Seal bypass systems

Seal bypass systems
AUTHORS
Joe Cartwright, Mads Huuse, and Andrew Aplin
ABSTRACT
We present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and
allow fluids to flow vertically or subvertically across the seal. In so
doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids
to bypass the pore network. If such bypass systems exist within a
given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by
the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting
types of sealing sequences using three-dimensional (3-D) seismic
data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through
the seal sequence, with leakage internally at higher levels or to the
surface as seeps.
We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe
related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior.
INTRODUCTION
The term ‘‘seal’’ is one of the most widely used in petroleum geology, but is perhaps, one of the most misleading. The definition suggests that the seal prevents flow, whereas all rock types possess
intrinsic permeability to both single and multiphase fluids. In this
article, the term ‘‘sealing sequence’’ refers to an assemblage of generally low-permeability lithofacies that halt or retard the flow of petroleum toward the basin surface. Although there is no doubt that
Joe Cartwright 3DLab, School of Earth,
Ocean and Planetary Sciences, Cardiff University,
Cardiff CF10 3YE, United Kingdom;
[email protected]
Joe Cartwright received his B.A. degree and his
D.Phil. in geology from Oxford University. He is a
research professor of geophysics and director
of the 3DLab at Cardiff University. His research
interests focus on three-dimensional seismic interpretation in basin analysis, with special emphasis on
seal integrity analysis, the genesis of polygonal
faults, the emplacement of sandstone and igneous
intrusions, and silica diagenesis.
Mads Huuse 3DLab, School of Earth, Ocean
and Planetary Sciences, Cardiff University, Cardiff
CF10 3YE, United Kingdom; present address:
Department of Geology and Petroleum Geology,
University of Aberdeen, Aberdeen AB24 3UE,
United Kingdom; [email protected]
Mads Huuse received his Ph.D. from the University of Aarhus in 1999. In 1999 – 2005, he was
a postdoctoral researcher, first at the University
of Aarhus, then at the University of Aberdeen,
working on injected sands, and finally, at Cardiff
University, working on cap rocks. Currently,
he is lecturer of geophysics at the University of
Aberdeen. His special interest is in the seismic
imaging of fluid-flow features, glacial deposits, and
cool-water carbonates.
Andrew Aplin NRG, School of Civil Engineering and Geosciences, University of Newcastle,
Newcastle upon Tyne NE1 7RU, United Kingdom;
[email protected]
Andrew Aplin received his Ph.D. from Imperial
College in 1983. He was a Royal Society European
Fellow at Centre de Recherches Pétrographiques
et Géochimiques Nancy from 1983 to 1984 and
worked at BP as a research geochemist from 1984
to 1990. Since 1990, he has worked at the
University of Newcastle, where he is currently
professor of petroleum geoscience. His main
research interests are in the physical and fluidflow properties of fine-grained sediments.
ACKNOWLEDGEMENTS
Copyright #2007. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received December 1, 2005; provisional acceptance December 7, 2005; revised manuscript
received February 2, 2007; final acceptance April 9, 2007.
DOI:10.1306/04090705181
AAPG Bulletin, v. 91, no. 8 (August 2007), pp. 1141 – 1166
1141
We acknowledge receipt of Natural Environment
Research Council grant NE/C516 487/1 to
A. Aplin.
petroleum migrates through sealing sequences on modest geological time scales (e.g., Macgregor, 1996), flow
paths and flow rates are poorly constrained and commonly conjectural (e.g., Weber, 1997). Faults, fractures,
and capillary pore systems are all possibilities and appear to be operative in specific cases (e.g., Caillet, 1993;
Leith et al., 1993; Leith and Fallick, 1997; Losh et al.,
1999; O’Brien et al., 1999; Gartrell et al., 2002; Nordgard Bolas and Hermanrud, 2003; Boles et al., 2004).
Deciding among these possibilities is clearly an important element of petroleum systems analysis, but is notoriously difficult and requires a detailed knowledge of
the material properties of seal units at a range of scales
and through geological time.
Much of the work on the physical properties of seals
has been undertaken in the context of either seal analysis or overpressure prediction. Seal analysis, both for
top seals and fault seals, has largely focused on the prediction of potential column heights and generally centers on the definition of a threshold capillary entry pressure, which must be overcome for leakage to occur (e.g.,
Berg, 1975; Schowalter, 1979; Watts, 1987; Vavra
et al., 1992; Brown, 2003). Once breached, or in situations where the pore system of the seal is oil wet (Bennett
et al., 2004; Aplin and Larter, 2005), the rate of leakage through a sealing sequence is fundamentally controlled by its (relative) permeability. Recent research has
provided useful permeability data for mudstone and
chalk seals (e.g., Schlömer and Krooss, 1997; Dewhurst
et al., 1998, 1999; Kwon et al., 2004; Mallon et al., 2005).
In situations where pore pressure is sufficiently high to
cause mechanical failure of the seals, petroleum may
leak through resulting fractures (Grauls and Cassignol,
1992; Gaarenstroom et al., 1993). However, rates and
mechanisms are imprecisely known (Ingram and Urai,
1999; Brown, 2000).
The analysis described above is limited in being
based largely on the core-scale properties of sealing sequences. The ultimate goal for any exercise in assigning risk levels to seals is the definition of the threedimensional (3-D) architecture of the seal combined
with a full description of the sealing properties in 3-D,
in the context of past and present-day stress regimes.
This includes a description of heterogeneities on any
observable scale, not merely those accessible from core
or well data. The most vulnerable parts of a seal are,
by definition, those that can act as fluid-migration pathways, i.e., the most permeable connected routes with
the lowest capillary threshold pressures. A major challenge for explorationists, therefore, is to identify and
predict these routes using a combination of geophys1142
Seal Bypass Systems
ical, petrophysical, and geological data. This approach
was fully appreciated over two decades ago by Downey
(1984), but can now be reevaluated using modern techniques such as 3-D seismic interpretation.
The focus of this article is on seismic-scale permeability heterogeneities and is based on a reappraisal of
Downey’s pioneering recognition that some high-quality
seals may be breached episodically or semipermanently
by a range of geological structures that we collectively
term ‘‘seal bypass systems.’’ We propose that if such
bypass systems exist within a given sealing sequence,
then predictions of sealing capacity based exclusively
on rock physical properties such as capillary entry pressure, hydraulic conductivity, and wettability may be
largely negated by the capacity of the bypass system to
breach the microscale sealing framework of the grains
and pore network. The degree to which the pore-scale
analysis might be invalidated depends critically on the
relative fluxes that can be transmitted via the pore network as opposed to the bypass systems. This article is
based largely on 3-D seismic observations of sealing sequences in which geological discontinuities are embedded within the seal and breach it by acting as highly
focused, vertical, or subvertical fluid-flow paths from
the underlying reservoirs. As such, it focuses mainly on
an object-based recognition of potential features that
could lead to significant bypass of the pore network. It
should be noted here that the recognition of seismicscale features does not automatically imply that fluxes
through these features would be greater than through
their subseismic equivalents.
Our approach is to define and classify different categories of pathways and to document these with a series
of examples based on the interpretation of modern 3-D
seismic volumes. Our classification is object based because it is intended as a guide to interpretation. Whereas many of these objects have been recognized previously, this article is a first attempt to group the range of
observed phenomena. It should therefore be regarded
only as a preliminary, qualitative scheme, to be refined
as more such features are recognized and their impact
on fluid-flow regimes is quantified. The article is intended to alert seismic interpreters to the significance
of the bypass features and to aid them in their task of
distinguishing them from the many subvertical to vertical artifacts present on seismic data. The article begins
with some formal definitions, sets out the classification
we propose, and presents examples for each of the main
categories. The article concludes with a discussion of
the wider significance of these features for petroleum
exploration.
SEAL BYPASS SYSTEMS
We define seal bypass systems (SBS) as ‘‘large-scale
(seismically resolvable) geological features embedded
within sealing sequences that promote cross-stratal fluid
migration and allow fluids to bypass the pore network.’’
The restriction to objects that are large enough to be
resolvable on seismic data is purely arbitrary and for
the purposes of developing a seismically based classification. It is not intended to exclude bypass systems
that fall beneath this arbitrary scale limit. Subseismicscale bypass systems can be more efficient than much
larger features, and it is important to recognize that the
size of any given feature does not necessarily correlate
with its permeability.
We recognize three main groups of SBS (Table 1),
consisting of (1) faults, (2) intrusions, and (3) pipes, described in detail below, and based on an extensive review
of published seismic data. We observe that SBS are extremely common in most petroliferous basins where
there is good 3-D seismic coverage. They are widespread
on a global scale: all petroliferous basins for which we
have access to data contain SBS. We have also found
that SBS are commonly unreported in basins where they
are well developed and may be significant fluid-flow
conduits. In many petroliferous basins, at least two of
the three classes of SBS are developed, and in some, all
three classes can be found widely distributed within a
basin.
In the section below, we review background data
for each of the groups in turn, providing examples of
seismic expression, and compare the relative efficiencies of each group and the different types of fluid-flow
behavior they exhibit.
FAULT BYPASS
This is the largest group of bypass systems, and their
function of conduits across sealing sequences has been
intensively studied (e.g., Sibson, 1981; Downey, 1984;
O’Brien et al., 1999, Finkbeiner et al., 2001; Nordgard
Bolas and Hermanrud, 2003). The function of hydraulic fractures in seal leakage has also been extensively researched (Secor, 1965; Silverman, 1965; Engelder and
Lacazette, 1990). Small fractures are known to be important as potential hydrocarbon conduits (Smith, 1966),
but because they are unlikely ever to be imaged individually on seismic data, we confine our discussion to
seismically resolvable faults as vertical fluid-migration
pathways. Because fault dimensions scale with fault
displacement, seismically resolvable faults with throws
greater than 10 m (33 ft) are commonly found to cross
hundreds of meters of stratigraphic section, and fault
conduits thus have great potential for long-range vertical fluid transmission (Hooper, 1991).
We subdivide the class into two families: trapdefining or supratrap, based on whether the faults define and delimit the trap with a lateral seal component,
or whether they are embedded within the sealing sequence (Table 1). The rationale behind this subdivision
is based on the notion that many trap-defining faults
may have grown by successive earthquakes, and that this
motion history would have involved a different mode
of fluid flow along the fault planes than for supratrap
faults. Examples of trap-defining faults include many
of the classic tilted fault block traps of the North Sea
Basin, or thrusted hanging-wall traps of foreland thrust
and fold belts. Examples of supratrap faults include,
for example, sets of synthetic and antithetic faults in
crestal collapse structures above rollover anticlines or
polygonal fault systems as widely developed in seals of
Cenozoic age in northwest Europe and on the Atlantic
margins.
In the case of trap-defining faults, the vertical permeability of the fault plane and adjacent damage zones
is dependent on the larger scale context of the fault and
its history of motion, in combination with the local hydrodynamic boundary conditions. In the case of supratrap faults, the behavior of the faults as fluid valves from
the trap is more closely coupled with the hydrodynamic conditions in the reservoir and in the sealing sequence.
