent faults, while, at greater depth, faults
propagation was not associated with tensile fracturing. Such a fault growth mechanism
explains the permeability variation with depth of the rock. The fact that a few tensile fractures
are also observed below 400-500 m would appear to be contradictory to this growth
mechanism. These fractures are assumed to be tensile fractures formed by the elevated pore
pressure raised at the early stage of folding following burial diagenesis. As such tensile
fractures are isolated, they do not enhance the permeability of the rock as much.
September 2008
Page 69
NOTES
September 2008
Page 70
Fault growth in mechanically layered sequences: a modelling approach
Michael Welch, Rob Knipe and Christian Tueckmantel
Mechanically layered sequences subjected to extensional strain often show complex fault
patterns. Some layers may contain populations of closely spaced, low throw, layer-bound
faults. Other layers act as mechanical barriers, with faults from adjacent layers terminating at
the layer boundaries. A significant portion of the extension is often taken up on a few large
faults which cut through the entire section, but which may be segmented. These patterns may
be seen at many scales, from outcrop to basin-scale; we will show examples from the Gulf of
Suez and the North Sea.
These fault patterns reflect the control of the mechanical layering on fault nucleation and
growth: in the early stages of deformation, faults may be confined within the layers in which
they nucleate, only later propagating outwards to form large through-cutting faults. To better
understand this process, we have developed a 2D hybrid numerical/analytical model that
uses an energy balance calculation to simulate fault nucleation and propagation through the
sequence. In this talk we will demonstrate two applications of this model:
•
Firstly we will show that in an elastic medium, the displacement on a static (nonpropagating) fault will be proportional to fault length. This may explain why the early layerbound faults tend to have low throws, while the later through-cutting faults have much
greater throws.
•
We will then model propagating faults. We will show that, once propagation starts, a fault
will continue propagating until it reaches a mechanical boundary. The main controls on
fault propagation are the friction coefficient and the differential stress. Faults may thus
nucleate first either in low friction layers (e.g. clays), or in brittle layers with high
differential stress (e.g. cemented sandstones). Similarly, faults will tend to terminate either
at high friction layers, or at ductile layers in which differential stress is low. If the
mechanical properties of the layers are known, it is possible to predict the differential
stress at which faults will finally cross these barrier layers to form through-cutting faults.
The model we present here therefore offers a method for predicting the fault distribution and
clustering within a mechanically layered succession.
September 2008
Page 71
NOTES
September 2008
Page 72
Geomechanical integrity of a sealing fault during late life depletion of a petroleum
reservoir
Hans Petter Jostad, Fabrice Cuisiat, Lars Andresen, Elin Skurtveit
Norwegian Geotechnical Institute, Sognsveien 72, N-0806 Oslo
The presentation will present the results from geomechanical analyses of fault behaviour
during depletion of a North Sea reservoir. The time-dependent geomechanical analyses were
performed with the commercial Finite Element program PLAXIS. A user defined poroelastic
material model was developed and implemented in order to take into account the
compressibility of the grains due to pore pressure changes (Biot effect) and the
compressibility of the pore fluid in undrained shale. Geological data were used to build
representative fault models. Results from laboratory experiments on reservoir and cap rock
materials were used to define input mechanical properties to be used in the analyses.
The present stress changes in the B-Fault, which is a hydraulic barrier between the reservoir
and a neighbouring field, were calculated in two characteristic vertical cross-sections through
the fault. The present pore pressure depletion of 30 MPa was used. A full consolidation
analysis was first performed which showed that the most critical conditions were those at
steady state pore pressure conditions in the fault. Further analyses were performed for
drained behaviour of the fault, i.e. with linear pore pressure profile within the fault zone. The
analyses showed that shear failure might have developed on the reservoir side of the B-Fault
especially in areas with high clay content and lower shear strength. Shear failure could
propagate along the fault and not through the fault. Local tension failure might occur, but
could not propagate though the fault because for the production time scale considered, the
fault core zone was drained and the effective mean stresses increased with depletion on the
depleted side of the fault. It was also found that tension zones did not propagate in the fault
height direction.
The results showed that fault sealing integrity was not much affected by the stress changes
caused by present pore pressure depletion at the field, in agreement with field observation.
Similar geomechanical analyses were performed for another bounding fault and expected
pore pressure changes for late life conditions. The calculated stress changes were equal or
less critical than the calculated present stress changes in the B-Fault. It was therefore
concluded that the stress changes due to the planned pore pressure depletion at the late life
of the Field would not change significantly the hydraulic resistance for the bounding fault.
An extensive parametric study was carried out to assess the sensitivity of the results to
uncertainty in geometry and mechanical properties. The maximum stress changes were found
to be very little sensitive to geometrical variations and uncertainties in the mechanical
stiffness distributions. The largest uncertainty was related to the peak shear strength of the
fault (core) zone.
September 2008
Page 73
NOTES
September 2008
Page 74
Experimental Study on Self-Sealing of Indurated Clay
Zhang, C.-L., Rothfuchs, T., Wieczorek, K. Herbert, H.-J., Gesellschaft für Anlagen- und
Reaktorsicherheit (GRS) mbH, D38120 Braunschweig, Germany
e-mail: [email protected]
The self-sealing potential of the Callovo-Oxfordian argillite and the Opalinus clay was
experimentally investigated on strongly damaged samples. Gas permeability as a function of
the confining stress before and after water resaturation was measured. Not only normallysized but also large-scale and cylindrical ring-shaped samples were tested. Each test lasted
over a time period of 5 to 16 months. The experimental findings are:
•
The permeability of the pre-damaged samples decreased significantly with a concurrent
increase of the confining stress due to fracture closure. The permeability measured in
radial direction on a hollow sample decreased from 10-15 m2 at a low confining stress of 1
MPa to 10-21 m2 at 28 MPa. The compression of the sample led to plastic closure of preexisting fractures, leading to a significantly lower permeability after unloading. A similar
permeability reduction with increasing confining stress was also observed in axial
direction, parallel to the bedding plane. But, at low confining stresses below 10 MPa, the
axial permeability parallel to the bedding was about one to two orders of magnitude
higher than the radial one perpendicular to the bedding. The hydraulic anisotropy
vanishes off with increasing the confining stress.
•
The permeability of fractured clay rocks was dominated by the confining stress normal to
the fracture plane. This was validated by gas permeability measurements on a large
sample (D=260mm/L=616mm) with fractures oriented parallel to the sample axis. The
increase of the lateral stress from 3 to 18 MPa at 19 MPa axial stress led to a decrease
of axial permeability from 10-13 to 10-19 m2.
•
The permeability of damaged clay rocks decreased also with time due to the timedependent compaction of pores and fractures. On the pre-damaged samples, a
permeability reduction by a factor of 4 to 8 was observed over two months at a low
confining stress of 1.5 MPa.
•
The high swelling potential of the studied clay rocks led to the closure of fractures when
water was injected into the sample. This was confirmed by a pronounced decrease of the
gas permeability from 10-16 to 10-21 m2 after water resaturation was reached.
•
The re-sealed samples exhibited low permeability to gas and water of less than
10-20 m2 as it is usually observed on undisturbed clay rocks.
All these experimental results provide evidence for the high self-sealing capacity of the
studied clay rocks under the combined impact of reconsolidation and resaturation.
September 2008
Page 75
NOTES
September 2008
Page 76
Kinetics of Time-dependent Processes in Fault Zones: Implications for Fault Seal
Analysis
Sankar Muhuri, Energy Technology Company, Chevron Corp., 1500 Louisiana, Houston, TX
77082, USA; e-mail [email protected]
Fault zone mixing models such as shale gouge ratio or clay smear potential has been the
cornerstone of fault seal analysis in hydrocarbon exploration. Drilling results in contractional
toes of gravity driven systems suggest gaps in this conventional methodology for predicting
nature and capacity of fault seals. The algorithms predict effective seals in traps with large
displacement thrust faults though actual observations prove otherwise. Moreover, the above
approaches do not effectively predict sealing behavior in sand rich startigraphic settings that
appears to be common in these exploration cases. A wealth of knowledge exists on the
deformation of porous sandstones that can be used to refine seal analysis in sand rich
settings. Outcrop analyses of deformation bands or small faults that occur in high porosity
sandstone sequences have enhanced our understanding of deformation processes including
grain comminution and its effect on permeability across these zones. Textural characteristics
similar to natural faults have been reproduced both in laboratory experiments on porous
sandstones as well as in numerical models. Petrographic and geochemical observations on
natural fault rocks however, reveal the critical role of time-dependent deformation
mechanisms in fault zone evolution. A review of results from “slip-hold-slip” rock mechanics
experiments highlighting observations on evolution of mechanical strength, textural
parameters and hydraulic properties are presented to illustrate the role of kinetics and
interaction between various stages in the life cycle of fault zones.
The importance of time-dependent processes such as grain growth and recrystallization,
pressure solution, Ostwald ripening, sub-critical crack growth and healing many of which
occur in tandem in the evolution of fault zones are apparent from the experimental data.
Microscopic features such as pore-collapse, triple junction grain boundaries, fluid inclusion
trails or healed fractures only occur in the presence of a reactive fluid (brine). Increases in
cohesive strength and peak strength of the synthetic fault zones also happen only in “wet”
experiments. Finally, decrease in porosity and permeability occurs during the “hold” period of
wet experiments in contrast to microfracturing and porosity increase during “slip” events.
