Seismic facies and hydrocarbon potential of carbonate reservoirs in

Transcription

Seismic facies and hydrocarbon potential of carbonate reservoirs in
Seismic facies and hydrocarbon potential of
carbonate reservoirs in ramp settings
Alexander Wunderlich
Institute for Geology, Gustav-Zeuner-Straße 12, 09599 Freiberg, Germany
Abstract. Carbonate ramps have a gently dipping surface less than 1° and show
homoclinal or distally steepend morphologies. Carbonate rocks make up only
20% of the sedimentary rock record yet account for more than 60% of the world’s
proven hydrocarbon reservoirs. Reservoirs in ramp settings make up only 10 to
15% of the whole carbonate reservoirs but show more subtle play types than many
rimmed shelves, with wide opportunities for stratigraphic and structural trapping
and lateral variations in reservoirs quality. 3-D seismic imaging represent a robust
methology for the interpretation of carbonate reservoirs and structures. Accurate
seismic imaging of the reservoir architecture has become an important predictive
tool for reservoir characterization because it helped to build 3-D geological
framework within which depositional facies can be distributed in time and space.
Calculation of volume-based attributes produced new volumes of data in addition
to reflectivity volumes to extract 3-D geometries within the reservoir.
Introduction
The original concept of the carbonate ramp (Ahr, 1973), as an alternative to the
steep-sloped, reef-rimmed shelf, was a carbonate slope with a low-gradient (<1°)
from shoreline to basin (Burchette and Wright, 1992).
Petroleum exploration in 1960s (e.g. Smackover Fromation, Bishop, 1969) led
the oil industry to the develop regional facies maps that showed extensive
grainstone belts along and parallel to the shoreline. Detailed analysis of seismic
lines from carbonate platforms from across the globe revealed major differences.
The most puzzling of these was the absence of a distinct shelf margin (shelf-slope
break) from seismic profiles. This shelf margin could be seen in the modern
Bahamas, Barrier Reef and other classic carbonate localities. Therefore facies
models of Wilson (1970) were calibrated to this specific morphology. The absence
of this shelf-slope break feature suggested that standard facies models could not be
applied to such platforms. Therefore another set of models as created that evolved
into the present-day ramp model(Fig. 2).
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Fig.1. A vintage-1973 sktech illustrating the physiography of a carbonate shelf (modified
after Burchette and Wright, 1992).
Fig.2. A vintage-1973 sketch illustrating the physiography of a carbonate ramp
(modified after Burchette and Wright, 1992).
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Classification of ramp settings
Read (1985) developed six types based on energy regimes and the distribution of
shallow-water facies. Burchette and Wright (1992) subdivided ramp settings
according to the degree of wave, tide and storm activity with reference to the
depostional processes (Fig.3).
The inner ramp is the zone above fair-weather wave base where wave and
current activity are almost continuous (Reading, 1996). This zone is dominated by
sand shoals or organic barriers and like siliciclastic shorelines they are marked by
shoreface, beach, lagoonal and tidal environments. The mid-ramp zone lies
between fair-weather wave base and storm wave base where the sea floor is
affected by storm waves but not by fair-weather waves. According to that,
sediments show evidence of frequent storm reworking (Burchette and Wright,
1992). Graded beds (e.g. tempestites) and hummocky cross-stratification
characterize these deposits (Reading, 1996). Aigner (1984), Burchette (1987) and
Faulkner (1988) recognized distinct proximal to distal trends within the
succession. The outer-ramp zone extends from below the normal wave base to the
basin floor. Storm-generated currents produce graded units or erode and rework
the sediment. In deeper zones restricted bottom conditions appear, producing
suboxic or anoxic environments.
Fig.3. The main environmental subdivisions on a homoclinal, carbonate ramp. The
pycnocline is not always originally present or identifiable in the rock record (from
Burchette & Wright, 1992).
Carbonate Seismic Facies Analysis
The appearance of carbonate rocks in general in seismic data contains a lot of
different information. The seismic data can contain evidence about their original
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depositional environment, lithofacies, diagenesis, and source rock and reservoir
potential (Macurda, 1997).