Smaller faults with throws typically less than 100 m
(330 ft) that detach or tip out within the seal sequence or
within the reservoir are likely to slip in small-magnitude
increments, so seismic pumping (Sibson, 1981) is less
likely to feature as a mechanism than for many basementlinked, trap-defining faults that are more likely to have
been seismogenically active.
Seismic examples of trap-defining faults versus supratrap faults are presented in Figure 1. Leakage of hydrocarbons is visible in both examples in the form of
distributed amplitude anomalies that can be directly
linked to small gas accumulations, but this does not by
itself prove that the faults have acted as the conduits,
as is sometimes assumed (Heggland, 1997).
Despite the wide appreciation of the potential function of faults as being potential leakage routes through
sealing sequences (O’Brien et al., 1999; Gartrell et al.,
2002; Nordgard Bolas and Hermanrud, 2003), there
are few published case studies documenting this function or quantifying their impact on fluid fluxes. The
Cartwright et al.
1143
Table 1. Classification of SBS
SBS
Faults: trap defining
Faults: supratrap
Intrusions: sand
Intrusions: mud
Intrusions: salt
Intrusions: igneous
Pipes: dissolution
Pipes: hydrothermal
Pipes: blowout
Pipes: seepage
Setting
Dimensions
Any tectonically active setting;
commonly seismogenic in
character
Faults embedded in sealing
sequence, generally not
lined to basement
Mainly in deep-water basins;
various tectonic settings;
very fine-grained seals
Variable, but lengths
commonly greater than
5 km (3.1 mi)
Usually < 10 km ( <6 mi)
in length, < 1–2 km
( <0.6 –1.2 mi) vertical extent
Aperture: centimeters to
meters; height: meters
to 1 km (0.6 mi); length:
meters to 10 km (6 mi)
Aperture: centimeters to
meters; height: meters to
1 km (0.6 mi); length: meters
to 10 km (6 mi)
Height: tens of meters to
kilometers; width: tens of
meters to kilometers
Height: tens of meters to
kilometers; width: tens of
meters to kilometers
Height: tens of meters to
kilometers; width: meters to
hundreds of meters
Height: tens of meters to
several kilometers; width:
tens of meters to 1–2 km
(0.6 –1.2 mi)
Mainly in tectonically active
basins, commonly with
highly overpressured
deeper sequences
Within salt basins
In volcanic margin basins
and other magmatically
active settings
Underlain by carbonate
or evaporite sequences
In volcanic margin basins
and other magmatically
active settings; associated
with magmatic intrusions
into basin fill sequences
Various basin types, but
commonly where there are
overpressured intervals
at depth
Various basin types, but
commonly where there are
overpressured intervals
at depth
Height: tens of meters to
1– 2 km (0.6 –1.2 mi);
width: tens to hundreds
of meters
Height: tens of meters to
1– 2 km (0.6 –1.2 mi);
width: tens to hundreds
of meters
theoretical basis for this type of analysis is well established (Secor, 1965; Sibson, 1981), but the question
of whether faults are semipermanently open pathways
for fluid migration is widely debated within the petroleum industry. That faults act as fluid-flow pathways is
hard to dispute, given the wide evidence for this in ore
1144
Seal Bypass Systems
Seismic Expression
Systematic offsets of stratal reflections
Systematic offsets of stratal reflections
Discordant amplitude anomalies,
localized forced folding
Cylindrical conduits with amplitude
anomalies distributed adjacent to or
within the conduit
Geometries range from diapirs to walls
to fault-controlled intrusions
Discordant amplitude anomalies,
localized forced folding, and
hydrothermal pipes
Cylindrical or steeply conical zones of
intense disruption of stratal reflections,
localized sag folding
Cylindrical or steeply conical zones of
intense disruption of stratal reflections
typically developed directly above
igneous intrusions and commonly
linked to sea-floor mounds
Cylindrical or steeply conical zones of
intense disruption of stratal reflections
typically developed directly above
localized breach points of underlying
fluid source interval; linked to
pockmarks; distributed amplitude
anomalies are common
As for blowout pipes, but no link
to pockmarks
deposits (Newhouse, 1942). More of the debate centers on which faults act as seals or as fluid pathways. If
some act as both, when do they exhibit their specific
functions? It may be most relevant to play analysis to
consider their sealing potential as a time-dependent parameter (Alexander and Handschy, 1998).
One of the major controls on the long-term behavior
of fault planes as flow conduits is the static permeability
of the fault rocks. Faults whose damage zones are more
permeable than their host sequences can be major flow
routes irrespective of their specific history of rupture
and displacement. In these rare cases, faults may act as
cross-stratal migration routes on a semipermanent basis
(Hooper, 1991; James, 1997). However, most fault zone
rocks are characterized by lower permeability than their
host rocks, and steady-state leakage rates would be commensurately small (Knipe, 1997). The physical properties of gouge in fault zones (Gibson, 1994) and the permeability of any connected fractures in any damage
zones surrounding the larger scale fault (Aydin, 2000)
will determine the extent to which steady-state flow
can occur along the fault zone during dormant periods
between slip events.
The capacity of seismogenic faults to act as flow
conduits needs to take account of the slip behavior of
the fault in addition to the static permeability of the
fault zone. Considerable deviation of the efficiency of
the conduit can be predicted for periods before, during,
and immediately after active rupture of the fault plane,
i.e., during the earthquake cycle. Faults most probably
act as valves for fluids during active rupture events (Sibson, 1981; Muir Wood, 1994; Hickman et al.,1995) and
could potentially bleed large volumes of fluid from a
reservoir in contact with the active fault.
Direct evidence of significant fluid flux upward
along fault planes is graphically revealed by trails of large
pockmark craters that are aligned vertically above the
upper tips of fault planes (Heggland, 1997; Ligtenberg,
2005). The link with pockmarks by itself suggests that
fluxes conducted along the fault planes were large enough
to excavate substantial volumes of sediment from the
seabed (Hovland and Judd, 1988). Other, less direct observational evidence that links fault planes to significant fluid flux includes (1) mud mounds or mud diapirs
forming directly above fault planes (Berryhill, 1986);
(2) hydrate mounds above fault planes (Roberts et al.,
2000); (3) temperature anomalies above faults (Kumar,
1977) or within fault rocks (Roberts et al., 1996; Losh
et al., 1999); (4) fluid pressure distributions across faults
(Berg and Habeck, 1982); (5) distribution of cementing
phases in fault-hosted veins (Eichhubl and Boles, 2002);
(6) pore-water salinity anomalies associated with faults
(Lin and Nunn, 1997); (7) and hydrothermal deposits
associated with faults (e.g., Hedenquist and Henley,
1985) and clustering of small gas-related amplitude anomalies stacked vertically in footwall or hanging-wall traps
along single faults (Heggland, 1997).
Polygonal Fault Systems
A good example of the supratrap family of fault bypass
systems is provided by polygonal fault systems. In the
past decade, it has been recognized that many ultralowpermeability sealing sequences are deformed by pervasively developed networks of polygonal faults, and it is
therefore pertinent to consider the likely impact of these
faults on seal integrity (Cartwright and Dewhurst, 1998;
Cartwright et al., 2003; Stuevold et al., 2003) (Figure 2).
These fault networks are ubiquitous within certain finegrained lithologies, such as smectitic claystones and siliceous or calcareous oozes. Sealing sequences of Eocene
age in the North Sea are deformed by extensive polygonal fault systems and yet overlie many prolific Paleocene reservoirs, suggesting that these faults do not compromise the seal integrity.
The typical range of lithofacies in which polygonal
faults occur possesses core-scale permeabilities that are
extremely low (<10 17 m 1), so the presence of such
a pervasive fault network formed during the earliest
stages of compaction and dewatering represents a significant potential permeability heterogeneity. Polygonal fault planes are known to transmit fluids, although
the fault planes have almost no fault gouge, and static
permeabilities are lower in the vicinity of the slip zone
than in the adjacent wall rocks ( Verschuren, 1992; A.
Bolton, 2001, personal communication). This suggests
that dilatancy occurs during active slip (Sibson, 1981),
and there is a transient flux during any given slip event
(Cartwright et al., 2003). In addition, indirect indicators for fluid flow along polygonal faults have been inferred from pockmark trails that are observed above
polygonal faults in diverse settings (e.g., Gay et al., 2004).
Stacked gas anomalies are commonly associated with
polygonal faults in North Sea Tertiary gas chimneys, but
as noted earlier, this does not imply that the faults bled
gas. Gas leakage from hydrocarbon reservoirs such as in
the Ormen Lange field has also been indirectly linked
to polygonal faults acting as the main conduits (Stuevold
et al., 2003; Berndt et al., 2003).
In summary, the fact that many prolific accumulations are successfully trapped by a top seal that is
polygonally faulted suggests that leakage through these
faults is insufficiently rapid to compromise the accumulation. However, there may be specific circumstances
where the polygonal faults are reactivated under later
stress conditions where they can potentially threaten
trap integrity. The recognition of polygonal faults in a
sealing sequence thus points to a high-quality seal being present because of their tendency to occur in very
Cartwright et al.
1145
1146
Seal Bypass Systems
low-permeability sediments, but evidence of leakage
via the faults needs to be scrutinized as part of the seal
risk assessment.
INTRUSIVE BYPASS
Intrusive bypass systems are a group of intrusive structures that breach the integrity of a sealing sequence in
one or another of three distinct ways. First, the intrusive event itself involves the puncturing of the seal, and
the transmission of fluids through the seal along with
the intrusive material. A good example of this behavior
is when mud volcanoes first form. Second, when the
intruded material possesses a markedly higher permeability compared to the sealing sequence, fluid flow
will be focused upward through the intrusion. Examples of this case are sandstone intrusions or highly fractured igneous intrusions. Third, when the intrusion process results in intense fracturing and deformation of the
sealing sequence, fluid flow can exploit the increased
permeability of the sealing sequence in the contact zone.
Good examples of this can be found in the sheath zone
around salt diapirs, or in metamorphic aureoles around
igneous intrusions. The magnitude of the permeability
perturbation in each of these three cases depends on
several factors related to the specific intrusion family.
Each of the four intrusive bypass families included in
Table 1 is discussed in more detail below, with reference to seismic examples and context.
Sandstone Intrusions
This family is a potentially significant element in regionalscale fluid flow in basins and a significant mode of seal
failure, whose existence at seismic scale has only recently
been recognized (Molyneux et al., 2002; Hurst et al.,
2003). Three-dimensional seismic data have revealed
kilometer-scale sandstone dikes and sills in several petroleum systems in northwest Europe (Hurst et al.,
2003; Schwartz et al., 2003; Huuse and Mickelson,
2004; Shoulders and Cartwright, 2004). They commonly intrude what would otherwise be regarded as
high-quality sealing sequences and, in so doing, fundamentally alter the seal integrity. The effect of a sandstone intrusion in this context is to insert a meters-wide
conduit with darcy permeability into a sequence with
nanodarcy permeability. Sandstone intrusions crossing
otherwise excellent sealing successions up to 1000 m
(3300 ft) thick are documented in the United Kingdom
Atlantic margin petroleum province (Shoulders and
Cartwright, 2004) (Figure 3).