Evolution of particle size distribution (psd) both during slip events and post-slip periods offers
insight into nature of deformation mechanisms that are operative during the life cycle of fault
zones. During slip both natural and synthetic fault gouge exhibits decrease in mean grain size
with characteristic trends in evolution of mean grain size and fractal dimension as a function
of shear strain or accumulated slip. In contrast, mean grain size increases in the post-slip
periods indicating grain growth. Ostwald ripening, a process of recrystallization growth tends
to reduce the proportion of smaller grain size fraction and grow the larger grains. Clear trends
emerge from experiments on the time-dependence of particle size distribution evolution.
Incorporating reaction-transport kinetics of post-deformation processes in our thinking of fault
zones is a step forward and may hold the key towards addressing issues in diverse research
areas such as fault seal capacity in hydrocarbon reservoirs and time-dependent (chemical)
compaction and earthquake recurrence intervals.
September 2008
Page 77
NOTES
September 2008
Page 78
Strong velocity weakening in fault gouges: results from rock analogue experiments
André Niemeijer1,2,3, Derek Elsworth1,3 and Chris Marone2,3
Department of Energy and Mineral Engineering, The Pennsylvania State University, USA
2
Department of Geosciences, The Pennsylvania State University, USA
3
G3 Center and Energy Institute, The Pennsylvania State University, USA
1
Fluids are important in deformation processes in the upper- to middle crust where they exert
strong influence on frictional behaviour of fault gouges via mechanical (pore fluid pressure)
and chemical effects (solution-transfer processes). Despite these observations, not much is
known about the interplay of chemical and mechanical processes, primarily since the required
conditions are difficult to simulate in the laboratory (i.e. high temperature, low strain rate and
high strain). In this study, we report results from an experimental study on the shear
behaviour of simulated fault gouges of rock salt under conditions where pressure solution is
known to be operative. The experiments extend conditions previously studied to higher sliding
velocities and allow for comparison between two different experimental methods (i.e. biaxial
vs. rotary shear).
We find that steady state friction is very similar for both the direct shear and rotary shear
configurations (for pure salt gouges in the presence of brine at a normal stress of 5 MPa, slip
rates of 0.03-10 m/s and shear strains up to 10). However, at sliding velocities higher than
previously obtained in the rotary shear configurations (i.e. > 10 m/s) and high strains, we
find that samples of rock salt weaken significantly and ultimately slide unstably (i.e. stick-slip)
in the double direct shear experiments. Sliding experiments on a chemically inert material (i.e.
quartz) under the same conditions do not show this significant weakening. Rate and state
frictional (RSF) parameters determined from velocity-stepping tests are large compared to
values reported on other materials (a >0.05 and b >0.05). The mechanical data suggest that
the gouges dilate significantly during sliding, with steady state porosity increasing with
increasing sliding velocity. We infer that steady state friction and the associated strong
velocity weakening are a results of competition between displacement-dependent dilation and
time-dependent compaction (pressure solution). Increasing sliding velocity leads to less net
compaction per unit displacement, resulting in higher porosities and less contact area,
resulting in lower friction.
These data document the need to expand the range of conditions for detailed experiments on
quartzose fault gouges to include the hydrothermal conditions expected in the upper- to
middle crust.
September 2008
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NOTES
September 2008
Page 80
Characterization of fault sealing for hydrocarbon migration and entrapment
Likuan Zhang1, Xiaorong Luo1, Dunqing Xiao2, Jianchang Liu3, and Changhua Yu2
1
Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese
Academy of Sciences, Beijing, China, 100029
2
Research Center of Exploration and Development, Dagang Oilfield Company, CNPC,
Dagang Tianjin, China, 300280
3
Energy Technology Company, Chevron, 1500 Louisiana St. Houston, TX 77002
Faults often act as pathways or/and seals to hydrocarbon migration and accumulation in
sedimentary basins. Quantification of fault properties and understanding their effects on
hydrocarbon migration and accumulation could significantly help reduce hydrocarbon
exploration risks. This presentation will introduce some of our recent work in this field based
on some case studies.
The interaction of hydrocarbon migration and the fault activities is a dynamic process in a
basin’s geologic history. The sealing/opening of faults may vary during the different periods of
the fault movement, and therefore may apply different influences on hydrocarbon migration
and accumulation in a basin’s history. In addition, fault plane in our fault modeling is not
treated as a simple surface, but a belt consisting of fault gouge, fault damaged zone, fault
derived fractures and failures.
Three factors are considered to be critical to fault sealability: shale gouge ratio (SGR) in the
vicinity of faults, pore pressure (P) in shale, and normal stress (s) on fault plane. To represent
the integrative effect of these factors, a fault opening coefficient (FOC) is defined as directly
proportional to P and inversely to s and SGR. By dividing a fault plane into small zones, the
value of FOC can be computed, and the sealability of fault at one zone (as indicated by
sealing probability, Ps) is identified by checking whether oil was found in the reservoirs over
one zone.
Some empirical relationships between FOC and Ps during migration are revealed in one of
our field case studies in the Chengbei Step-Fault Zone of the Bohaiwan Basin, China: Ps
tends to be 1 when FOC is smaller than 1; a power relationship exists between FOC and Ps
when FOC is between 1 and 3.5; and Ps tends to be 0 when FOC is larger than 3.5. They
also showed that faults varied its behaviors (open or close) during hydrocarbon migration in
different time and locations.
Finally, it is indicated that to make economic accumulations of hydrocarbons in a basin, faults
and the accompanied folds have to work together to increase the accessible fetches and to
provide driving force for hydrocarbons so as to make up the desirable volumes of
hydrocarbons.
September 2008
Page 81
NOTES
September 2008
Page 82
Constrained Inversions of Geophysical Data in the Parkfield Region of California
Ninfa Bennington, Clifford Thurber, University of Wisconsin
Model nonuniqueness and imperfect resolution are pervasive problems in the inversion of
geophysical data. We are exploring the utility of structural constraints employing a crossgradients penalty function to improve models of fault zone structure and fault slip along the
San Andreas fault in the Parkfield, California area. Previously, individual seismic and
resistivity models at SAFOD were completed that showed significant spatial similarity
between main features. In the first study, we will capitalize on this likeness by developing a
joint inversion scheme which uses the cross gradient penalty function to achieve structurally
similar images that fit both the resistivity and seismic models without forcing model similarity
where none exists. With the occurrence of the 2004 M~6 Parkfield earthquake, geodetic
observations for an entire earthquake cycle are now available, spanning the 1966 and 2004
events. In the second study, we develop constrained geodetic slip models based on the
assumption that aftershocks occur preferentially along the edges of slip patches. These
aftershock constraints will be applied in the development of models of coseismic and
postseismic slip for the 1966 and 2004 Parkfield events, as well as the interseismic slip.
September 2008
Page 83
NOTES
September 2008
Page 84
The role of slip-weakening friction in damage zone geometry
Michele Cooke, University of Massachusetts, Amherst and Heather Savage, University of
California, Santa Cruz
Numerical models of a linear fault show the generation of off-fault tensile failure that results
from inelastic slip along the fault. We explore models with slip-weakening friction to assess
the effects of variable friction on the damage patterns. Tensile fractures form where tangential
stresses along the fault exceed the tensile strength of the rock. These stresses result from
locally high slip gradients. Because faults of different displacement history and rock type
should have varying slip-weakening distances (L), we examine the effect of changing the slip
weakening distance on the damage pattern and find that this parameter is of paramount
importance in determining off-fault fracture orientation, intensity and distance from the original
fault. These results could guide field studies of small faults as to whether the fault failed in
small seismic events or in creep. In addition to the study of fracture development, we
investigate the amount of energy available for additional damage generation through a work
budget analysis. We compare the work budget of two faults, which are identical except that
one can generate off-fault fractures and the other cannot. Because off-fault fractures can slip
frictionally after forming in mode I failure, the presence of a damage zone makes the fault
system more efficient, with less stored internal work and less external work at the boundaries
once fractures have formed.
September 2008
Page 85
NOTES
September 2008
Page 86
The Nucleation of Large Earthquakes within Overpressured Fault Zones in Evaporitic
Sequences
N. De Paola1, C. Collettini2, D.R. Faulkner3
1
RRG, Earth Sciences Department, University of Durham,UK
2
GSG, Dipartimento di Scienze della Terra, Universita’ di Perugia, Italy
3
Rock Deformation Lab, Earth and Ocean Sciences Department, University of Liverpool, UK
Email: [email protected]
Telephone: +44-0191-3342333
The integration of seismic reflection profiles with well-located earthquakes show that the
mainshocks of the 1997-1998 Umbria-Marche seismic sequence (Central Italy) nucleated at a
depth of ~6 km within the Triassic Evaporites (TE, anhydrites and dolostones), where CO2 at
near lithostatic pressure has been encountered in two deep boreholes (about 4 km). In order
to investigate the deformation processes operating at depth in the source region of the
Colfiorito earthquakes we have characterized: 1) fault zone structure by studying exhumed
outcrops of the TE; 2) rheology and permeability by performing triaxial loading tests on
borehole samples of anhydrites at room temperature, 100 MPa confining pressure (Pc), and
range of pore fluid pressures (Pf). Permeability and porosity development was continuously
measured prior to and throughout the deformation experiments.
The architecture of large fault zones within the TE is given by a distinct fault core of very finegrained fault rocks (cataclasites and fault gouge), where most of the shear strain has been
accommodated, surrounded by a geometrically complex and heterogeneous damage zone.
Brittle deformation within the fault core is extremely localized along principal slip surfaces
associated with dolomite rich cataclasite seams, running parallel to the fault zone. The
damage zone is characterized by adjacent zones of heavily fractured rocks (dolostones) and
foliated rocks displaying little fracturing (anhydrites).