To get these very usefull informations, it is necessary to have methods, which
produces them. While surface mapping, paleontology, palynology, oxygen and
carbon isotope studies, petrography, X-ray diffraction, gravity, magnetics, and
core and well log analyses all play essential roles in oil and gas exploration, a vast
majority of the data is obtaoined by reflection seismology. In addition to being
relatively inexpensive, reflection seismology provides the only practical method of
obtaining a 3-D image of the earth’s susbsurface and of course of carbonate
environments (Palaz and Marfurt, 1997).
Recent advances in seismic data acquisition, processing, and visualization
techniques provide the opportunity to image carbonate reservoir architecture with
unprecented resolution. However, the analysis of seismic is a developing
metodologhy to quantify the volumes and rock properties of carbonate rocks. The
additional advantage of seismic data is that the imaged deposits can be displayed
at various stratigraphic levels and so it is possible to document the evolution of the
depositional environment through time. The paleogeomorphology can now be
accurately imaged for carbonate systems constructed by extinct reef builders that
have no modern analogs. This capability offers the unique opportunity to exploit
3-D images for questions regarding the growth pattern of different reef
communities, their paleoecology and reservoir heterogeneities in ancient systems,
for example ramp systems.
In the past decade, 3-D reflection seismology has replaced 2-D seismology
almost entirely in the seismic industry, and helped to understand the reflectors
within carbonate reservoirs. One very usefall advantages is the more accurate
imaging in 3-D seismology. With the help of depth-slices it is possible get
horizontal planes of the carbonate platform. And with inclusion of out-of-plane
diffractor it is possible to get more informations about the dip of the circumjacent
layers.
Variations of seismic reflections patterns of carbonate sediments result from the
combined effect of several parameters (Eberli et.al). These parameters can be
grouped into three categories that have a direct influence on the seismic image and
are the same for every carbonate depositional environment: a) physical properties
of the rock, in particular sonic velocity and bulk density, which together define
acoustic impedance; b) scale and geometrical relations of the seismic survey; c)
technical parameters and quality of the seismic survey.
Physical Properties
Porosity is the most important physical factor that influences velocity. Vp and Vc
increase with decreasing porosity, but large departures from this general trend are
possible. Choquette and Pray (1970) classified carbonate porosities into 15 basic
types which, combined with other elements, provide a detailed geologic
characterization. The most important and frequently observed types of porosity are
Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings
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interparticle, intercrystalline, moldic, vug, intraparticle, fracture and fenestral
(Wang, 1997).
Fig.4. Velocities (Vp and Vs) versus porosity in 173 gassaturated carbonate core samples.
Overall, dolomites show higher velocities than limestones at a given porosity
Geometrical relations
There are four basic reflections configurations found in seismic data. These are
parallel or subparallel, prograding, mounded or draped, and onlap fill (Macurda,
1997).
Parallel or subparalle reflections (Fig. 4) imply that during the deposition a
regional increase in accommodation potential was essential and that there is
nothing inherent in the reflection which gives us the information about how fast or
slow it was. Progradational reflections are really good to differentiate.
Progradation takes many forms as for example sigmoid, oblique (Fig. 5) and
shingled reflections. And all of these have a consequence for reservoir prediction
(Macurda, 1997). Mounded and draped reflections assume to be reefs (Fig. 6) or
buildups. But the question is first, if they are real geological feature or if they are
just geophysical artifacts like sideswipes, noise trains or overmigration. Seismic
patterns of onlap and onlap fill imply a termination of low-angle strata against a
steeper stratigraphic surface (Catuneanu, 2002). The onlap pattern is most
commonly found in two settings: at the base of a slope (Fig. 7) or on the shelf
according to transgression.
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Fig.4. Seismic profile from the northeastern Gulf of Mexico, perpendicular to the Early
Cretaceous shelf edge, showing parallel and subparallel reflections (modified after
Macurda, 1997).
Fig.5.Northwest-southeast seismic profile across the Campeche Bank north of the
Yucatan Peninsula, Mexico. The shelf margin is to the left. A series of inclined
oblique reflections (1) record northwestward progradation of this carbonate shelf
platform. Flat reflections (2) mark shelfal carbonate sediments (modified after
Macurda, 1997).
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Fig.6. Seismic profile depicting Cretaceous Sligo buildups (1,2) on the shelf margin of the
northeastern Gulf of Mexico (modified after Macurda, 1997).