The dimensions of the intrusions and, therefore, the
amount of sealing section they transect depend on several factors: (1) the size of the parent sand body supplying sand to the intrusion; (2) the overpressuring mechanism needed to liquefy and fluidize the sand and to
drive the fluid flow; (3) the burial depth; and (4) the
scale of the hydrodynamic regime imposing the overpressure. Reservoir-scale sandstone intrusions are not
uncommon: single sandstone dikes of up to 10 m (33 ft)
width and 30 km (18 mi) length have been described
from the coastal ranges of California (Schwartz et al.,
2003). In addition, discordant sandstone intrusions of
up to 45 m (147 ft) thick with a characteristic conical
geometry have been identified on 3-D seismic data
from a large area of the central and northern North Sea,
crossing up to 700 m (2296 ft) of otherwise highly impermeable seal of Eocene age (Molyneux et al., 2002;
Huuse et al., 2003; Huuse and Mickelson, 2004). These
sandstone intrusions certainly compromise the seal, but
in a more positive vein, they also provide secondary migration pathways into isolated deep-water sandstone
reservoirs within the Eocene (Huuse et al., 2005).
The emplacement of sandstone intrusions is a form
of natural blowout similar to that envisaged for blowout pipes (see below), although the geometry of the two
bypass systems is quite distinct. High fluid pressures
approaching lithostatic are involved in the process of
sandstone intrusion and result in high fluid fluxes, particularly during the intrusion event. The fluxes most
likely scale with the pore volume and pressure state of
the parent sand body supplying the intrusion network.
The fluid flux also likely scales with the volume of sand
emplaced in the intrusions. As a first-order estimate,
we suggest that the total fluid flux during an intrusion
Figure 1. Seismic profiles illustrating hydrocarbon leakage associated with faults. (A) Leakage from the crestal region of a large
tilted fault block expressed as vertically distributed amplitude anomalies (arrowed) in the footwall to this major trap-defining fault. A
bottom simulating reflection (BSR) associated with local development of gas hydrates can be seen near the top of the profile. Data
shown with permission of NAMCOR. (B) Extensive leakage of gas from a large anticlinal trap with a reservoir of submarine-fan
sandstone top sealed by polygonally faulted claystone (TF is top reservoir, A/CT is the Opal A to Opal CT boundary). The supratrap
faults are implicated indirectly in the leakage process. Data courtesy of Petroleum Geo-Services (UK) Ltd.
Cartwright et al.
1147
Figure 2. Seismic expression of polygonal fault
systems. (A) Horizontal
slice through a variance
attribute volume showing
a classical polygonal network embedded within a
biosiliceous mudstone
seal. Note the mud volcano conduit (MVC) toward
the bottom of the image.
(B) Profile from a 3-D
volume showing intense
polygonal faulting developed in sealing sequences
offshore Norway. The
change in amplitude response across the marker
labeled A/CT is caused
by silica diagenesis of the
lower part of the faulted
interval. Data courtesy
of Norsk Hydro.
1148
Seal Bypass Systems
Figure 3. Seismic expression of large-scale, interconnected sandstone intrusions from the Faeroe-Shetland Basin, offshore United
Kingdom. The sandstone intrusions cross a low-permeability sealing sequence comprised of mainly smectite-rich mudstones,
rendering them ineffective as a top seal for the major submarine-fan reservoir seen below the top fan (TF) marker. Some of the
sandstone intrusions are associated with forced folds, allowing them to be dated (Shoulders and Cartwright, 2004). Data by permission
of Exxon-Mobil.
event might be an order of magnitude larger than the
intrusion volume (Shoulders, 2005). From simple
considerations of Stoke’s law, likely flow velocities
during intrusion are on the order of 1–2 cm s 1 (0.4–
0.8 in. s 1) (Shoulders, 2005).
The impact of sandstone intrusions on sealing sequences is not restricted to the period of the intrusion
event (which may last only a few days). After formation,
sandstone intrusions can remain open as highly permeable conduits for many millions of years, until their
vertical continuity is broken by deformation, or the pore
space becomes cemented (Hurst et al., 2003; Huuse
et al., 2004; Jonk et al., 2005). They may thus have a
significant long-term impact on seal integrity. Wellcalibrated examples of large-scale sandstone intrusions
in the North Sea are partially cemented (Løseth et al.,
2003), but rarely completely tight, and there is abundant evidence from fluid-inclusion data of long-term
(>10 m.y.) and episodic flux of aqueous and hydrocarbon
fluids via these intrusive pathways (Jonk et al., 2005).
Igneous Intrusions
In contrast to sandstone intrusions where it is the intrusive lithology (sand) that provides the permeable pathway to allow focused fluid migration along the intrusion,
igneous intrusions have generally much lower permeability than the host medium typical sealing facies. However, the intrusion of hot magma at greater than 1000jC
into cold and wet sediments results in major changes in
host rock properties for tens of meters away from the
immediate contact zone (Einsele et al., 1980). In addition
to fracturing associated with forceful intrusion, fracture
sets also form during prograded metamorphism in the
contact aureole, during hydrothermally driven fluid loss
from surrounding sediments (Einsele et al., 1980) and
also in the thermal contraction fracturing during longer
term cooling of the intrusive body itself. These different
fracture sets thus provide a fracture permeability network at various scales surrounding the intrusion and
occasionally within the body of the intrusion itself.
Cartwright et al.
1149
Figure 4. Seismic expression of major igneous sills from the Rockall Basin, offshore United Kingdom. The sills are easily recognizable as high-amplitude bodies with a generally concave- or bowl-shaped geometry (Hansen et al., 2005). They are interconnected and transect 2 – 3 km (1.2 – 1.8 mi) of sealing successions of late Mesozoic and early Cenozoic age. Data courtesy of
Petroleum Geo-Services (UK) Ltd.
The fractures that develop during initial intrusion
will significantly modify fluid-flow behavior around the
intrusion during and immediately after the intrusion
event. The drive for this phase of fluid flow comes from
the thermal energy of the intrusion. Einsele (1992), for
example, estimated a total flux caused by hydrothermal
activity resulting from sill intrusion in the Gulf of California on the order of 107 m3/km2 (5.7 107 ft3/mi2)
of sill surface area based on the observed reduction in
porosity in sediments surrounding the sills. Fractures
within the metamorphic contact aureoles can provide
potentially longer term flow conduits, but hydrothermal fluids are highly mineralizing, and the fracture sets
have a high probability of closing quickly because of cementation. Vein systems observed surrounding igneous sills are commonly filled with mineral phases linked
to the initial stages of hydrothermal activity (Einsele
et al., 1980).
1150
Seal Bypass Systems
The scale and distribution of the fractured country
rock associated with any given intrusion depends mainly
on intrusion size, geometry, and mechanism. In many
petroliferous sedimentary basins, the most important
types of intrusion likely to feature in any assessment of
seal bypass are mafic dikes and sills, which are widely
developed, for example, on many volcanic continental
margins (Planke et al., 2000). These range in size from
a few meters to many kilometers in dimension and, in
the case of dikes, can cross many kilometers of sealing
sequences (Figure 4) (Rubin, 1995). The fracture systems associated with these types of forcible intrusions
are particularly complex and are known to be mainly
controlled by pore fluid –magma interactions and intrusion geometry (Pollard, 1973; Delaney, 1987). The dimensions and typical context of these types of intrusions
mean that they may be important in providing secondary migration routes in addition to having a negative
impact on seal integrity. As exploration campaigns increasingly target volcanic continental margin domains
such as west Africa, Brazil, India, and the northeast Atlantic margin deep water, the recognition of igneous
intrusions as major fluid-flow conduits will likely become more important.
Mud Diapirs and Diatremes
Mud diapirs and diatremes are an important and widespread subgroup of SBS, most widely known from tectonically active settings such as convergent margins,
foreland basins, and strike-slip provinces (Kopf, 2002;
Dimitrov, 2002), but they are increasingly recognized
in passive-margin settings (e.g., Graue, 1999; Hansen
et al., 2005; Frey Martinez et al., 2007). Mud diapirs or
diatremes are commonly associated with mud volcanoes. Collectively, this group of intrusive and extrusive
structures is widely documented, and the structures are
increasingly being identified from producing regions on
3-D seismic data (Van Rensbergen and Morley, 2003;
Davies and Stewart, 2005). Modern studies have largely
focused on methane flux during mud vulcanism in the
context of global greenhouse gas budgets (Dimitrov,
2002; Kopf, 2002). However, mud volcanoes are also
recognized as an extremely efficient mechanism for dewatering rapidly buried and overpressured clay-rich
sedimentary sequences (Kopf and Behrmann, 2000).
The main flux of fluid linked to mud intrusions is
associated with the intrusion event itself, i.e., in the
form of the fluids that comprise the ascending mud column. Intrusive activity is highly episodic, and this episodicity most likely depends on the hydrodynamic conditions in the source layer providing the flow of mud
into the diapir or diatreme (Brown, 1990). Our knowledge of fluxes associated with mud intrusion comes
mainly from studies of mud volcanoes, and accurate
quantitative data are limited (Kopf and Behrmann, 2000).
Nevertheless, mud volcanoes are clearly episodic (Dimitrov, 2002), which in turn implies that the underlying
mud intrusions are also episodic. Fluid flow upward
along a mud intrusion in the intervening periods separating eruption is likely to be limited to small volumes
flowing via fracture networks that develop through the
forceful intrusion of the mud into its host medium or
through intercalations of coarser material entrained in
the conduit fill (Morley, 2003).
Known or extrapolated fluxes from mud volcanoes
span a large range and are commonly based on extrapolations of modern eruptions. The most detailed quantitative analysis of flux is that of Kopf and Behrmann
(2000), in an analysis of a suite of large mud volcanoes on
the Mediterranean Ridge. They found that flux ranged
on the order of 103 – 104 m3/day (3.53 104 –3.53 105 ft3/day). Scales of mud intrusions and extrusions
vary considerably from meters to kilometers. The depth
of origin of mud extruded in volcanoes gives a good indication of the vertical distances that mud can be transported and, hence, the vertical extent of any potential seal
breach. In the mud volcanoes in Trinidad, clasts of Eocene limestone and Cretaceous silicified shale are commonly erupted from depths of greater than 2 km (1.2 mi)
(Kugler, 1933). Mud volcanoes in the Vøring Basin,
Norway, ascend through greater than 1 km (0.6 mi) of
otherwise highly intact sealing lithologies.