Mechanical results after triaxial loading tests show that the brittle-ductile transition occurs for
Pe < 20 MPa and is almost independent of fabric orientation and grain size. Brittle failure is
localized along discrete fractures and is always associated with a sudden stress drop.
Conversely, ductile failure occurs by distributed deformation along cataclastic bands. In this
case no stress drop is observed.
The static k of the anhydrites, measured prior to loading for Pe = Pc-Pf = 10-60 MPa, is
generally low, k = 10E-21 - 10E-19 m2, and, for a given Pe, is controlled by grain size and
fabrics orientation with variations up to 2 orders of magnitude.
The dynamic k measured at failure under constant Pe = 10-40 MPa (k = 10E-20-10E-17 m2)
is controlled by the grain size, fabrics and Pe, as k increases up to about 1-2 orders of
magnitude for decreasing Pe. All samples, independently whether deforming in a brittle or
ductile way, show dilatancy after yielding. The onset of dilatancy coincides with the first
increase in k, which increases dramatically prior to localized failure (upward concave curve),
whilst tends to stabilize prior to distributed deformation (downward concave curve). Our
experiments show that, during sample loading, the pattern of the permeability evolution is
controlled by the mode of failure.
Overall the integration of our field observations and laboratory data suggests that fault zones
within the TE can act as barrier to deep seated CO2 rich crustal fluid flow, and favour the
build up of fluid overpressures. During the seismic cycle, the maintenance of fluid
overpressures within the fault zone, as far as the co-seismic period, is possible as long as
localized brittle failure is prevented within the anhydrites. Brittle failure within the anhydrites
occurs at the effective pressure Pe < 20 MPa, which signs the rheological transition from
distributed (ductile) to localized deformation (ductile), associated with a dramatic increase in
permeability.
The formation of patches of pressurised fluids within the fault zone, may favour slip instability
and trigger seismic rupture nucleation.
September 2008
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NOTES
September 2008
Page 88
The nature of the San Andreas Fault at seismogenic depths: Insight from direct access
via the SAFOD boreholes
Evans, James. P., Bradbury, Kelly E., Jeppson, Tamara, Springer, Sarah D1, and Solum, J.2,
Department of Geology, Utah State University, Logan, UT 84322-4505
1
Now at: Chevron Overseas, Houston, TX. 2U. S. Geological Survey, Menlo Park, CA
2
Now at: Shell Exploration Research, Houston, TX.
We characterize the physical properties, microstructures, and composition of faulted rocks of
the San Andreas fault encountered in the SAFOD borehole, which intersected the San
Andreas fault zone at depths of 2.4 to 3. 1 km vertical depth, and over a map distance of ~ 1
km. These data provide a window into large-scale fault structure from the surface to 4 + km
depth. We combine petrography and XRD of cuttings and a small amount of core, detailed
analysis of electric image log data, and borehole geophysical data to constrain the structure
and composition of the faulted rocks at depth. The westernmost fault is the largest fault
encountered and correlates to the Buzzard Canyon fault is approximately 45 m wide,
separates Salinian granodiorite on the southwest from a Salinian-derived arkosic section on
the northeast and contains fine-grained quartzofeldspathic cataclasites and calcite. The
middle fault zone lies at 2530 mmd, is localized in a clay-rich sedimentary unit between the
upper and lower arkoses and is a diffuse >65 mmd steeply dipping wide, low-velocity, high
gamma, clay-rich fault zone with numerous sheared clay fabrics. The deepest faults
juxtaposes arkosic rocks and fine-grained sedimentary rocks, and was cored during phase
one and phase 3 drilling. It is brittly damaged with little textural or mineralogic evidence of
fluid driven alteration, and may be a small fault within the active San Andreas Fault zone.
Each fault zone is marked by an increased abundance of altered and cataclastically deformed
grains as seen in cuttings. Analysis of image logs indicates the presence of structural blocks
with distinctly different bedding orientations, and fracture distributions throughout the section
roughly correlate with the presence of faults and low Vp and Vs values. The seismic
velocities and other geophysical signatures, and their relationships to the rock types are
highly variable. The Buzzard Canyon fault at depth contains abundant calcite and iron-oxide
alteration; and the middle fault has numerous clay-filled veins, features consistent with
extensive subsurface fluid flow. The deepest fault does not show evidence of alteration
resulting from extensive fluid flow. The deepest faults appear to correlation with the region
where the borehole is actively deforming via creep [Zoback et al], and up dip from the
hypocenters of the small earthquakes that appear to occur below the borehole. The entire
zone between the Buzzard Canyon and San Andreas [senso stricto] faults at depth appear to
contain a series of southwest-dipping faults and damage zones that bound blocks with a
variety of bedding and fracture orientations. If the deeper zone of cataclasite and alteration
intensity connect to the surface trace of the San Andreas fault, then this fault zone dips 80–
85° southwest, and consists of multiple slip surfaces in a damage zone up to 250–300 m
thick. This is supported by borehole geophysical studies, which show this area is a region of
low seismic velocities, reduced resistivity, and variable porosity. The microstructures and
alteration textures observed in the borehole are clearly associated with slip at the top of the
seismic region of the SAF, and are similar to textures observed in exhumed faults, lending
credence to using exhumed faults as proxies for faults at depth.
September 2008
Page 89
NOTES
September 2008
Page 90
Displacement Field In The Borderlands Of The San Andreas Fault, Durmid Hill, Ca and
the Origin of Late Sinistral ‘Faults’
Steven Wojtal, Department of Geology, Oberlin College, Oberlin, OH 44074
[email protected]
Adjacent to the San Andreas fault (SAF) in the Salton Trough, hinges of folds are consistently
oblique to the SAF trace. Folds formed above thrust faults with ramp-flat geometries.
Boudined marker beds record hinge-parallel stretching concurrent with folding. Within 2 km of
the SAF where folds are tight to isoclinal, thrust faults are locally vertical to overturned.
Continuous deformation contributes to fold flattening here. Farther from the SAF, thrust faults
are gently folded or subhorizontal, suggesting footwall imbrication prevailed during
deformation. Continuous deformation is not apparent here. Sequential reconstructions of the
deformation yield a displacement field with consistent with progressive dextral shearing
parallel to the SAF and flattening perpendicular to the fault.
The latest structures here are broad zones of sinistral shearing - sinistral ‘faults’ - that trend
~15-35° to the SAF. Sinistral shear zones have orientations comparable to Riedel X
fractures, but (1) they are not discrete faults, and (2) no other macroscopic Riedel shears
occur here. The late origin, diffuse character, and orientation of the sinistral shear zones are
consistent with formation parallel to directions of maximum sinistral shearing within a general
shear zone.
Folding, boudinage, and continuous deformation are likely products with interseismic
displacement. Folds, thrust faults, and late sinistral ‘faults’ may be active during seismic
events, but their geometries and character are also consistent with incremental transpressive
dextral shearing.
September 2008
Page 91
NOTES
September 2008
Page 92
Fault Interactions and the Growth of Faults on Earthquake and Geological Timescales
A. Nicol1,2, J. Walsh2, C. Childs2, V. Mouslopoulou2 and M. Schöpfer2
1
GNS Science, Lower Hutt, New Zealand
2
Fault Analysis Group, University College Dublin, Dublin, Ireland
Fault interactions are an essential feature of the vast majority of fault systems, whether they
are characterised by soft-linkage or hard-linkage (Walsh & Watterson 1991). Fault
interactions in soft-linked fault systems are reflected in the ductile deformations that
accommodate displacement transfer (e.g. relay ramps in normal fault systems), whilst
interactions in hard-linked fault systems arise from physical linkage of faults and the related
coupling of their growth. Whether a fault system is hard-linked or soft-linked, fault interaction
reflects the strain concentrations and shadows arising from short-term stress-dependent
behaviour of faults. In this talk, using fault growth constraints from both ancient and active
rifts, we show that this short-term behaviour, which is associated with variable displacement
rates and earthquake clustering, is responsible for the emergence of interdependent
displacement histories. Each fault is a vital element of a system that displays a remarkable
degree of kinematic coherence which produces, and maintains, a hierarchy of fault size
throughout deformation. As a consequence, on spatial scales greater than an individual fault
and over temporal scales greater than several earthquake cycles, the behaviour of individual
faults can be relatively predictable, with all faults in an array interacting to produce a system
that is geometrically relatively simple and coherent. A key to improving our understanding of
earthquake occurrence and fault growth is establishing the temporal and spatial length scales
over which this order occurs.
Walsh, J.J., Watterson, J. 1991. Geometric and kinematic coherence and scale effects in
normal fault systems, In The Geometry of Normal Faults, Roberts, A.M., Yielding, G.&
Freeman, B., eds, Geol. Soc. Lond. Sp. Pub. 56, 193-203.