Hydrocarbon Potential and Economic Aspect
The focus by explorationists lies on depositional models for petroleum exploration
and development in carbonate rimmed shelves or in large isolated buildups than in
ramp system (Burchette and Wright,1992). Carbonate ramps often form more
subtle play types than many rimmed shelves, with wide opportunities for
stratigraphic and structural trapping and lateral variations in reservoirs quality.
Little is known about factors that control the distribution of source and reservoir
rocks in ramp settings.
Low-energy ramps are sparse in potential reservoir facies unless downslope
buildups or incipient organic rims are developed (Read, 1985), or they lie in situations
where the timing of diagenesis in relation to petroleum migration have been
particularly favourable (Burchette and Wright, 1992). High-energy ramps in contrast
commonly possess a wide range of reefoid facies and carbonate sandbody types.
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Reservoirs in organic build-ups
Organic build-ups are a common location for petroleum reservoirs (Fig. 7). These
features have exhibited a wide range of organic and sedimentary facies through time
and this diversity is reflected in variations in their reservoir potential. Buildup shape
and location may reflect tectonic, topographic or hydrographic control (Burchette
and Wright, 1992). The thickness of ramp buildup reservoirs is seldom greater than
100-200m.
Fig.7. Miocene buildup, Luconia Province, Malaysia. This buildup is not situated on a
carbonate ramp, instead it is on platform. But the main geometries are the same (modified
after Masaferro et al., 2004)
Example 1: Morgan, W.A, 1985; Silurian Reservoirs in UpwardShoaling Cycles of the Hunton Group, Mt. Everette and Southwest
Reeding Fields, Kingfisher County, Oklahoma
This Oil-field is characterized by two main productive facies. One of them is a
skeletal buildup (Fig. 8) from the Clarita Formation (Hunton Group).
Porosity within the Clarita buildup facies ranges from 0 to 15% and is mainly
biomoldic and solution-enhanced. The trap is formed by a combination of updip
loss of porosity (associated with a facies change from porous, dolomitized crinoiddominated packstone and wackestone to non-porous arthropod packstone and
wackestones of the shallow shelf facies). The trap is also formed by an overlying
seal provided by the non-porous deep ramp facies of the overlying Henryhouse
Formation (Morgen, 1985).
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Fig.8. Depositional model for the Clarita Formation. During the Late Silurian, crinoid-rich
skeletal build-ups accumulated near a shelf edge. Subaerial exposure resulted in local
solution brecciation of some build-ups, and probably was influential in their dolomitization.
Vertical scale is exaggerated (modified after Morgan, 1985).
Reservoirs in grain-to wackestone dominated ramps
Grainstone reservoirs in ramp settings are widespread and occur in a number of
variations. The composition of the grainstone sediments may also vary (e.g. oolitic
or bioclastic), depending on the age and the location of the ramp (e.g. whether
windward or leeward). Shoreline carbonate sandbodies to major detached shoal
complexes or shoal complexes over offshore highs represent the major reservoir
facies . Nevertheless, a general characteristic of many ramp grainstone reservoirs
is that they are relatively thin or layered, seldom with more than a few tens of
metres of reservoir facies in any one zone, and commonly of wide lateral extent
(Burchette and Wright, 1992).
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Alexander Wunderlich
Example 2: Lindsay and Kendall, 1985, Depositional facies,
Diagensis, and Reservoir Character of Mississippian Cyclic
Carbonates in the Mission Canyon Formation, Little Knife Field,
Williston Basin, North Dakota
This oil-field is characterized by a dolomitized skeletal wackestone and pellet
wackestone-packstone with an average porosity of 14% and an average
permeability of 30md. Key beds were identified on the basis of openhole well
logs, which permitted subdivision of the section into six informal zones designated
as A, B, C, D, E and F (Lindsay and Kendall, 1985). Zone D is the main
productive portion (Fig. 9) due to the well developed porosity.
Fig.9. Idealized depositional setting of Mission Canyon Formation at Little Knife Field.
Zone D within the transitional open to restricted marine facies is the main productive
portion (modified after Lindsay and Kendall, 1985).
Conclusion
(1) Carbonate ramps have a gently sloping surface with a dip of less than 1°. They
can also be subdivided into inner-, mid-, and outer-ramp environments.