The key requirements for mud diapirism and vulcanism are a means to liquefy the fine-grained parent
sedimentary unit. This can be achieved by inflation (addition of fluid under pressure), in-situ overpressuring,
or external triggering (e.g., earthquakes). The liquefied
mud can ascend rapidly (in kilometers per day), but the
rates of ascent vary widely depending on conduit geometry and viscosity (Kopf, 2002). The vent geometry
is the least well-understood part of the system, but the
most important (Kopf and Behrmann, 2000; Davies
and Stewart, 2005). Not only does vent geometry control the surface expression and flux, but it also forms
the main breach of the seal. Seismic imaging of vents is
generally poor (Dimitrov, 2002), although modern 3-D
seismic data commonly reveal a much narrower vent
geometry than commonly apparent on two-dimensional
(2-D) seismic data (Figure 5). Prestack depth migration is commonly required to obtain the best results
(Davies and Stewart, 2005). Conduits are generally narrower than commonly apparent on seismic data (Van
Rensbergen and Morley, 2003). Rare field examples
of vents indicate a large degree of fracture propagation
and stoping for the ascent process (Morley, 2003).
Several major hydrocarbon accumulations have reservoirs that are pierced by mud diapirs or mud volcano
conduits. Excellent examples are known from Trinidad
(e.g., the Forest Reserve field; Deville et al., 2003) and
also the Caspian Sea hydrocarbon province, where fields
such as ACG (Azeri Chirag Gunashli) and Shah Deniz
are giant oil and gas fields with penetrative mud structures close to their respective crests (Fowler et al.,
2000; Davies and Stewart, 2005). In both these fields,
although there is considerable leakage of hydrocarbons
at the surface above and surrounding the mud structures, the major accumulations are, nonetheless, intact.
This may be an indication that the passage of a mud
diapir through the reservoir was largely self-sealing, or it
Cartwright et al.
1151
Figure 5. Seismic expression of a buried mud volcano (MV) and underlying cylindrical conduit. The conduit is associated with
localized folding and with vertically stacked amplitude anomalies, probably indicative of gas-filled porous units or cemented zones.
Thickness changes related to the associated folds have been used to identify two to three phases of fluid venting along this and
similar conduits in the area (Hansen et al., 2005). Data courtesy of Norsk Hydro.
could imply that hydrocarbon charge is very recent, and
the rate of influx of hydrocarbons to the reservoir is
greater than the rate of loss via the mud vent.
Fracture and fault systems are widely observed in
association with mud diapirs and diatremes, either forming through forceful intrusion mechanisms, stoping, or
caldera-style collapse of the mud chambers supplying
the vent (Kopf, 2002; Morley, 2003; Davies and Stewart, 2005). These faults and fractures represent an additional means for upward fluid flow and, where intimately linked to the mud structures, can be considered
as part of the intrusion-related bypass system.
Salt Diapirs
Many hydrocarbon provinces are located in areas where
salt tectonics is an integral part of the deformational
regime and where diapirism is a common structural
1152
Seal Bypass Systems
style (Jackson, 1995). The growth of salt diapirs commonly involves forced folding and radial and concentric
faulting, and this associated deformation can exert a
major impact on fluid-flow regimes and seal integrity.
Salt diapirs are widely associated with focused fluid
flow as evidenced by (1) localized development of mud
mounds above diapirs in the Gulf of Mexico (Roberts
and Carney, 1997); (2) shallow gas anomalies clustered
around and above salt diapirs in the North Sea (Heggland,
1998); and (3) localized salinity anomalies around salt
diapirs, offshore Louisiana (Esch and Hanor, 1995), and
with large pockmarks above diapir margins in west Africa.
In the North Sea, gas chimneys in the Tertiary overburden
are widely observed above salt diapirs, and these contribute to imaging problems at depth and to drilling hazards
in the shallow subsurface (Heggland, 1997, 1998).
Hydrocarbon leakage in the vicinity of salt diapirs
occurs via complex fracture networks that are developed
in the sheath of drag folds in the immediate contact
zone between the salt body and the forcibly intruded
host sediments and in the carapace immediately above
the crest of the diapir (Alsop et al., 2000; Davison et al.,
2000). Despite this obvious vertical permeability to hydrocarbons, major oil accumulations occur in Paleocene
sandstone and Upper Cretaceous chalk reservoirs trapped
against salt walls and diapirs, and the large column heights
of these accumulations testify to the effectiveness of
both lateral seals and top seals in this setting (Davison
et al., 2000). It is probable, therefore, that the fault and
fracture systems surrounding the intrusive salt bodies
are only permeable for finite periods of time and are
not open for flow long enough to deplete reservoired
hydrocarbons.
Diapir growth is commonly episodic (Jackson,
1995), and commonly follows the cyclicity established
by regional tectonics (Davison et al., 2000). Hence, the
fluid flow associated with salt diapirism is also likely
to be episodic. Finally, it should be borne in mind that
the deformation of local aquifers and seals caused by
the intrusion of a diapir has the potential to dramatically alter the local hydrogeology, and this, in turn, can
lead to seal failure resulting from excess pressure heads
linked to lateral pressure transfer (Evans et al., 1991).
PIPE BYPASS
Pipes are the least well documented of the SBS groups,
and they have only recently been described in detail
with the benefit of 3-D seismic data (Løseth et al., 2001,
2003; Berndt et al., 2003). They can best be defined
seismically as columnar zones of disturbed reflections
that may or may not be associated with subvertically
stacked amplitude anomalies. Pipes are commonly ignored on seismic data because they tend to exhibit a vertical to subvertical geometry (Figure 6) and can therefore
be confused with seismic artifacts such as migration
anomalies, scattering artifacts, lateral velocity anomalies,
and attenuation artifacts related to shallow diffractors
(Løseth et al., 2001, 2003; Davies, 2003). Care is therefore needed in differentiating true pipes from seismic
artifacts, which is best done by considering the structural
and stratigraphic context of any candidate pipe. They
are commonly seen to emanate from crestal regions,
e.g., tilted fault block crests, fold crests, or crests of
sand bodies with positive topography, but many pipes
are also documented from flat-lying units or synclinal
regions, albeit with some focusing element at depth.
Pipes are commonly circular or subcircular in planform, and they are therefore easiest to identify in 3-D
seismic volumes using slice-based or horizon-based attributes (Figure 7). As with any attribute-based interpretation, the usual caveats apply regarding artifacts.
The detailed structure of pipes is poorly understood
at present and may be highly variable. In some cases,
pipes consist of zones of deformed reflections related to
minor folding and faulting. In others, they simply appear to consist of stacked pockmark craters or stacked
localized amplitude anomalies that are likely to be small
gas accumulations or zones of cementation, but with
no resolvable deformation (Figure 6). Although there
is commonly no visible systematic offset of reflections
that would support an interpretation of associated smallscale faulting (and fracturing), outcrop analogs suggest
that intense fracturing is indeed likely to occur within
the pipe (Løseth et al., 2001, 2003). This fracturing is
responsible for the enhanced permeability and loss of
seal integrity (Bryner, 1961). By analogy with many
published accounts of breccia pipes at outcrop and in
mines, the pipes seen on seismic data cutting across sealing sequences (e.g., Figure 6) are considered most likely
to consist of brecciated seal facies with zones of high
fracture intensity, variably intruded by small dikes of
material transported from the base and from along the
flow route.
We suggest a subdivision of the pipe bypass systems into four families based on their contextual setting
( Table 1). This should be regarded only as a preliminary classification because of the infancy of research into
these features. The use of a context-based scheme specifically for pipe bypass systems requires a significant
input from the interpretation of geologic setting before
a classification can be made, but this scheme does have
the advantage that the context offers strong clues as to
pipe genesis. Suggestions are made below on how to approach this interpretation. These four families are dissolution pipes, hydrothermal pipes, blowout pipes, and
seepage pipes. The latter two are closely related and
distinguished on the basis of presence or absence of surface features denoting a more dynamic flow regime
along the pipe.
Dissolution Pipes
Dissolution pipes form by dissolution of rock units at
depth to form subsurface cavities that promote instability in the overburden leading to collapse (Stanton,
1966; Cooper, 1986). They are thus likely to occur in
areas of evaporite or carbonate karst. The dimensions
Cartwright et al.
1153
Figure 6. Seismic expression of pipes crossing sealing sequences. (A) A large pipe from the Faeroe-Shetland Basin, offshore United
Kingdom, characterized by a vertical zone of disrupted reflections, with small offsets, and localized brightening of reflections within or
adjacent to the pipe. The profile is from a 3-D survey, but even with 3-D migration, the imaging cannot be trusted to give an accurate
portrayal of internal structure: the feature is too close in diameter to the horizontal resolution limit. The pipe is seen to emanate from
a small fold structure deeper in the profile, which is inferred to be the fluid source unit for the genesis of the pipe. This is a regional
aquifer of early Eocene age. Data courtesy of CGGVeritas. (B) Pipes seen on a seismic profile emanating from the crest of a low-relief
anticlinal structure and reaching the seabed, where they terminate in small pockmarks (Berndt et al., 2003). The source of the fluid is
unknown, but most likely from within the underlying polygonally faulted sealing sequence, which is a regional top seal for important
hydrocarbon accumulations in this basin. Data courtesy of Statoil A/S.
of the collapse pipe are directly related to the dimensions
of the solution cavity, but are also influenced by overburden strength and heterogeneity (Branney, 1995).
An example of a group of dissolution pipes is shown
in Figure 8, from the Levant Basin in the eastern Mediterranean where circular cavities were dissolved into
the Messinian evaporite sequence and the overburden
collapsed to form tall cylindrical zones of sagging, intense faulting, and fracturing. The high-quality evaporite seal is thus breached by the localized dissolution
1154
Seal Bypass Systems
structure, and fluids could therefore exploit this depleted region to migrate up through the overburden,
further assisted in their vertical migration by the fracture
pathways opened up in the dissolution pipe (Bertoni
and Cartwright, 2005). This type of migration window
through evaporite seals could represent a significant component of play modeling in areas such as the raft domain of offshore Angola, where the main source and
reservoirs are in the pre- and postevaporite sequences,
respectively (Duval et al., 1992).
Figure 6. Continued.
Dissolution pipes form at a rate controlled by the
rate of solution: this could be gradual or rapid (Stanton,
1966). The strain rate of the collapse will dictate the
extent of fracturing and, hence, the permeability of
the pipe (Branney, 1995). The highest permeabilities
are likely to be developed during the phase of active
deformation of the pipe; hence, the greatest potential
for fluid expulsion probably occurs during pipe formation. Once formed, however, fracture networks could
remain as potential fluid escape pathways for long periods thereafter, subject to the local history of fracture
closure and healing caused by cementation or confining
stress (Aydin, 2000).