September 2008
Page 93
NOTES
September 2008
Page 94
KEYNOTE:
Quantifying Fault Slip rates and Earthquake Clustering along Active Normal Faults in
Central Italy: Insights from Cosmogenic Exposure Dating and Numerical Modelling
Patience Cowie1, Richard Phillips1, Gerald Roberts2, Ken McCaffrey3 and Tibor Dunai1
1
School of GeoSciences, Edinburgh University, Drummond Street, Edinburgh, EH8 9XP
Joint Research School of Earth Sciences, University of London- Birkbeck College, Malet
Street, London, WC1E 7HX
3
Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE
2
An outstanding challenge to our understanding of fault array evolution remains the
appropriate characterisation and mechanistic understanding of episodic fault activity and
temporal variations in slip rate. This gap in understanding not only inhibits our ability to predict
the timing and location of future earthquakes, it also limits our ability to interpret the surface
process response to active tectonics. For an area of active extensional deformation in the
Italian Apennines, we have been using a combination of field data collection and numerical
modelling to address this challenge. The field area is characterised by a number of active
normal faults, up to 30-35 km in length with total geologic offsets up to 2 km, which have been
developing since ~ 3 Ma. The average Holocene (12-18 ka) slip rate along the length of each
active fault segment has been derived from offset sediments and landforms whose ages are
constrained via dating of tephra at many sites. Comparing these Holocene rates with rates
inferred from basin fill reveals that, over the last 0.7-1.0 Myr the slip rate on some faults has
increased, while other faults have become inactive. Furthermore, comparing the Holocene
rates to paleoseismic observations and earthquake catalogues, suggests that earthquake
activity may switch back and forth between adjacent fault segments on timescales of 103-104
years. These variations are consistent with elastic interaction, i.e., stress transfer between
neighbouring fault segments. To investigate these phenomena, we are using surface
exposure dating of striated bedrock scarps, formed since the last glacial maximum, to derive
the slip rate variability and earthquake recurrence on these faults over multiple earthquake
cycles. The exposure age depends on the concentration of 36Cl, which is produced by the
interactions of cosmic ray secondary neutrons and muons with Ca within the scarp limestone.
The number of earthquakes, their timing and the magnitude of the associated slip are
revealed by cusps in the overall increase in 36Cl concentration (and thus exposure age) from
the base to the top of each scarp. Using ground-based LiDAR to map the scarps at different
scales, we are also able to characterise fault geometry and kinematics in 3D, and confirm that
the exposure ages we derive are the result of tectonic exhumation rather than erosion/burial
by surface processes. The aim of this project is to test a key prediction of numerical models of
elastic interaction between growing normal faults. These models predict that there should be
a systematic variation in rupture history on faults depending on their position and orientation
relative to neighbouring faults and the overall regional tectonic loading. We will present an
overview of the field program, plus results from numerical simulations of fault array evolution
that look specifically at the spatial and temporal variations in stress loading of fault segments
in different geometric configurations and the impact that this has on earthquake recurrence.
September 2008
Page 95
NOTES
September 2008
Page 96
September 2008
Page 97
KEYNOTE:
Seismogenic Permeability
Pradeep Talwani, Dept. of Geological Sciences, Univ. South Carolina. USA
The role of fluids in triggering seismicity was first realized following the impoundment of the Hoover
dam on the early 1940s. As the number of examples of reservoir induced seismicity (RIS) grew in the
1960s, the role of increases in fluid pressures in their occurrence came from the studies of seismicity
that followed high-pressure fluid injections in the deep Arsenal well near Denver. Colorado. As the
spatio-temporal pattern of RIS began to be accurately monitored with increasingly dense seismic
networks, it was found that the seismicity was primarily related to the diffusion of pore pressures along
critically stressed, saturated fractures. The hydraulic property controlling pore pressure diffusion is its
hydraulic diffusivity c, which is directly related to its intrinsic permeability k. For nearly 100 cases of
induced seismicity, c was found to lie between 0.1 and 10 m2/s, and k between 5x10-16 and 5x10-14 m2
a range we have named seismogenic permeability ks. Theoretical analysis of these observations shows
that the diffusion of pore pressures can be identified with Biot’s slow compressional wave through
porous, saturated media. The relative amounts of fluid mass transfer and pore pressure diffusion
depend on the fracture permeability. For fractures with k=ks pore pressure diffusion dominates, leading
to a build up in pore pressure. For k<ks there is no increase in pore pressure, and for k>ks fluid mass
transfer dominates without an increase in pore pressure. Support for the empirically inferred ks came
from dedicated experiments near Nice, France, and Kobe, Japan. In both cases when water was added,
faults with k>ks were associated with aseismic fluid flow, while the nearby fractures with k=ks became
seismic. These results suggest that seismogenic permeability is an intrinsic property of fractures where
pore pressure diffusion results in seismicity.
September 2008
Page 98
NOTES
September 2008
Page 99
Energy partitioning during seismic slip in pseudotachylyte-bearing faults (Gole Larghe
Fault, Adamello, Italy)
L. Pittarello1, G. Di Toro1,2, A.Bizzarri3, G. Pennacchioni1, J. Hadizadeh4, M. Cocco5
1
Dipartimento di Geoscienze, Università degli Studi di Padova, via Giotto, 1, 35137-Padova,
Italy
2
Istituto di Geoscienze e Georisorse, Unità operativa di Padova, CNR, via Giotto, 1, 35137Padova, Italy
3
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, via Donato Creti, 12,
40128-Bologna, Italy
4
Department of Geography & Geosciences, University of Louisville, Louisville, 40292
Kentucky, USA
5
Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma, via di Vigna Murata, 605,
00143-Roma, Italy
The determination of the energy budget of an earthquake is a challenging problem in the
Earth Science community as understanding of the partitioning of energy is a key towards the
comprehension of the physics of earthquakes. However, the energy budget cannot be
estimated by seismological analyses while field and experimental studies might yield some
hints. [Here] we propose to estimate the energy budget from field and microstructural
analyses of an exhumed fault segment decorated by pseudotachylyte (solidified frictioninduced melt produced during seismic slip). In particular, we determined the partition of the
mechanical work density (Ef, energy adsorbed in the fault plane during a seismic rupture per
unit area) into frictional heat (Q) and surface energy (Us, energy required to create new
fracture surface) (Kostrov and Das, 1988):
Ef = Q + Us [J m-2]
Comment [GDT1]: Credo che
Chester e Wilson et al. non
facciano una stima del
partitioning, ma solo dell’energia
di superficie.
The selected fault segment belongs to the Gole Larghe Fault Zone, which crosscuts the
tonalitic Adamello batholith (Italian Alps) (Di Toro et al., 2005). The fault segment was
exhumed from ~10 km depth, typical for earthquake hypocenters in the continental crust, and
records a single seismic rupture, as proved by field and microstructural evidences. Frictional
heat per unit fault area was estimated from the pseudotachylyte average thickness (Di Toro et
al., 2005) and Q results ~27 MJ m−2. Surface energy was estimated from internal microcrack
density (Chester et al., 2005) in several plagioclase clasts entrapped in the pseudotachylyte,
and Us ranges between 0.10 and 0.85 MJ m−2. Since the internal fragmentation of the
plagioclases clasts is negligible in the wall rocks, this estimate is considered as representative
of the surface energy adsorbed by coseismic fragmentation in the slipping zone.
It follows that, in the studied fault segment, about 97–99% of the mechanical energy was
dissipated as heat, and less than 3% adsorbed as surface energy. We conclude that at 10 km
depth most of the energy exchanged during an earthquake is heat.
Comment [GDT1]: Non
chiuderei un lavoro o un abstract
dicendo che quanto diciamo ci
trova d’accordo con qualcun altro,
anche perchè le loro stime non
riguardano i terremoti, ma dati di
laboratorio in condizioni non
sismiche.
Chester, J.S., Chester, F.M., Kronenberg, A.K., 2005. Fracture surface energy of the
Punchbowl Fault, San Andreas System. Nature 437, 133–136.
Di Toro, G., Pennacchioni, G., Teza, G., 2005. Can pseudotachylytes be used to infer
earthquake source parameters? An example of limitations in the study of exhumed faults.
Tectonophysics, 402, pp. 3-20.
Kostrov, B., Das, S., 1988. Principles of earthquake source mechanics. Cambridge University
Press, London.
September 2008
Page 100
NOTES
September 2008
Page 101
Particle size distribution analysis in pristine and faulted quartz-rich, poorly cohesive
sandstones: influence of analytical procedures in laser diffraction analysers
Fabrizio Balsamo1, Fabrizio Storti1, Marco Congia2 and Valentino Polchi2
1
Dipartimento di Scienze Geologiche, Università “Roma Tre”, Roma, Italy
Alfatest, Roma, Italy
2
Particle size distributions of pristine clastic rocks are modified by comminution and cataclasis
during faulting and, in particular, they undergo a generalized size shift towards finer values.
This tectonically-induced progressive size decrease is governed by several factors including
the environmental conditions of deformation, cleavage and microfracture sets inherited within
particles, the degree of cementation etc. Particle size distributions play a first order role on the
frictional and hydraulic properties of fault zones, particularly when the clay content is very low.
Establishing field relationships between fault displacement and the related evolution of
particle sizes plays a first order role for making predictions of fault zone hydrology. Availability
of laser diffraction analysers provides the possibility of fast and detailed particle size analysis
in poorly cohesive or loose materials. Modern particle size laser analysers adopt different
technical solutions and provide the possibility to use a wide variety of analytical methods. This
implies accurate investigations on the possible influence that analytical methods may exert on
the final results obtained by different procedures from the same instrument.
In this contribution, we present particle size data from pristine and faulted poorly cohesive
Pliocene sandstones of the Crotone basin, in Southern Italy. Particle size data were obtained
by a Malvern Mastersizer 2000 laser diffraction analyser, spanning in size from 0.00002 to 2.0
mm. We performed specific test analyses by using different analytical procedures including
the comparison of results from wet and dry methods, the variation of duration and intensity of
ultrasonic particle mobilisation preceding laser activation, the variation of centrifugal pump
speed etc. Results highlight the need of preliminary methodological tests to set up the most
appropriate analytical procedures before planning systematic particle size analyses by laser
diffraction.