(2) Carbonate ramps have characteristic combinations of seismic facies that aid in
their recognition and in the evaluation of their hydrocarbon potential.
(3) Ramps and their associated sediments form prolific petroleum source and
reservoir systems and offer a range of subtle stratigraphic play types and lateral
facies variations (Burchette and Wright, 1992). The reservoirs can be found in
mid- or outer ramp isolated buildups. Grainstone and packstone reser-voirs are
common as well, but they show difficult reservoir heterogeneity.
(4) Seismic imaging of carbonate depositional architectur (in 2-D as well as in 3D) has seen marked improvement over the last 10 years and has allowed
interpreters to better delineate the complex histories of carbonate platform
sequences.
Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings
11
References
Ahr W.M. (1973) The carbonate ramp--an alternative to the shelf model. Trans., Gulf
Coast Assoc. Geol. Soc., 23: 221-225
Ahr W.M. (1989) Sedimentary and tectonic controls on the development of an early
Mississippian carbonate ramp, Sacramento Mountains area, New Mexico. In: Crevello,
Wilson, Sarg, Read Controis on Carbonate Platform and Basin Development. Soc.
Econ. Paleontol. Mineral., Spec. Publ., 44: 203-212
Aigner, T (1984) Dynamic stratigraphy of epicontinental carbonates, Upper Muschelkalk
(M. Triassic), South-German Basin. Neues Jahrb. Geol. Pal~iontol., Abh., 169: 127159
Burchette T.P. (1987) Carbonate-barrier shorelines during the basal Carboniferous
transgression: the Lower Limestone Shale Group, South Wales and western England.
In: Miller, Adams, Wright (Editors) European Dinantian Environments. Wiley,
London, pp. 239-263
Burchette T.P, Wright, (1992) Carbonate ramp depositional systems. Sediment. Geol., 79 .
3-57
Catuneanu O (2002) Sequence stratigraphy of clastic systems: concepts, merits, and pitfalls
Journal of African Earth Sciences, Volume 35, Issue 1, Pages 1-43
Choquette P.W., L.C. Prag (1970) Geologic nomenclature and classification of porosity in
sedimentary carbonates: AAPG Bull., v. 54, p. 207-250
Faulkner T.J (1989.) Carbonate Facies on a Lower Carboniferous Storm-Influenced Ramp
in SW Britain. Ph.D. thesis, University of Bristol (unpublished)
Lindsay, Kendall (1985 Depositional facies, Diagensis, and Reservoir Character of
Mississippian Cyclic Carbonates in the Mission Canyon Formation, Little Knife Field,
Williston Basin, North Dakota. In Roehl P, Choquette W.P (1985) Carbonate Petroluem Reservoirs, Springer Verlag
Macurda D (1997) Carbonate Seismic Facies Analysis. in Carbonate Seismology edited by
Palaz, Marfurt (1997) SEG Special Publication
Masaferro J.L, Bourne R, Jauffred J. C. (2004) Three-Dimensional Seismic Volume
Visualization of Carbonate Reservoirs and Structures. In Seismic Imaging of
Carbonate Reservoirs, AAPG Memoir 81
Morgan W (1985) Silurian Reservoirs in Upward-Shoaling Cycles of the Hunton Group,
Mt. Everette and Southwest Reeding Fields, Kingfisher County, Oklahoma. In Roehl
P, Choquette W.P (1985) Carbonate Petroluem Reservoirs, Springer Verlag
12
Alexander Wunderlich
Palaz I, Marfurt KJ (1997) Carbonate Seismology: An Overview. SEG Special Publication
Read J.F. (1982) Carbonate platforms of passive (extensional) continental margins: types,
characteristics and evolution. Tectonophysics, 81: 195-212
Read J.F (1985) Carbonate platform facies models. Bull. Am. Assoc. Pet. Geol., 69: 1-21
Reading HG (1996) Sedimentary Environments and Facies. Blackwell Scientific Oxford
Wang Z (1997) Seismic Properties of Carbonate Rocks. in Carbonate Seismology edited by
Palaz, Marfurt (1997) SEG Special Publication
Wilson Jl (1970) Depositional facies across carbonate shelf margins. Transactions, Gulf
Coast Association of Geological Societies, 20, 229-223
Seismic facies and hydrocarbon potential of carbonate reservoirs in ramp settings
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