Hydrothermal Pipes
Hydrothermal pipes form by the release of a high flux
of hydrothermal fluids associated with certain kinds of
igneous intrusions, particularly mafic sills or laccoliths
(Svensen et al., 2004), and can therefore be expected
to affect sealing sequences when they are breached by
igneous intrusions. The hydrothermal fluids are derived
from the magma by devolatilization and from the host
sediments by localized heating, metamorphism, or sim-
ply thermal pumping of pore fluids (Delaney, 1987;
Einsele, 1992). The volumes of fluids involved depend
primarily on magma composition, temperature, and intrusive volume (Delaney, 1987).
Subsurface examples of hydrothermal pipes were
first interpreted from regional 2-D seismic data (Skogseid and Eldholm, 1989), but their geometry and relationship to underlying mafic sills was only recognized
more recently using 3-D seismic data (Bell and Butcher,
2002; Davies et al., 2002). Subsequently, their ubiquitous development in volcanic continental margins has
become more apparent (Trude et al., 2003; Hansen,
2004; Svensen et al., 2004), and they can be predicted
to occur in any petroliferous basin where there is a phase
of mafic intrusion in the form of dikes and sills. This
encompasses such active petroleum systems as offshore Brazil, the entire northeastern Atlantic margin,
and offshore India, all regions of intensive deep-water
exploration.
Hydrothermal pipes are recognized on seismic data
from two main types of observation: (1) from their character as columnar or steep-sided, downward-tapering
conical zones of disturbed or even collapsed stratal
reflections and (2) from their direct connection with
Cartwright et al.
1155
Figure 7. Use of attribute maps to recognize pipes. Pipes are seen clearly on this display of the variance attribute on a stratal slice
taken within a sealing sequence that has been penetrated by a suite of pipes (P). These have a characteristic circular to oval planform
and can easily be distinguished from survey acquisition artifacts (A) and faults (F ).
igneous sills (Figure 9). They are almost exclusively observed to emanate from the inclined lateral margins of
sills or from ridge-like junctions within sills, i.e., local
structural crests (Hansen et al., 2004). They exhibit a
considerable range in dimensions, with diameters of
about 100 m (330 ft) to 3 km (1.8 mi), and heights of
hundreds of meters to 2.5 km (1.5 mi) (Figure 9).
Hydrothermal pipes associated with mafic sills are
common sites of mineralization and are referred to in
the mining literature as breccia pipe bodies (Barrington and Kerr, 1961; Bryner, 1961). These breccia pipe
bodies are widely considered to have formed catastrophically by high-velocity focused fluid flow with large fluxes
(Rove, 1947; Sohnge, 1963). The fracture networks are
1156
Seal Bypass Systems
opened within the columnar pipe body by hydraulic
fracturing and by stoping and collapse (Phillips, 1972).
The dynamics of fluid flow likely lead to pipe enlargement, wall rock abrasion, and collapse (Barrington and
Kerr, 1961), which are associated with the formation
of complex multiphase vein networks (Newhouse,
1942). From studies of ore petrogenesis, breccia pipes
are known to act as fluid-flow conduits for many millions of years after the initial intrusion-related phase
of hydrothermal activity (Barrington and Kerr, 1961).
Much complementary evidence of this is seen on seismic data for hydrothermal pipes acting as fluid conduits after a gap of many millions of years after their
initial formation (Svensen et al., 2004; Hansen et al.,
Figure 8. Seismic profile across a series of three dissolution pipes from the eastern Mediterranean. The regionally sealing Messinian
evaporite succession (labeled ME, and approximately 250 ms in vertical extent) is locally depleted beneath three vertical collapse
structures (labeled 1 –3) (Bertoni and Cartwright, 2005). The reflection continuity and amplitude are almost lost within these collapse
zones. These dissolution pipes represent a window for fluid migration across the otherwise high-quality regional seal of the
evaporites. Data courtesy of BG Group.
2005). This propensity for durability as flow conduits
means that these types of pipes have important implications for seal integrity and also for secondary hydrocarbon migration.
Blowout Pipes
This group of pipes is perhaps the most enigmatic and
most difficult to classify. Some affinities exist between
our classification of blowout pipes and the definition of
mud-related diatremes by Brown (1990, p. 8970), who
considered that ‘‘widespread diatreme formation is an
important indication of vigorous and regionally extensive fluid advection systems.’’ Blowout pipes can have
similar horizontal and vertical dimensions to the other
types of pipe, and likewise as with other pipe families,
they are typically seen on seismic data as a columnar
zone of disturbed reflections or vertically stacked localized amplitude anomalies (Figure 10). They can be dis-
tinguished, however, on the basis of their association
with surface or paleopockmarks (Løseth et al., 2001).
Localized deformation of reflections in the form of small
faults and folds is also commonly seen either on the
margins of the columnar disturbance or within the core
of the column (Figure 10).
Blowout pipes can be differentiated from dissolution and hydrothermal pipes by their context: they are
not specifically associated with underlying igneous intrusions or with karstified units. Instead, they tend to
be localized at natural leakoff points for overpressured
pore fluids, for example, at the crests of structures,
above gas reservoirs, or at the updip limits of aquifers.
This contextual link to obvious positions of focus of
overpressure is strong supporting evidence justifying
the use of ‘‘blowout’’ in the terminology because it
hints at a link to the genetic process of a high pressure
gradient driving the fluid flow in such a way as to produce the columnar conduit.
Cartwright et al.
1157
Figure 9. Seismic expression of a hydrothermal vent and underlying pipe fed from a shallow-level igneous sill, offshore Norway.
The sill is clearly recognizable from its high-amplitude reflections, and the pipe is a subvertical zone of disrupted reflection character
located directly above the upper tip of the sill. This is a typical position for the location of hydrothermal pipes. The seabed extrusion of
hydrothermal fluids and entrained sediment built a sea-floor mound. Amplitude anomalies are distributed at shallower stratigraphic
levels directly above the mound and pipe, suggesting later rejuvenation of the pipe as a fluid conduit. Data courtesy of Statoil A/S.
The formation of blowout pipes is poorly understood at present because only a few examples have been
described, and none have been calibrated by drilling.
They were first described and named by Løseth et al.
(2001), who linked their genesis to catastrophic breaching of top seals on shallow gas reservoirs. They can be
regarded as a natural blowout to surface or to some
shallower aquifer (internal blowout). The diagnostic
link between blowout pipes and pockmarks is the main
argument to support the concept that these pipes represent a discrete blowout event, instead of a longer term,
slower flux process or seepage (Løseth et al., 2001) (see
Seepage Pipes section).
1158
Seal Bypass Systems
The large flux most likely involved in the formation of some of the larger pockmarks requires a high
transient permeability along the pipe. We therefore
consider that they form by a highly dynamic process involving a combination of hydraulic fracture under elevated pore-fluid pressures and stoping or fluid-driven
erosion and collapse (Figure 11). The fact that they
have been observed to ascend across more than 1000 m
(3300 ft) of highly impermeable units suggests that
the driving processes involved in pipe development
are extremely energetic and may be analogous to the
formation of igneous diatremes, where gas expansion
is a major factor in fracturing the conduit ahead of the
Figure 10. Example of blowout pipe and typical seismic characteristics. This seismic profile is from offshore Namibia, showing
vertically stacked pockmarks as a testament to longevity of blowout pipe activity. The association with methane from deeper sources
(thermogenic) is suggested by the shallow anomalies and the interpreted presence of gas hydrate in the shallow interval. Data
courtesy of NAMCOR.
rising fluid (magma) column (Lorenz, 1985). We suggest that blowout pipes represent the first stage in
an evolutionary sequence that could ultimately lead
to the development of conduits for mud volcanoes
because the theoretical conditions are similar for both
cases (Karakin et al., 2001). This potential link to
mud volcano conduits is evident from the similarities of
scale and seismic expression of some pipes and mud
volcano conduits (e.g., compare Figures 5, 10). Possibly,
the different outcomes could be controlled by differences in the source layer of the fluids. If there is insufficient flux of gas-saturated mud to reach the surface
and build an edifice, then we might expect the surface
expression to be a pockmark crater instead of a mud
volcano.
Blowout pipes are commonly, but not universally, circular in planform, even when they are seen to
emanate from fault planes or from linear ridge crests
or pinch-outs. This circular planform is common to
all diatremes and has been explained by Novikov and
Slobodskoy (1978) as a means of minimizing energy
losses caused by wall friction in the contact zone between the upward-flowing and expanding fluids and
granular particles in suspension and the static country
rock.
The fluid flux associated with blowout pipes is
likely to be a maximum at the time of formation based
on their highly dynamic mode of formation. Many examples do, however, show evidence of episodic flow
behavior, notably in the vertical stacking of pockmark
craters (Figure 10), and significant permeability anisotropy may thus persist between the pipe and the
host strata for long periods of time. Some of this permeability differential must be caused by fracturing, but
Cartwright et al.
1159
Figure 11. Model for
pipe-type conduit development, based on existing
models for diatreme
formation and breccia pipe
formation (e.g., Novikov
and Slobodskoy, 1978).
Gas expansion and highvelocity gas jets are intrinsic to most models of
diatreme and breccia
pipe formation, so we
propose a model for
pipes where seal failure
by hydraulic fracturing
is followed by the ascent
of fluids with a gas
phase separating at some
point, expanding and
focusing the energy on
wall rock degradation in
a cylindrical or thin, tapering conical conduit.
the remainder could be the result of lithological heterogeneity by transport of higher permeability rock types
upward along the pipe during flow episodes.
Seepage Pipes
Seepage pipes encompass a range of structures that resemble blowout pipes in their seismic character and
dimensions, but which lack the blowout craters diagnostic of a violent outburst of fluid at the upper pipe
termination (Figure 12). Seepage pipes tend to form
in similar settings to blowout pipes, i.e., above gas reservoirs, on structural crests, and along updip margins
of aquifers. The main difference between seepage and
blowout pipes appears to be the physical properties of
the host rock. Blowout pipes occur almost exclusive1160
Seal Bypass Systems
ly in fine-grained sealing sequences, whereas seepage
pipes mainly seem to occur in sand or silt-dominated
sequences. The higher bulk permeability of the latter
host sequences promotes vertical seepage of fluids
through the pore network and, thus, prevents overpressure buildup by bleeding off fluids before they reach
the host-rock fracture gradient.
DISCUSSION
The brief review of bypass systems presented above
demonstrates that they are embedded into many sealing
sequences worldwide. Evidence on seismic data of focused fluid flow through low-permeability sedimentary
units is ubiquitous in many petroliferous sedimentary
Figure 12. Example
of seepage pipes (arrow)
from the Faeroe-Shetland
Basin, offshore Scotland.
Pipes are recognizable
as columnar zones of disrupted reflections with
localized amplitude anomalies. This pipe emanates
from a small fold on the
upper surface of a deeper
buried aquifer. The lack
of any pockmark located
at the upper termination of
the pipe is suggestive
of low fluid flux and possible seepage, but some
uncertainty is attached
to this interpretation because of limited seismic
resolution. Data courtesy
of CGGVeritas.
basins. The realization that many sealing sequences are
transected partly or entirely by features that facilitate cross-stratal fluid flow raises several fundamental
questions that should be addressed in play development.