September 2008
Page 102
NOTES
September 2008
Page 103
Rigidity of Tectonic Faults and their Temporal Variation
Alexander A. Spivak, Institute of Geospheres Dynamics, Russian Academy of Sciences,
Leninsky pr. 38, bld. 1, Moscow, 119334 Russia
[email protected]
The block-hierarchic division, nonlinearity, and Earth's crust geodynamics as a whole are
determined in many respects by the deformability of the tectonic faults. One of the main
characteristic of the tectonic faults is its rigidity (normal kn and shear ks):
kn =
dσ n
dτ
, ks =
,
d wn
d ws
where σn and τ are the normal and shear effective stresses, respectively, at the edges of the
tectonic fault; wn and ws are the relative normal and shear displacements of the edges,
respectively.
We present the results of experimental measurement of the mechanical rigidity of the deep
Nelidovo-Ryazan tectonic fault (extension is about 800 km) and auxiliary faults of order II and
III relatively Nelidovo-Ryazan fault. The kn and ks values were determined on basis of the
recording nonlinear effects of the propagation of low-amplitude seismic waves across the
faults. In order to determine kn and ks values we used the seismic method of diagnosis based
on recording the amplitude variation of seismic waves propagating through a fault. Tectonic
faults were considered as a flat layer, whose elastic properties differ from the corresponding
characteristics of the enclosing rock massif. In the case of normal incidence of longitudinal or
transverse wave, the normal kn (correspondingly, shear ks) rigidity of the fault is determined
according to the following formulas:
kn =
π ρCP
TP K 2 − 1
,
ks =
π ρ CS
TS K 2 − 1
,
where ρ, CP, and CS are the density of medium and the velocity of propagation of longitudinal
and transverse waves, respectively; TP, and TS are the periods of the corresponding waves; K
is the ratio of maximum amplitudes of displacement velocities in the seismic waves before
and after the fault.
Seismic waves caused by chemical explosions in open pits mines of the Moscow district and
local impulse microoscillations of relaxation type.
In studied faults: kn = 0.05-0.19, ks = 0.012-0.034 MPa/mm for fault of order I; kn = 0.28-0.1,
ks = 0.08-0.29 MPa/mm for fault of order II, and kn = 0.5-2.0, ks = 0.2-0.5 MPa/mm for fault
of order III (over a period of measurements).
The research shows that rigidity of the faults varies in time (in the ranges indicated above).
Moreover, temporal variations of the rigidity of tectonic faults of the same periodicity correlate
with time variations in the microseismic background amplitude in the frequency band 0.1-2 Hz
(the correlation coefficient ranges from – 0.52 to – 0.63 at a significance level not less than
0.95).
September 2008
Page 104
NOTES
September 2008
Page 105
The Role of Fluids in Triggering Earthquakes: Observations from Reservoir Induced
Earthquakes
El Hariri, M., Abercrombie, R. E., Boston University, Boston, MA 02478, USA., [email protected];
Rowe, C. A., Los Alamos National Laboratory, Los Alamos, NM 87545, [email protected]; do
Nascimento, A. F., Universidade Federal do Rio Grande do Norte, Natal, RN 59078-970,
Brazil, [email protected]
We relocate micro-earthquakes induced by the Açu reservoir in Brazil to investigate the
spatio-temporal evolution and triggering of earthquakes caused by fluid diffusion. Fluid flow is
believed to play a major role in triggering tectonic earthquakes. Reservoir induced seismicity
provides a natural laboratory in which to investigate and characterize earthquakes triggered
by fluid flow. Our results can be used to quantify and model pore-pressure diffusion, and to
investigate the role of fluids in triggering earthquakes in other tectonic settings.
Do Nascimento et al. (2004) recorded and located 267 earthquakes (M ≤ 2.1) beneath the
Assu reservoir between 1994-1997. The seismicity increased several months following annual
water level peaks, implying that fluid pressure diffusion is the principal triggering mechanism.
The small station spacing and very low-attenuation, Precambrian basement rock enabled
them to locate the earthquakes with uncertainties of only a few hundred meters. The
earthquakes were located in three clusters, and the time delay to activation of each cluster
increased with the depth of the cluster. The location uncertainties were too large to resolve
any seismicity migration within a single cluster.
We relocate 173 earthquakes from the largest cluster Açu using waveform cross-correlation
to obtain groups of similar events. We use these groups to improve the pick accuracy (to subsample accuracy in 200 sample/s data), and then invert for more accurate hypocentral
locations. Our uncertainties are on the order of 10 - 50 m, and our locations are more tightly
clustered. We observe temporal migration of the earthquakes, both along strike, and to
increasing depth. We observe a seismicity migration rate between 32 and 57.5 m/day. The
rate is highest during the time of peak seismicity rate, and there is some suggestion that the
rate decreases with increasing depth.
September 2008
Page 106
NOTES
September 2008
Page 107
Results from field pumping experiments testing connectivity across deformation
bands in Tucano Basin, NE Brazil
W. E. Medeiros (Departamento de Física, UFRN, Natal, Brazil, [email protected]), A. F. do
Nascimento (Departamento de Física, UFRN, Natal, Brazil, [email protected]) & F. C. A.
da Silva (Departamento de Geologia, UFRN, Natal, Brazil, [email protected])
Sandstones of the Ilhas Group in Tucano Basin, NE Brazil, commonly present outcrops
exhibiting intense deformation mainly as deformation bands. In two of these outcrops, we
performed pumping tests in order to verify connectivity across the deformation bands. In both
outcrops, the macroscopic deformed zone has approximately 1km length and 15m thick.
Several wells were drilled in each side of the deformed area as well as within the deformed
area and in situ permeability measurements were done across the outcrop surface in one of
the cases. In the pumping tests whilst one well was pumped downdraws were monitored at
the other wells. The permeability profiles revealed a huge variation of permeability of up to
four orders of magnitude. Nonetheless, well tests in both cases revealed a moderate
connectivity across the deformation band since the observed stationary downdraw at
monitoring wells on the opposite side of each fault zone was a considerable fraction (~1/8) of
the downdraw observed in the pumped well. Water conductivity measurements during the
pumping tests showed large variation, which can be interpreted as partial
compartimentalization of the aquifer These results indicate that there is 3D connectivity in the
field scale across the deformation bands.
September 2008
Page 108
NOTES
September 2008
Page 109
Different scales of fracturing in the Callovo-Oxfordian argillite of the Meuse /HauteMarne Andra URL area, France
D. Guillemot (Andra), P. Lebon (Andra)
The Andra Underground Research Laboratory (URL) is located in the eastern part of the Paris
basin within a Callovo-Oxfordian argillite ranging from 420 to 550 m between underlying
Dogger limestone and overlying Oxfordian limestone. The site has been chosen away from
the main regional lineaments as the Metz-Hunsrück fault and the Vittel fault.
A comprehensive geological and geophysical surveying has been carried out in this area in
order to identify and quantify fracturing at different scales: from satellite images and 2D
seismic (pluri-km scale) to outcrops survey and 3D seismic (km to hm scale) and to core and
bore-hole imaging as well as underground observations in the shafts and URL drifts (dm to m
scale).
Major faults visible on geological maps and 2D seismic can be divided into (i) NW-SE and
NE-SW cover faults directly rooted in the basement (as Marne trough) (ii) NW-SE cover
structures de-coupled from basement structures (as Poissons fault system), (iii) NE-SW cover
faults disconnected from any basement faults (as Gondrecourt trough). Recent hydrogeology
results show that the flow gradient orientation greatly differs in the Dogger and in the
Oxfordian limestone in the vicinity of Marne and Poissons fault systems, what confirms the
lack of communication between the two and the sealing role of the faulted Callovo-Oxfordian
argillite.
Subseismic fractures (a few meters throw) are only visible on 3D seismic images. None of
them extend shallower than the Bathonian.
Minor brittle structures (mainly tectonic joints) have been observed on limestone outcrops.
Their azimuth distribution is bimodal as for major faults (N030-050 and N130-150) in both
Oxfordian and Dogger limestones with different trends in Liasic and Infra-Liasic levels. The
density (2 to 4 joints/m) is higher near the regional faults. Surveys in drillings and shafts have
confirmed these azimuth distributions. Their density at depth and away from faults is 1 to 2
joints/m, frequently filled with calcite. The lithology controls the nature and the density of
these tectonic features. The Callovo-Oxfordian argillites are characterized by the absence of
tectonic features and the scarcity of joints (a few tens along 1300 m of core drilled). Joints
origin is clearly related to early diagenetic phenomena (compaction figures, S-shaped subvertical joints, compaction cone). They are systematically filled up with calcite or celestite.
It can be taken advantage to establish a parallel between the tectonic structuring and the
regional stress field. A great number of stress measurements (by hydraulic fracturing
techniques in various boreholes, by shaft convergence monitoring and by systematic
analyses of borehole breakouts) were carried out within the limestone-clay-limestone
sequence. The orientation of the minor horizontal stress (N65°E) is found to be in good
agreement with geological considerations. Its magnitude is higher in the central part of the
argillite formation than in the limestones even though the major horizontal stress is almost
constant or increases slightly with depth. Models suggest that limestone formations are more
deformable over long time than predicted by laboratory tests, due to slow rate non-elastic
deformation processes such as pressure-solution. A de-coupling is suspected between
stresses within the upper part of the sedimentary pile and the basement at the triasic salt
level.
In conclusion, this part of the Paris basin area recorded only weak brittle deformation. Major
faults do not allow significant fluid transfer. Off of the fault areas the Callovo-Oxfordian argillite
shows only scarce diagenetic mineral filled joints. Both tectonic structures and stress field
seem differently behave above and below the triasic salt level.
September 2008
Page 110
NOTES
September 2008
Page 111
Fault imaging in the western US using high resolution seismic reflection methods
Lee M. Liberty, Department of Geosciences, Boise State University, 1910 University Dr.
Boise, Idaho 83725-1536 USA
[email protected]
208-426-1166
High resolution seismic reflection studies show faults act as barriers to lateral flow and
conduits to vertical flow in alluvial aquifers. These data also constrain fault geometries and
slip rates for neotectonic studies. I present three examples from the western US. At an
underground nuclear blast site in Nevada, the water table reflector is offset in alluvium more
than 10 m across both pre- and post-blast fault scarps. Five axial seismic profiles that cross
near the blast zone show shallow groundwater is strongly influenced by 40 year old blastrelated faulting. Groundwater flow at blast depths is controlled by the permeability distribution
in the deeper alluvium and underlying volcanic rocks. The >2km offset between the structural
center of the basin and the topographic center of the basin implies axial channel migration
with basin formation. These higher permeability fluvial channels intersect the blast chimney
and may influence contaminant migration rates and directions. Faults that strike normal to
regional groundwater flow directions also may imply anomalous deep groundwater flow
directions. In the Pahsimeroi Valley, Idaho, seismic images from the upper few hundred
meters show depth to impermeable basement rocks and fault geometries correlate with
gaining and losing reaches of local streams. Surface springs align with cross-basin faults,
mixing deep and shallow groundwater. Here, adequate stream flow for fish and groundwater
for irrigation requires an accurate water budget. Seismic imaging of the upper few hundred
meters across the Seattle fault zone, Washington State show steeply dipping Tertiary and
younger strata separate the Seattle and Tacoma Basins. The blind tip of the Seattle fault
forms a synclinal growth fold into the Seattle Basin and a fault propagation fold with a forelimb
breakthrough on the Seattle uplift. Lidar-identified lineaments from a M7 or greater
earthquake in about 900-930 A.D. represent folding along a backthrust and a forelimb
breakthrough fault along the south edge of the Seattle Basin.
September 2008
Page 112
NOTES
September 2008
Page 113
The Influence of Regional Stress on Geostatistical Patterns of Fault Permeability at
Smith Creek Hot Springs, Nevada, USA
Scott Brinton and Jerry P Fairley, Dept of Geological Sciences, University of Idaho, Moscow,
Idaho 83844-3022 USA
There are many factors that influence fault permeability, including amount and type of offset,
country rock, history of fluid flow, and the nature of the rocks juxtaposed across the fault
plane. In addition, it is widely recognized that a fault’s orientation in the regional stress field is
important for determining its transmissivity. Here we consider not the overall transmissivity of
a fault segment at a given orientation, but the way in which the spatial distribution of
permeability varies as segment orientation changes. We examine permeability distributions in
three segments of the Smith Creek fault, inferred on the basis of hydrothermal discharge
temperatures. Two of the segments investigated are at approximately the same orientation
(ENE) and share a similar spatial permeability structure; the intervening segment, oriented
NNE, demonstrates significantly different spatial organization. We attribute these differences
to the orientation of the segments in the regional stress field, and discuss the implications for
numerical simulation of subsurface fluid flow.
September 2008
Page 114
NOTES
September 2008
Page 115
Large scale Hydraulic Properties of Faults and Fault Zones of the Central Aar and
Gotthard Massifs (Switzerland)
Olivier Masset & Simon Loew
The present paper focuses on the role of fault and fault zones on larger scale hydraulic
properties of the crystalline rocks of the central Aar and central Gotthard “massifs” in
Switzerland, based on a compilation and interpretation of inflow data from 25 long and deep
traffic tunnels and hydropower galleries. The Aar and Gotthard “massifs” belong to the
European crust and are both composed of pre-Variscan polyorogenic and polymetamorphic
basement rocks intruded by Variscan magmatic rocks of granitic and granodioritic
composition. They strike parallel to the Alpine edifice and are separated by PermoCarboniferous and Mesozoic sediments, and locally, by a third smaller crystalline “massif”
named Tavetsch massif. Although they were initially interpreted as autochthonous and thus
coined as massifs, they both have been thrusted over underlying sediments, strongly
internally deformed, rotated and uplifted during the Tertiary Alpine orogeny.
It is commonly admitted that in crystalline rocks by far most of the groundwater flow takes
place in fractures. Among these fractures, faults and fault zones can produce outstanding
inflow rates when intersected by deep underground excavations. In the studied underground
excavations, the total early time and late time inflow rates are controlled disproportionally by
brittle faults and their damage zones. However, faults and particularly fault zones are very
heterogeneous features in terms of geometry and hydraulic properties and their local
properties are extremely difficult to predict without local predrillings. This study provides not
only data about fracture geometries and statistics, which are normally used to constrain flow
models of fractured rocks, but direct information about the spatial and rate distribution of
groundwater flow as determined from tunnel observations.
This paper first compares fault and fault zone architectures from both massifs in varying
lithologies. We show that faults and fault zones of the central Gotthard massif are in general
more brittle than faults or fault zones of the central Aar massif and that lithology impacts the
conductance of faults, as derived from tunnel and gallery inflows, to a lesser extent than brittle
tectonic overprint. In the second part, inflow data from 25 tunnels and galleries are used to
derive large scale equivalent rock mass hydraulic conductivities based on simple analytical
flow models. We discuss the influence of all fracture and inflow types on the derived
equivalent hydraulic conductivity and their spatial distributions. We show that clear trends in
the evolution of the large scale hydraulic conductivity with depth can only be seen, when
singular large inflows from brittle faults are excluded. The transmissivities derived from larger
fault inflows show no decrease with depth in the first 1500 meters below ground surface.
September 2008
Page 116
NOTES
September 2008
Page 117
Buoyancy driven gas dispersion along an inclined low permeability boundary:
Andrew Woods, BP Institute, Cambridge, England, CB3 OEZ
Simon Norris, NDA, Harwell, England
We develop an approximate model for the dynamics of a spreading plume of gas released
into a permeable layer of rock. We assume the source gradually wanes over time, and that
the rock lies below an extended inclined boundary of lower permeability, fractured rock. The
model accounts for (i) the slow drainage of gas through the low permeability layer and the
fractures, (ii) the capillary retention of gas in the pore spaces as it is displaced by water on the
trailing face of the current, and (iii) the impact of a background hydrological flow of water.
The model is used to develop some approximate analytic expressions for the evolving shape
of the gas plume with time, and also to describe the zone which is, at some point, invaded by
gas. The expressions are then combined with probability distributions to describe the
uncertainty in the value of different parameters, and thereby produce probabilistic estimates
for the lateral extent of the gas plume at different times subsequent to the start of the release.
The modelling illustrates how the shape of the trapped gas plume depends on uncertainties in
the drainage rate through the fractures, the capillary retention, and the strength of any
background flow.
September 2008
Page 118
NOTES
September 2008
Page 119
3D Structures of Permeable and Impermeable Faults in Granite: A Case Study in the
Mizunami Underground Research Laboratory, Japan
Kenji Amano (Japan Atomic Energy Agency)
Japan Atomic Energy Agency (JAEA) is conducting two URL projects in Japan, Mizunami
URL (crystalline rocks) and Horonobe URL (sedimentary rocks), to demonstrate practicability
and reliability of the technologies for a geological disposal of high-level radioactive waste
through application to relevant geological environments. The Mizunami URL project, main
topic here, started in 2002 and have been planned and partly executed since the late 1990’s.
The project is divided into three phases, surface-based investigations (Phase I),
investigations during shaft/tunnel excavation (Phase II) and investigations in underground
facilities (Phase III). In 2008, the two shafts are sinking below 200mGL during the Phase II.
An intensive multidisciplinary survey (surface reconnaissance, reflection seismic, shallow and
deep borehole investigations, cross-hole tomography, cross-hole hydraulic tests, shaft-wall
mapping, long-term hydraulic monitoring etc) has been carried out to date in order to evaluate
the subsurface structural and hydrogeological conditions of the sedimentary cover and the
basement granite. Central in the field work was to identify or infer fault distributions which
provide the geometrical context in terms of a model of deformation zones and the rock mass
between the zones. Using the geological and geometrical description data as a basis,
hydraulic properties (K, T, Ss, hydraulic heads) of each fault were combined with the fault
model.
The 3D fault models made in parallel with the corresponding hydrogeological concepts
around the Mizunami URL have indicated following geological and hydrogeological settings:
Mizunami URL seems to be located in the small scale pull-apart basin related to bend or
stepover structures of the Tsukiyoshi fault which is one of the major faults in this area.
More than 20 minor normal or strike-slip faults are developed in the basin by extensional
deformation. The closer fault of the basin axis the more deformed and displaced.
Transmissivities of faults show more than 2 orders of magnitude higher or lower compared to
those of the intact rock masses (Averaged T =10-8m2s-1) without exception. Therefore, it
seems possible that almost faults in this area are distinguished into either permeable
structures or impermeable structures to affect the local (or sub-regional) groundwater flow
system in vary degree.
The impermeable faults are limited only to three particularly-large faults in the pull-apart
basin, the uppermost master fault (the Tsukiyoshi fault), the centre fault forming the basin axis
and the lowermost fault. Since the hydraulic heads from the boreholes enclosed by the
uppermost master fault and the centre fault show little change during the cross-hole hydraulic
tests, those faults may act as hydraulic barriers and create a hydraulic compartment.