For example, if a prospect has a sealing sequence where
such features can be identified on seismic data, when
did they form relative to the charge history? If they
were present prior to hydrocarbon charge, do they represent a significant risk to the seal?
The recognition of SBS is also important in a wider
context in basin analysis. For example, they are important
to factor in to any modeling of basinal fluid flow because
the distribution and continuity of low-permeability units
is a prime factor in flow routes and magnitudes (Bethke,
1985; Garven, 1995). At present, basin fluid-flow models do not implicitly incorporate SBS mainly because
their importance is underappreciated and their function
is poorly quantified. The modeling of overpressure development might be significantly impacted if bypass
systems are incorporated into a fluid-flow model because they can clearly function as long-term drains preventing pressure buildup, and the recognition of blowout pipes over many structural crests testifies to this
function (Figure 10). Many models would be compromised by the presence of even a small number of pipes
or sandstone intrusions were these to connect the overpressured cell with shallower hydrostatically pressured
compartments. An integrated analysis of pressure valves
within any basin where substantial overpressure is predicted would therefore greatly benefit from the incorporation of SBS.
Given that significant pathways for vertical fluid
migration over distances greater than 1 km (0.6 mi) are
widely developed in many sedimentary basins, future
research relating to the wider significance of these pathways and reducing risk and uncertainty in play analysis
should focus on quantifying the bulk permeability of
specific families of bypass structure and defining scaling
relationships of value in prediction. Only limited quantification of fluxes has been attempted in this article
simply because there is little public domain data to address this issue. Future work should involve physical
and numerical modeling, and it would be invaluable to
explore whether there are any scaling relationships between dimensions and permeability or mode of formation and permeability.
Cartwright et al.
1161
The key issue for risking any particular SBS is to
ascertain whether the cumulative flux over geologic
time scales through the bypass system is significantly
greater than that through the pore network of the neighboring sealing sequences. Estimating the maximum
fluxes for specific periods of time is also important,
particularly for those where hydrocarbon charge into
reservoirs is at its peak or where there is a risk of subsequent leakage. The geomechanical context is important to consider in risking bypass phenomena because
host lithology and in-situ stress both are major factors
in most of the bypass-forming processes discussed above,
particularly where hydraulic fracturing is involved
(Hubbert and Willis, 1957).
In our review of seismic examples, we have encountered many large structures that have been drilled
and found to be water bearing or have residual hydrocarbons where there are SBS developed throughout the
main sealing sequence down to the reservoir interval.
We consider that in many, if not all, of these cases, the
development and activity of the SBS is the main reason
for leakage instead of capillary leakage. A recent case
history of the Snorre field (Nordgard Bolas and Hermanrud, 2003) is a definitive example of this, where a
supratrap fault defined the leakage limit for the field
and, hence, dictated the position of the present-day
oil-water contact of this underfilled trap.
The emphasis in this article has been on the function of bypass systems as fluid conduits, and this might
present too negative a picture that the presence of a
bypass feature in a top seal, for example, means inevitable failure. Certainly, top seal failure is one of the
most commonly cited reasons for failed accumulations
or dry holes (Grunau, 1987). Also important is that
many hydrocarbon accumulations occur in areas where
SBS have formed after the hydrocarbons were emplaced
and where those SBS crosscut the reservoirs. Good examples are the ACG and Shah Deniz fields of the
Caspian Sea petroleum province (Fowler et al., 2000;
Davies and Stewart, 2005), where mud volcanic activity is very recent, and where the conduits for the giant
mud volcanoes cross the main reservoir units. That
major accumulations can survive the formation and
subsequent activity of the SBS involved implies that
whatever leakage has occurred is less than the incoming
flux of hydrocarbons, so that a dynamic fill-and-spill
equilibrium is maintained despite the SBS.
In conclusion, although many critical elements of
bypass systems need much further investigation, their
potential function in determining the overall integrity
of a seal is clear. Where they can be identified within a
1162
Seal Bypass Systems
sealing sequence, an attempt should be made to assess
their short- and long-term impact on fluid flow. Modern 3-D seismic data goes some way to enabling such
an assessment to be conducted, but much work remains
to be done on characterizing their permeability before
risking of seals containing bypass systems can be truly
quantitative.
CONCLUSIONS
1. Seal bypass systems are shown to encompass a range
of geologic features that promote cross-stratal fluid
migration and allow fluids to bypass the pore network of the dominant seal lithology.
2. Seal bypass systems are commonly developed in petroliferous basins around the world.
3. To aid in seismic interpretation of these systems, a
threefold, object-based classification scheme has been
introduced, in which SBS are divided into fault bypass, intrusive bypass, and pipe bypass groups.
4. The most important need when risking the impact
of SBS is to date their formation and to estimate their
contribution to fluid flow across a sealing sequence.
5. Bypass systems are responsible for many seal failures
and failed accumulations, but they are also present in
top seals of large petroleum accumulations.
6. The presence of a bypass system does not inevitably mean that the seal has completely failed: seal
failure is relative, and if the hydrocarbon flux into
the reservoir exceeds the capacity of the bypass system to bleed off fluids, significant accumulations will
occur.
REFERENCES CITED
Alexander, L. L., and J. W. Handschy, 1998, Fluid flow in a faulted
reservoir system: Fault trap analysis for the Block 330 field in
Eugene Island, South Addition, offshore Louisiana: AAPG Bulletin, v. 82, p. 387 – 411.
Alsop, G., N. G. Evans, and M. Safaricz, 2000, Observations on
deformation pattern and mechanics of salt diapir penetration
in the central North Sea: Marine and Petroleum Geology, v. 17,
p. 601 – 618.
Aplin, A. C., and S. R. Larter, 2005, Fluid flow, pore pressure and
leakage in mudstone cap rocks, in P. Boult and J. Kaldi, eds.,
Evaluating fault and cap rock seals: AAPG Hedberg Series 2,
p. 1 – 12.
Aydin, A., 2000, Fractures, faults and hydrocarbon entrapment,
migration and flow: Marine and Petroleum Geology, v. 17,
p. 797 – 814.
Barrington, J., and P. F. Kerr, 1961, Breccia pipe near Cameron,
Arizona: Geological Society of America Bulletin, v. 72, p. 1661 –
1674.
Bell, B. R., and H. Butcher, 2002, On the emplacement of sill
complexes: Evidence from the Faeroe-Shetland Basin, in D. W.
Jolley and B. R. Bell, eds., The North Atlantic igneous province:
Geological Society (London) Special Publication 197, p. 67 –
94.
Bennett, B., J. O. Buckman, B. F. J. Bowler, and S. R. Larter, 2004,
Wettability alteration in petroleum systems: The role of polar non-hydrocarbons: Petroleum Geoscience, v. 10, p. 271 –
277.
Berg, R. R., 1975, Capillary pressure in stratigraphic traps: AAPG
Bulletin, v. 59, p. 939 – 956.
Berg, R. R., and M. H. Habeck, 1982, Abnormal pressures in the
lower Vicksburg, McAllen Ranch field, south Texas: Gulf
Coast Association of Geological Societies Transactions, v. 32,
p. 247 – 253.
Berndt, C., S. Bunz, and J. Mienert, 2003, Fluid expulsion from
polygonally faulted clays, Norwegian margin, in P. van Rensbergen, R. Hillis, A. Maltman, and C. Morley, eds., Subsurface
sediment mobilisation: Geological Society (London) Special
Publication 216, p. 223 – 243.
Berryhill, H. J., 1986, Late Quaternary facies and structure, northern Gulf of Mexico: Interpretations from seismic data: AAPG
Studies in Geology 23, 289 p.
Bertoni, C., and J. A. Cartwright, 2005, Submarine dissolution of
buried evaporites in the Levant margin (eastern Mediterranean): Journal of the Geological Society (London), v. 158,
p. 765 – 786.
Bethke, C. M., 1985, A numerical model of compaction-driven
groundwater flow and heat transfer and its application to the
paleohydrology of intracratonic sedimentary basins: Journal of
Geophysical Research, v. 90, p. 6817 – 6828.
Boles, J. R., P. Eichhubl, G. Garven, and J. Chen, 2004, Evolution
of a hydrocarbon migration pathway along basin-bounding
faults: Evidence from fault cement: AAPG Bulletin, v. 88,
p. 947 – 970.
Branney, M. J., 1995, Downsag and extension at calderas: Bulletin
of Volcanology, v. 57, p. 303 – 318.
Brown, A., 2000, Evaluation of possible gas microseepage mechanisms: AAPG Bulletin, v. 84, p. 1775 – 1789.
Brown, A., 2003, Capillary effects on fault-fill sealing: AAPG Bulletin, v. 87, p. 381 – 395.
Brown, K. M., 1990, The nature and hydrogeological significance of
mud diapirs and diatremes for accretionary systems: Journal of
Geophysical Research, v. 95, p. 8969 – 8982.
Bryner, L., 1961, Breccia columns associated with epigenetic ore
deposits: Economic Geology, v. 56, p. 488 – 508.
Caillet, G., 1993, The cap rock of the Snorre field, Norway: A
possible leakage by hydraulic fracturing: Marine and Petroleum
Geology, v. 10, p. 42 – 50.
Cartwright, J. A., and D. Dewhurst, 1998, Layer-bound compaction faults in fine grained sediments: Geological Society of
America Bulletin, v. 110, p. 1242 – 1257.
Cartwright, J. A., D. M. D. James, and A. J. Bolton, 2003, The
genesis of polygonal fault systems: A review, in P. van Rensbergen, R. Hillis, A. Maltman, and C. Morley, eds., Subsurface
sediment mobilisation: Geological Society (London) Special
Publication 216, p. 223 – 242.
Cooper, A. H., 1986, Subsidence and foundering of strata caused by
dissolution of Permian gypsum, North Yorkshire, in G. M.
Harwood and D. B. Smith, eds., The English Zechstein and related topics: Geological Society (London) Special Publication 22,
p. 127 – 139.
Davies, R., B. Bell, J. A. Cartwright, and S. Shoulders, 2002, Threedimensional seismic imaging of dike-fed submarine volcanoes:
Geology, v. 30, p. 223 – 226.
Davies, R. J., 2003, Kilometer-scale fluidization structures formed
during early burial of a deepwater slope channel on the Niger
Delta: Geology, v. 31, p. 949 – 952.
Davies, R. J., and S. A. Stewart, 2005, Emplacement of giant mud
volcanoes in the South Caspian Basin: 3D seismic reflection
imaging of their root zones: Journal of the Geological Society
(London), v. 162, p. 1 – 4.
Davison, I., G. I. Alsop, N. G. Evans, and M. Safaricz, 2000, Overburden deformation patterns and penetration mechanisms of
salt diapirs in the Central Graben, North Sea: Marine and Petroleum Geology, v. 17, p. 601 – 618.
Delaney, P. T., 1987, Heat transfer during the emplacement and
cooling of dykes, in H. C. Halls and W. H. Fahrig, eds., Mafic dyke
swarms: Geological Association of Canada Special Paper 34,
p. 31 – 46.
Deville, E., A. Battani, R. Griboulard, S. Guerlais, and J. P. Herbin,
2003, The origin and processes of mud volcanism: New insights
from Trinidad, in P. van Rensbergen, R. Hillis, A. Maltman,
and C. Morley, eds., Subsurface sediment mobilisation: Geological Society (London) Special Publication 216, p. 475 – 490.
Dewhurst, D. N., A. C. Aplin, J. P. Sarda, and Y. L. Yang, 1998,
Compaction-driven evolution of poroperm in natural mudstones: An experimental study: Journal of Geophysical Research, v. 103, p. 651 – 661.
Dewhurst, D. N., Y. L. Yang, and A. C. Aplin, 1999, Permeability
and fluid flow in natural mudstones, in A. C. Aplin, A. J. Fleet,
and J. H. S. Macquaker, eds., Muds and mudstones: Physical
and fluid-flow properties: Geological Society (London) Special
Publication 158, p. 22 – 43.
Dimitrov, L. I., 2002, Mud volcanoes — The most important pathway for degassing deeply buried sediments: Earth Science
Reviews, v. 59, p. 49 – 76.
Downey, M. W., 1984, Evaluating seals for hydrocarbon accumulations: AAPG Bulletin, v. 68, p. 1752 – 1763.
Duval, B., C. Cramez, and M. P. A. Jackson, 1992, Raft tectonics in
the Kwanza Basin, Angola: Marine and Petroleum Geology,
v. 9, p. 389 – 404.
Eichhubl, P., and J. R. Boles, 2000, Rates of fluid flow in fault
systems — Evidence for episodic rapid fluid flow in the Miocene
Monterey Formation, coastal California: American Journal of
Science, v. 300, p. 571 – 600.
Einsele, G., 1992, Sedimentary basins, evolution, facies and sediment budget: Berlin, Springer-Verlag, 628 p.
Einsele, G., J. M. Gieskes, J. Curray, and D. M. Moore, 1980, Intrusion of basaltic sills into highly porous sediments, and resulting
hydrothermal activity: Nature, v. 283, p. 441 – 445.
Engelder, T., and A. Lacazette, 1990, Natural hydraulic fracturing,
in N. Barton and O. Stephansson, eds., Rock joints: Proceedings of the International Symposium on Rock Joints in Loen,
Norway, Rotterdam, A. A. Balkema, p. 35 – 43.
Esch, W. L., and J. S. Hanor, 1995, Fault and fracture control of
fluid flow and diagenesis around the Iberia salt dome, Iberia
Parish, Louisiana: Gulf Coast Association of Geological Societies Transactions, v. 45, p. 181 – 187.
Evans, D. G., J. A. Nunn, and J. S. Hanor, 1991, Mechanisms
driving groundwater flow near salt domes: Geophysical Research Letters, v. 18, p. 927 – 930.
Finkbeiner, T., M. Zoback, P. Flemings, and B. Stump, 2001, Stress,
pore pressure, and dynamically constrained hydrocarbon columns in the South Eugene Island 330 field, northern Gulf of
Mexico: AAPG Bulletin, v. 85, p. 1007 – 1035.
Fowler, S. R., J. Mildenhall, S. Zalova, G. Riley, G. Elsley, A.
Desplanques, and F. Guliyev, 2000, Mud volcanoes and structural development on Shah Deniz: Journal of Petroleum Science
and Engineering, v. 28, p. 189 – 206.
Frey-Martinez, J. M., J. Cartwright, B. Hall, and M. Huuse, 2007,
Clastic intrusion at the base of deep-water sands: A trap-forming
Cartwright et al.
1163
mechanism in the eastern Mediterranean, in A. Hurst and J. A.
Cartwright, eds., Sand injectites: Implications for hydrocarbon exploration and production: AAPG Memoir 87, p. 49 –
63.
Gaarenstroom, M., R. A. J. Tromp, M. J. de Jong, and A. M.
Brandenburg, 1993, Overpressures in the central North Sea:
Implications for trap integrity and drilling safety, in J. R. Parker,
ed., Petroleum geology of northwest Europe: Proceedings of the
4th Conference, London, Geological Society, p. 1305 – 1313.
Gartrell, A., M. Lils, and J. Underschultz, 2002, Controls on the
trap integrity of the Skua oil field, Timor Sea, in M. Keep and
S. J. Moss, eds., The sedimentary basins of Western Australia 3:
Proceedings of the Petroleum Exploration Society of Australia
Symposium, Perth, p. 389 – 407.
Garven, G., 1995, Continental-scale groundwater flow and geologic
processes: Annual Reviews in Earth and Planetary Sciences,
v. 24, p. 89 – 117.
Gay, A., M. Lopez, P. Cochonat, and G. Sermondadaz, 2004, Polygonal fault-furrows system related to early stages of compaction,
upper Miocene to present sediments of the lower Congo Basin:
Basin Research, v. 16, p. 101 – 116.
Gibson, R. G., 1994, Fault zone seals in siliciclastic stata of the
Columbus Basin, offshore Trinidad: AAPG Bulletin, v. 78,
p. 1372 – 1385.
Graue, K., 1999, Mud volcanoes in deep-water Nigeria: Marine and
Petroleum Geology, v. 17, p. 959 – 974.
Grauls, D., and C. Cassignol, 1992, Fluid pressure-induced open
fracture anomaly-characterization from well data and seismic
velocities — Mechanisms and major implications: Bulletin des
Centres de Recherches Exploration-Production Elf Aquitaine,
v. 16, p. 275 – 284.
Grunau, H. R., 1987, A worldwide look at the caprock problem:
Journal of Petroleum Geology, v. 10, p. 245 – 266.
Hansen, D. M., 2004, 3D seismic characterisation of igneous sill
complexes in sedimentary basins: Ph.D. thesis, Cardiff University, Cardiff, United Kingdom, 312 p.
Hansen, D. M., J. A. Cartwright, and D. Thomas, 2004, The development of intersecting igneous sills from 3D seismic, in R. J.
Davies, J. Cartwright, S. A. Stewart, J. R. Underhill, and M.
Lappin, eds., 3D seismic technology: Application to the exploration of sedimentary basins: Geological Society (London) Memoir 29, p. 301 – 316.
Hansen, J. P. V., J. A. Cartwright, M. Huuse, and O. R. Clausen,
2005, 3D seismic expression of fluid migration and mud remobilization on the Gjallar Ridge, offshore mid-Norway: Basin
Research, v. 17, p. 123 – 140.
Hedenquist, J. W., and R. Henley, 1985, Hydrothermal eruptions in
the Waitopu geothermal system, New Zealand: Economic
Geology, v. 80, p. 1640 – 1668.
Heggland, R., 1997, Detection of gas migration from a deep source
by the use of exploration 3D seismic data: Marine Geology,
v. 137, p. 41 – 47.
Heggland, R., 1998, Gas seepage as an indication of deeper prospective reservoirs: A study based on exploration 3D seismic
data: Marine and Petroleum Geology, v. 15, p. 1 – 9.
Hickman, S., R. Sibson, and R. Bruhn, 1995, Introduction to special
section: Mechanical involvement of fluids in faulting: Journal
of Geophysical Research, v. 100, p. 12,831 – 12,840.
Hooper, E. C. D., 1991, Fluid migration along growth faults in compacting sediments: Journal of Petroleum Geology, v. 14, p. 161 –
180.
Hovland, M., and A. G. Judd, 1988, Seabed seeps and pockmarks:
London, Graham and Trotman, 283 p.
Hubbert, M. K., and D. G. Willis, 1957, Mechanics of hydraulic
fracturing: Transactions of American Institute of Mining Engineers, v. 210, p. 153 – 168.
1164
Seal Bypass Systems
Hurst, A., J. A. Cartwright, M. Huuse, R. Jonk, A. Schwab, D.
Duranti, and B. Cronin, 2003, Significance of large scale sand
injectites as long-term fluid conduits: Evidence from seismic
data: Geofluids, v. 3, p. 263 – 274.
Huuse, M., and M. Mickelson, 2004, Eocene sandstone intrusions in
the Tampen Spur area imaged by 3D seismic data: Marine and
Petroleum Geology, v. 21, p. 141 – 155.
Huuse, M., D. Duranti, C. Guargena, A. Hurst, B. T. Cronin, and
J. A. Cartwright, 2003, Sandstone intrusions: Detection and
significance for exploration and production: First Break, v. 21,
p. 33 – 42.
Huuse, M., D. Duranti, N. Steinsland, C. G. Guargena, P. Prat, K.
Holm, J. Cartwright, and A. Hurst, 2004, Seismic characteristics of large-scale sandstone intrusions in the Paleogene of the
south Viking Graben, U.K. and Norwegian North Sea, in R. J.
Davies, J. Cartwright, S. A. Stewart, J. R. Underhill, and M.
Lappin, eds., 3D seismic technology: Application to the exploration of sedimentary basins: Geological Society (London)
Memoir 29, p. 263 – 277.
Huuse, M., J. Cartwright, R. Gras, and A. Hurst, 2005, Km-scale
sandstone intrusions in the Eocene of the outer Moray Firth
(U.K. North Sea): Migration paths, reservoirs, and potential
drilling hazards, in A. G. Doré and B. Vining, eds., Petroleum
geology of NW Europe: Proceedings of the 6th Conference,
Geological Society, London, p. 1577 – 1594.
Ingram, G. M., and J. L. Urai, 1999, Top seal leakage through faults
and fractures: The role of mudrock properties, in A. C. Aplin,
ed., Muds and mudstones: Physical and fluid-flow properties:
Geological Society (London) Special Publication 158, p. 125 –
135.
Jackson, M. P. A., 1995, Retrospective salt tectonics, in M. P. A.
Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics, a
global perspective: AAPG Memoir 65, p. 1 – 28.
James, D. M. D., 1997, Discussion on a model for the structure and
development of fault zones: Journal of the Geological Society
(London), v. 154, p. 366 – 368.
Jonk, R., J. Parnell, and A. Hurst, 2005, Aqueous and petroleum
flow associated with sand injectites: Basin Research, v. 17,
p. 241 – 257.
Karakin, A. V., S. A. Karakin, and G. N. Kambarova, 2001, Movement of a mud mixture through a mud volcano channel: Izvestiya,
Physics of the Solid Earth, v. 37, p. 817 – 824.
Knipe, R. J., 1997, Juxtaposition and seal diagrams to help analyze
fault seals in hydrocarbon reservoirs: AAPG Bulletin, v. 81,
p. 187 – 195.