In the meeting, the most recent 3D permeable and impermeable structures around the
Mizunami URL will be presented with the multiple comparisons based on the results from
other investigations (ex mineralogy, geochemistry, hydrochemistry etc) and groundwater flow
simulations.
September 2008
Page 120
NOTES
September 2008
Page 121
Volumetric fault zone modelling using fault facies
Tveranger1, J., Cardozo1, N., Kjeldaas1, 2, G.C, Nøttveit1, H. Røe3, P.
1
Centre for Integrated Petroleum Research, University of Bergen, Norway
2
Department of Earth Sciences, University of Bergen, Norway
3
Norwegian Computing Center, 0314 Oslo, Norway
Limitations in existing methodologies for fault representation in industrial reservoir models
constrain the type and amount of structural features that can be included. This may seriously
affect simulation results and uncertainty evaluations as well as making drilling through fault
zones hazardous and unpredictable as volumetric fault zone properties are not included
explicitly in the model.
We present the results of a first effort at including fault zone structures as discrete,
volumetrically expressed features in field sized models using the fault facies modeling
concept. By defining and generating fault zone grids, the complete suite of existing tools used
for sedimentary facies modeling can be employed to implement volumetrically described fault
envelopes in standard reservoir models. The main difference compared to sedimentary facies
modeling consist of the use of conditioning parameters derived from strain modeling, which
helps to constrain the position and petrophysical properties of fault facies inside the fault
zone.
Although still at an early stage of development, with significant scope for improvement, the
fault facies modeling method is demonstrated as a viable approach, allowing explicit
representation of fault affected rock volumes and their petrophysical properties at any scale.
September 2008
Page 122
NOTES
September 2008
Page 123
Using outcrop observations, 3D discrete feature network (DFN) fluid-flow simulations,
and subsurface data to constrain the impact of normal faults and opening mode
fractures on the migration and concentration of hydrocarbons in an active asphalt
mine
Wilson, Christopher E., Aydin, Atilla, Durlofsky, Louis J., and Karimi-Fard, Mohammad
An active quarry near Uvalde, TX which mines asphaltic limestone from the Anacacho
Formation offers an ideal setting to study fluid-flow in fractured and faulted carbonate rocks.
Semi-3D exposures of normal faults and fractures in addition to visual evidence of asphalt
concentrations in the quarry help constrain relationships between geologic structures and the
flow and transport of hydrocarbons. Furthermore, a subsurface dataset which includes hand
samples and wireline logs from the surrounding region provides a basis to estimate asphalt
concentrations in both the previously mined portions of the quarry and the un-mined
surrounding rock volume.
We characterized a series of normal faults and opening mode fractures at the quarry and
documented a correlation between the intensity and distribution of these structures with
increased concentrations of asphalt. We mapped normal fault and fracture exposures within
the quarry in order to conceptualize their mechanical evolution, delineate their associated
damage zones, and document their dual impact as conduits (which assist asphalt migration)
and zones of enhanced porosity (which increase asphalt storage). Then we determined
relationships between the orientations and intensities of normal faults and the dips and
lithological layering of the Anacacho Formation. Combining these relationships with the fault
maps and the depositional architecture of the Anacacho Formation provided a basis to
construct a quarry-scale, geologically realistic, three-dimensional Discrete Feature Network
(DFN) which represents the geometries and material properties of the matrix, normal faults,
and fractures within the quarry. We then performed two-point flux, control-volume finitedifference fluid-flow simulations with the DFN to investigate the 3D flow and transport of
hydrocarbons. The results were compared and contrasted with available asphalt
concentration estimates from the mine and the aforementioned data from the surrounding drill
cores.
September 2008
Page 124
NOTES
September 2008
Page 125
Differential fracturing pattern in clay/limestone alternations at Tournemire (Aveyron,
France) and in the Maltese Islands
Rocher M.1, Missenard Y.2, Bertrand A.3, Cabrera J.2, Cushing M.2
1, 2
IRSN (Institute for radiological protection and nuclear safety), 1DSU/SSIAD, 2DEI/SARG,
B.P.17, 92262 Fontenay-aux-Roses Cedex, France
3
Lab. Tectonique, Univ. Paris XI-Orsay, 91405 Orsay Cedex, France
IRSN has reviewed ANDRA’s 2005 clay dossier on the feasibility of deep geological disposal
of high level, long-lived radioactive waste in the Callovo-Oxfordian clay formation (COX)
located in the Meuse/Haute-Marne (MHM) area, Eastern Paris Basin, and investigated by
ANDRA using the Bure Underground Research Laboratory (URL). An important task was to
deal with the influence of tectonic structures, which is a key question of the safety evaluation
as they may have potential for locally leading to a channelling of flows through the
sedimentary beds and thus for affecting transfer times, dilution factors and outlet positions.
Using available data from ANDRA’s geological survey, IRSN suspected the existence of faults
belonging to the same fault family, but with a different expression within the Mesozoic
sedimentary pile: near the Bure URL, faults suspected under the Lower Bathonian (using a
3D seismic survey) do not seem to have propagated through the COX (according to oblique
boreholes), whereas a few kilometres to the SW, structural and hydrogeological data (from
boreholes) allow to postulate that neighbouring faults hydraulically impact the entire
sedimentary pile, including the COX. Therefore, given these possible differences in fault
propagation, the fracturing pattern in the COX can not be straightforwardly extrapolated to the
whole MHM area.
The propagation of fractures from limestones to clays is related to the so-called “differential
fracturing phenomenon”, which means that ductile layers (clays) show a less intense and
concentrate fracturing than do breaking layers (such as limestones). This is commonly
observed at a decimetre scale and has been studied for joints, but it is still insufficiently
understood for faulting at the scale of tens to hundreds metre thick formations. This poorly
understood phenomenon is being addressed by IRSN on the basis of field observations at
analog sites at various scales, so as to elaborate a model of differential fracturing in
clay/limestone alternations. First fracturing observations were carried out in two analog sites,
Tournemire (Aveyron, France) and the Maltese Islands.
The present paper intends to describe the results obtained from these observations and
summarized below, as well as IRSN’s perspectives to continue this work.
Differential fracturing was revealed at a plurihectometric scale in IRSN’s Tournemire
experimental station, an ancient railway tunnel crossing a 150-m thick Toarcian clay formation
at 250 m depth. The following features were observed (3D seismic survey, tunnel, boreholes):
(i) in the limestones underlying the Toarcian clays, a normal fault re-activated as strike-slip is
associated with a narrow fractured zone, (ii) this structure continues into the clays as a wide
diffuse zone of thin, strike-slip faults, (iii) the structure is once more expressed within a narrow
fracture zone in the overlying limestones and then widens as it fades toward the surface.
In the Maltese Islands, along the seashore, the 2 to 20 m thick blue clays outcrop between
two tens of metres thick limestone formations of Oligo-Miocene age affected by slight
extensional tectonics. The underlying limestone layers show numerous joints and faults,
whereas few of them affect (partially or fully) the clay layer, usually with a smaller dip. Some
fractures have conducted fluids from limestones to clays; others seem to have been
generated by fluid overpressure.
September 2008
Page 126
NOTES
September 2008
Page 127
Contrasting Styles of Faults and Fault Rocks in the Rio Grande Rift of Central New
Mexico, USA: Their Relationships to Rift Architecture and Groundwater Resources
Jonathan Saul Caine, Scott A. Minor, and Mark R. Hudson
U.S. Geological Survey, Denver, CO, USA
[email protected]
Fault zones within and flanking Neogene basins of the Rio Grande rift show great diversity in
architectural style, orientation, scale, age, displacement magnitude and direction, fault rocks,
cements, permeability structure, and geochemistry. These parameters were characterized in
detail for a number of representative fault zones throughout the central rift to understand their
role in the evolution of the rift and their influence on paleo-fluid-flow and present-day
groundwater resources. Several characteristic groups of faults were recognized: 1) Steep
ENE- and NE-striking faults involving Proterozoic crystalline basement and Paleozoic
sedimentary rocks. These faults show possible pre-Neogene strike-slip and possible normalslip reactivation. Their late extensional reactivation postdates hydrothermal alteration
extensional strain, suggesting thermal weakening had a role in localizing these faults possibly
during the early stages of rift evolution. 2) NNW- to NNE-striking, steep normal faults in
poorly lithified Neogene basin-fill sediments. These faults have pervasive clay-rich cores that
preserve little evidence of cataclasis as well as unusual, deeply incised grooves and
convolutions within the clays, particularly at the margins of the fault cores. When present,
sparse damage zone structures are deformation bands and no open, fault-related fractures
were observed. Yet, siliciclastic sediments adjacent to the uncemented fault cores are
variably and commonly asymmetrically cemented by coarse calcite and occasional silica. The
ubiquitous presence with the basin-fill, northerly trend, and dominantly normal slip of these
faults is consistent with accommodation of E-W extensional strain throughout the evolution of
the rift. 3) NW- to NE-striking, steep, normal and strike-slip small displacement faults in
Pliocene basaltic rocks. These faults form distributed networks of slip surfaces within
monoclinal fold limbs, no development of central clay-rich fault cores, and are related to the
later stages of rift evolution.