Kopf, A., 2002, Significance of mud volcanism: Reviews of Geophysics, v. 40, p. 1005, doi:10.1029/2000RG000093.
Kopf, A., and J. H. Behrmann, 2000, Extrusion dynamics of mud
volcanoes on the Mediterranean Ridge accretionary complex,
in B. Vendeville, Y. Mart, and J. L. Vigneresse, eds., Salt, shale
and igneous diapirs in and around Europe: Geological Society
(London) Special Publication 174, p. 169 – 204.
Kugler, H. G., 1933, Nature and significance of sedimentary volcanism: Science of Petroleum, v. 1, p. 297 – 299.
Kumar, M. B.,1977, Geothermal and geopressure patterns of Bayou
Carlin-Lake Sand area, south Louisiana: AAPG Bulletin, v. 61,
p. 65 – 78.
Kwon, O., A. K. Kronenberg, A. F. Gangi, B. Johnson, and B. E.
Herbert, 2004, Permeability of illite-bearing shale: 1. Anisotropy
and effects of clay content and loading: Journal of Geophysical
Research, v. 109, Art. No. B10205, doi:10.1029/2004JB003052.
Leith, T. L., and A. E. Fallick, 1997, Organic geochemistry of caprock hydrocarbons, in R. C. Surdam, ed., Seals, traps and the
petroleum system: AAPG Memoir 67, p. 115 – 134.
Leith, T. L., I. Kaarstad, J. Connan, J. Pierron, and G. Caillet,
1993, Recognition of caprock leakage in the Snorre field,
Norwegian North Sea: Marine and Petroleum Geology, v. 10,
p. 29 – 41.
Ligtenberg, J. H., 2005, Detection of fluid migration pathways
in seismic data: Implications for fault seal analysis: Basin Research, v. 17, p. 141 – 154.
Lin, G., and J. A. Nunn, 1997, Evidence for recent migration of
geopressured fluids along faults in Eugene Island, Block 330,
offshore Louisiana, from estimates of pore water salinity: Gulf
Coast Association of Geological Societies Transactions, v. 47,
p. 419 – 424.
Lorenz, V., 1985, Maars and diatremes of phreatomagmatic origin:
A review: Transactions of the Geological Society of South
Africa, v. 88, p. 459 – 470.
Løseth, H., L. Wensaas, B. Arntsen, N. Hanken, C. Basire, and K.
Graue, 2001, 1000 m long gas blow-out pipes: 63rd European
Association of Geoscientists and Engineers Conference and
Exhibition, Amsterdam, Extended Abstracts, p. 524.
Løseth, H., L. Wensaas, B. Arntsen, and M. Hovland, 2003, Gas and
fluid injection triggering shallow mud mobilization in the Hordaland Group, North Sea, in P. van Rensbergen, R. Hillis, A.
Maltman, and C. Morley, eds., Subsurface sediment mobilisation: Geological Society (London) Special Publication 216,
p. 139 – 157.
Losh, S., L. Eglinton, M. Schoell, and J. Wood, 1999, Vertical and
lateral fluid flow related to a large growth fault, Eugene Island
Block 330 field, offshore Louisiana: AAPG Bulletin, v. 83,
p. 244 – 276.
Macgregor, D. S., 1996, Factors controlling the destruction or preservation of giant light oilfields: Petroleum Geoscience, v. 2,
p. 197 – 217.
Mallon, A. J., R. E. Swarbrick, and T. J. Katsube, 2005, Permeability of fine-grained rocks: New evidence from chalks: Geology, v. 33, p. 21 – 24.
Molyneux, S., J. A. Cartwright, and L. Lonergan, 2002, Conical
amplitude anomalies as evidence for large scale sediment intrusions: First Break, v. 20, p. 123 – 129.
Morley, C. K., 2003, Outcrop examples of mudstone intrusions
from the Jerudong anticline, Brunei, and inferences for hydrocarbon reservoirs, in P. van Rensbergen, R. Hillis, A. Maltman,
and C. Morley, eds., Subsurface sediment mobilisation: Geological Society (London) Special Publication 216, p. 381 –
394.
Muir Wood, R., 1994, Earthquakes, strain cycling and the mobilisation of fluids, in J. Parnell, ed., Geofluids: Geological Society
(London) Special Publication 78, p. 85 – 98.
Newhouse, W. H., 1942, Ore deposits as related to structural features: Princeton, University Press, 240 p.
Nordgard Bolas, H. M., and C. Hermanrud, 2003, Hydrocarbon
leakage processes and trap retention capacities offshore Norway: Petroleum Geoscience, v. 9, p. 321 – 332.
Novikov, L. A., and R. M. Slobodskoy, 1978, Mechanism of formation of diatremes: International Geology Review, v. 21, no. 10,
p. 1131 – 1139.
O’Brien, G. W., M. Lisk, I. R. Duddy, J. Hamilton, and P. Woods,
1999, Plate convergence, foreland development and fault reactivation: Primary controls on brine migration, thermal histories and trap breach in the Timor Sea, Australia: Marine and
Petroleum Geology, v. 16, p. 533 – 560.
Phillips, W. J., 1972, Hydraulic fracturing and mineralization: Journal of the Geological Society (London), v. 128, p. 337 – 359.
Planke, S., P. Symonds, E. Alvestad, and J. Skogseid, 2000, Seismic
volcanostratigraphy of large volume basaltic extrusive complexes
on rifted margins: Journal of Geophysical Research, v. 105,
p. 19,335 – 19,351.
Pollard, D. D., 1973, Derivation and evaluation of a mechanical
model for sheet intrusions: Tectonophysics, v. 19, p. 233 – 269.
Roberts, H. H., and R. Carney, 1997, Evidence of episodic fluid, gas
and sediment venting, northern Gulf of Mexico continental
slope: Economic Geology, v. 92, p. 863 – 879.
Roberts, H. H., R. Sassen, and A. Milkov, 2000, Seafloor expression
of fluid and gas expulsion from deep petroleum systems,
deepwater Gulf of Mexico, in P. Weimer, ed., Petroleum systems
of deepwater basins: Gulf Coast Section SEPM 21st Annual
Research Conference, p. 401 – 422.
Roberts, S. J., J. A. Nunn, L. M. Cathles, and F. D. Cipriani, 1996,
Expulsion of abnormally pressured fluids along faults: Journal
of Geophysical Research, v. 101, p. 28,231 – 28,252.
Rove, O. N., 1947, Fluid flow and the genesis of breccia pipes: Economic Geology, v. 42, p. 161 – 193.
Rubin, A. M., 1995, Propagation of magma filled cracks: Annual
Reviews of Earth and Planetary Sciences, v. 23, p. 287 – 336.
Schlömer, S., and B. M. Krooss, 1997, Experimental characterisation of the hydrocarbon sealing efficiency of caprocks: Marine
and Petroleum Geology, v. 14, p. 565 – 580.
Schowalter, T. T., 1979, Mechanics of secondary hydrocarbon migration and entrapment: AAPG Bulletin, v. 63, p. 723 – 760.
Schwartz, H., J. Sample, K. D. Weberling, D. Minisini, and J. C.
Moore, 2003, An ancient linked fluid migration system: Coldseep deposits and sandstone intrusions in the Panoche Hills,
California, U.S.A.: Geofluids, v. 23, p. 340 – 350.
Secor, D. T., 1965, Role of fluid pressure in jointing: American
Journal of Science, v. 263, p. 633 – 646.
Shoulders, S., 2005, Mechanics of sandstone intrusions: Ph.D. thesis, Cardiff University, Cardiff, United Kingdom, 287 p.
Shoulders, S., and J. A. Cartwright, 2004, Constraining the depth
and timing of large-scale conical sandstone intrusions: Geology,
v. 32, p. 661 – 665.
Sibson, R. H., 1981, Fluid flow accompanying faulting, field evidence and models, in D. W. Simpson and P. G. Richards, eds.,
Earthquake prediction an international review: American Geophysical Union Maurice Ewing Series, v. 4, p. 593 – 603.
Silverman, S. R., 1965, Migration and segregation of oil and gas, in
A. Young and J. E. Galley, eds., Fluids in subsurface environments: AAPG Memoir 4, p. 53 – 65.
Skogseid, J., and O. Eldholm, 1989, Voring Plateau continental margin: Seismic stratigraphy and vertical movements: Proceedings
of the Ocean Drilling Programme, Scientific Drilling Results,
v. 104, p. 993 – 1030.
Smith, D. A., 1966, Theoretical considerations of sealing and nonsealing faults: AAPG Bulletin, v. 50, p. 363 – 374.
Sohnge, P., 1963, Genetic problems of pipe deposits: Geological
Society of South Africa Transactions, v. 66, p. 19 – 22.
Stanton, R. J., 1966, The solution brecciation process: Geological
Society of America Bulletin, v. 77, p. 843 – 848.
Stuevold, L., R. Faerseth, L. Arnsen, J. A. Cartwright, and N. Moller,
2003, Polygonal faults in the Ormen Lange field, offshore
Norway, in P. van Rensbergen, R. Hillis, A. Maltman, and C.
Morley, eds., Subsurface sediment mobilisation: Geological
Society (London) Special Publication 216, p. 263 – 282.
Svensen, H., S. Planke, A. Malthe-Sorensen, B. Jamtveit, R. Myklebust,
and S. Rey, 2004, Release of methane from a volcanic basin as a
mechanism for initial Eocene global warming: Nature, v. 429,
p. 542 – 545.
Trude, J., J. A. Cartwright, R. J. Davies, and J. Smallwood, 2003,
New technique of dating igneous sills: Geology, v. 31, p. 813 –
816.
Van Rensbergen, P., and C. Morley, 2003, Re-evaluation of mobile
shale occurrences on seismic sections of the Champion and
Baram deltas, offshore Brunei, in P. van Rensbergen, R. Hillis,
A. Maltman, and C. Morley, eds., Subsurface sediment mobilisation: Geological Society (London) Special Publication 216,
p. 395 – 410.
Cartwright et al.
1165
Vavra, C. L., J. G. Kaldi, and R. M. Sneider, 1992, Geological applications of capillary pressure: A review: AAPG Bulletin, v. 76,
p. 840 – 850.
Verschuren, M., 1992, An integrated approach to clay tectonic deformation, and development of a new 3D surface modelling
method: Ph.D. thesis, University of Ghent, Ghent, Belgium,
359 p.
Watts, N. L., 1987, Theoretical aspects of cap-rock and fault seals
1166
Seal Bypass Systems
for single- and two-phase hydrocarbon columns: Marine and
Petroleum Geology, v. 4, p. 274 – 307.
Weber, K. J., 1997, A historical overview of the efforts to predict
and quantify trapping features in the exploration phase and in
field development planning, in P. Moller-Pedersen and A. G.
Koestler, eds., Hydrocarbon seals: Importance for exploration
and production: Norwegian Petroleum Society (NPF) Special
Publication 7, p. 1 – 13.