Characteristics of fault zone architecture indicate fundamental differences between faults in
the rift flank, basin-fill, and volcanic tablelands. Crystalline rocks tend to have well-developed
damage zones composed of open fracture networks surrounding cataclastic, clay-rich fault
cores. The resulting architectural style could make these faults combined conduit-barriers to
present-day groundwater flow. In contrast, the architecture and macroscopic textures in faults
in poorly lithified basin-fill sediments suggest particulate flow in the paleo-saturated zone was
an important deformation mechanism. This resulted in these faults being partial barriers that
cause anisotropic paleo- and present-day groundwater flow. The distributed, uncemented,
open slip-surfaces of faults in the volcanic rocks suggests they may act as conduits for
present-day groundwater flow. Lack of evidence for major faults at the mountain front-basin
interface suggests recharge to the basin is not impeded, whereas faults in the basin fill may
compartmentalize the aquifer under pumping stresses from domestic groundwater use.
Individual fault groups have distinctive geochemistry related to their origin and evolution. For
example, illite dominates hydrothermally altered fault cores in crystalline rocks. In contrast,
kaolinite is dominant in faults cutting both Proterozoic and Paleozoic rocks, and smectite
dominates faults in basin sediments. Al and Fe are prevalent in Proterozoic basement faults,
whereas Ca and Ba are prevalent in basin faults.
September 2008
Page 128
NOTES
September 2008
Page 129
Assessing Temporal Changes in Fault Permeability for Radioactive Waste Disposal
Lunn R.J. 1, Brady P. 1, Kirkpatrick J. 2, Shipton Z.K. 2
1
Department of Civil Engineering, University of Strathclyde, Glasgow, Scotland
Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, Scotland
2
The geologic disposal of radioactive waste raises uncertainties regarding radionuclide
transport through fault zones. This paper presents a method of determining the hydraulic
conductivity of a fault zone at various stages in its temporal evolution. Outcrop data detailing
faults at various stages in their temporal evolution have been collated from the Mount Abbott
Quadrangle study area, Sierra Nevada. The groundwater flow simulation program
Groundwater Vistas has then been employed to model flow through each of these fault zones,
using several permeability scenarios that describe the relative permeability of the fault slip
surface as compared to the surrounding host rock and damage zone fractures. A complex
network of flow paths has been observed through the detailed architectural features of the
different fault zones, ranging from concentrated, tortuous flow paths to flow paths which are
more direct and distributed throughout the host rock. Results show that the presence of a fault
has a significant effect on the flow of groundwater and that the hydraulic conductivity of a fault
zone may increase significantly with further deformation.
September 2008
Page 130
NOTES
September 2008
Page 131
Fault-Zone Control of Fluid Flow in Extensional Basins
Michael A. Simms, Department of Earth and Planetary Sciences, Johns Hopkins University
Baltimore, Maryland 21218 USA
([email protected])
The syn-rift setting is a dynamic environment for fluid-flow processes because of elevated
heat flow, active extension and development of fault zones, and generation of relief in fault
blocks and the rift flanks. Buoyancy-driven flow (thermal convection, thermohaline
convection, and haline convection) and topography-driven flow are flow processes that can
occur in extensional basins and can be influenced or controlled by the locations and
characteristics of fault zones. Numerical modeling of variable-density fluid flow is used to
simulate the dependence of basin-scale flow patterns on fault-zone properties including faultzone spacing, fault-zone permeability, permeability anisotropy and structure, and fault-zone
extent and continuity. Fault zones can be the locus for the onset of thermal convection and
thermohaline convection, can control convection-cell widths and the spacings of thermohaline
plumes, and define the locations of fluid discharge to the sea floor. The presence of
sufficiently permeable fault zones can allow thermal convection to occur under subcritical
conditions of basin thickness, sediment permeability, and heat flow. The spacings of morepermeable fault zones can control the size and sense of flow of thermal convection cells with
fault-bounded cells occurring between major faults with spacings of up to 10 to 20 km. Faultbounded convection cell pairs can form between wider-spaced fault zones. Wide recharge
zones of down flowing fluid are centered on a fault zone and up-flow zones are closely
aligned with fault zones depending on their dip and permeability. Major permeable sand
zones in the basin or the underlying basement sequence can allow convection cells to be
bounded by more widely-spaced fault zones. The fault zones associated with rift-flank uplift
and the uplift of foot-wall blocks can provide migration pathways for topography-driven flow of
meteoric water to enter the basin sequence and underlying basement. Foot-wall crests can
function as rift-interior areas of meteoric recharge in which the elevation of the water table will
be related to the amount of uplift above sea level and the climatic conditions in the basin.
Higher permeability of the bounding fault zone or higher water-table elevations can drive
meteoric recharge to greater depths in the fault zone so that deeper permeable zones can be
affected and can transport meteoric water further into the submarine portion of a basin.
Adjacent fault zones also can provide pathways for upward discharge of meteoric water that
has penetrated the basin. The numerical simulations provide a tool for identifying and
assessing the characteristics of fault zones that could influence fluid-flow patterns in a
particular basin setting.
September 2008
Page 132
NOTES
September 2008
Page 133
Pull-aparts, scaling and fluid flow
D.C.P. Peacock1, X. Zhang2 & M.W. Anderson3
1
Fugro Robertson Ltd., Llandudno LL30 1SA, UK
Schlumberger Reservoir Geomechanics Centre of Excellence, 10 The Courtyard, Eastern
Road, Bracknell, RG12 2XB, UK
3
School of Earth, Ocean and Environmental Sciences, University of Plymouth, Plymouth PL4
8AA, UK
2
Steps between strike-slip fault segments appear to obey a power-law scaling relationship. If a
region displays seismically-resolvable pull-aparts, there are also likely to be a predictable
number of sub-seismic pull-aparts. Pull-aparts can play an important role in controlling fluid
transportation in low porosity rocks. A flow model is developed to estimate the flow rate
through individual pull-aparts. The pull-apart flow model produces similar flow rates to the
pipe flow model with a length to displacement (L/W) ratio of 1, but is closer to the fracture flow
with an L/W ratio of 10. The flow rate is smaller in the pipe flow model than in the fracture flow
model where the aperture of a fracture or the radius of a pipe is small, because the impact of
fluid viscosity on the average flow rate is significant. The flow rate is larger in the pipe flow
model than in the fracture flow model where the aperture of a fracture or the radius of a pipe
is large, because the impact of fluid viscosity on the average flow rate is insignificant. The
flow model is expanded to incorporate the power-law scaling of a population of pull-aparts, to
show how different scales of structures can contribute to overall fluid movement through a
faulted rock mass.
September 2008
Page 134
NOTES
September 2008
Page 135
Fault formation in uncemented sediments. Insight from laboratory experiments.
F. Cuisiat and E. Skurtveit
Norwegian Geotechnical Institute, Sognsveien 72, N-0806 Oslo
Faulting mechanism in un-cemented sediments is addressed through laboratory experiments
in a newly developed high stress ring shear apparatus. The main objective is to investigate
basic mechanisms involved in the deformation process of sediments during faulting. An
understanding of these processes and how they affect fluid flow is important for the
development of fault models and their implementation into reservoir simulators.
The experimental test program comprised three types of ring shear tests: shearing of
homogenous sand, shearing of layered sand - clay sequence and shearing of unclean sand
with varying clay content. Visual inspection of the samples after testing, analyses of thin
sections and permeability measurements across the shear zone during testing were used to
describe shear band characteristics and properties like geometrical continuity, thickness and
sealing potential. Deformation processes such as grain reorientation, clay smear and
cataclasis were identified from the tests.
For shearing of layered sand-clay sequence an increasing shear zone complexity was
observed with increasing depth at time of faulting. The experiments suggest that at shallow
burial depth, in clay rich sediments, clay smear is the most efficient mechanism for
permeability reduction. At this depth, sand-sand juxtaposition shear is dominated by grain
rolling causing only minor permeability reduction. At greater burial depths, permeability
reduction is dominated by grain crushing. Measurements of permeability both across clay
smear and sand-sand juxtaposition yield similar values. Shearing of multiple clay layers (3
layers) produced a composite clay smear 2-3 times thicker than for a single clay layer,
whereas when reducing the clay layer thickness to one half of the reference layer, a thin and
discontinuous clay smear was produced. The permeability across the clay smear was found
to increase as the thickness of the clay source decrease for single clay layers, but the
permeability for composite smear was more complex.
Shearing of unclean sand was performed in order to study the formation of phyllosilicate
framework faults. Due to limited testing, it was difficult to establish trend lines in the data set.
Based on one thin section the phyllosilicate framework shear band consisted of a central
shear zone with grain crushing and sand grains embedded in a matrix of clay and crushed
sand. Outside the shear zone little grain crushing occurred. Clay was found along the
margins of the sand grains and only locally filling the pore space between the sand grains.
The observed clay enrichment within the shear zone was likely to be an effect of porosity and
grain size reduction during shearing. For similar initial sand porosity the residual strength
decreased with increasing clay content, but more data with comparable initial porosity would
be needed for a better understanding of the formation and properties of phyllosilicate
frameworks faults. The changes in permeability of sand with clay content measured before
shearing was in agreement with published available models in the literature. A significant
permeability reduction during shearing was observed together with clay enrichment within the
shear zone.
September 2008
Page 136
NOTES
September 2008
Page 137
KEYNOTE:
Extraordinary permeability associated with major W-E rock-mass discontinuities
cutting Carboniferous strata in northern England and central Scotland - some
cautionary tales
Younger, P., University of Newcastle
Please see insert.
September 2008
Page 138
NOTES
September 2008
Page 139
Burlington House
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September 2008
Page 140
September 2008
Page 141

Fault Zones: Structure, Geomechanics and Fluid Flow

Transcription

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