Click to view images shown during oral presentation
in China, May, 2002.
Development Case Study of a Karsted Carbonate “Island” Hydrocarbon Reservoir: West Carney Hunton Field, Oklahoma*
By
James R. Derby1, F. Joe Podpechan2, Jason Andrews3, and Sandeep Ramakrishna4
Search and Discovery Article #20008 (20002)
1Consulting geologist, Tulsa, Oklahoma ([email protected])
2Independent geologist, Tulsa, Oklahoma ([email protected])
3Independent geologist, Tulsa, Oklahoma ([email protected])
4University of Tulsa ([email protected])
West Carney Hunton
Field (WCHF) produces oil and gas from the Silurian Hunton Formation or Group, a
carbonate unit lying between the Ordovician Sylvan Shale below, and the Upper
Devonian Woodford Shale above. Field production history is complicated by early
production of water with hydrocarbons and an unusual distribution of dolomite
and limestone across the field area. As large quantities of water are produced,
reservoir pressure drops, produced water volume decreases, and volume of
hydrocarbons produced increases. Distribution of porosity types
, lithologies,
and production is best understood by a model for two distinctly different
reservoirs in the same stratigraphic horizon and of slightly different geologic
ages.
The geologically
older reservoir occupies the center of the field. It is composed of vuggy,
open-marine, brachiopod biostromes and crinoid/coral packstones and wackestones
developed in the Cochrane Formation. Following deposition, the Cochrane was
exposed and eroded, leaving an irregular topographic highland or island in the
subsequent Silurian sea. The younger reservoir, the Clarita Formation, is
developed lateral to, as well
as over, this “island”. The Clarita is a dolomite
or partly dolomitized limestone that formed in a shallow-water to sabkha
setting. The two reservoirs have different production characteristics and
pressure regimes. The principal producing zones are dependent initially on the
depositional facies of the “island” and its flanking sediments, followed by
subaerial exposure, weathering, and karst development. Fracturing and faulting
contemporaneous with exposure, karst dissolution, cave development, and collapse
have combined with later tectonic faulting to create compartments of hydrocarbon
accumulation that cut across the depositional and diagenetic reservoir
types
.
Further, fluid saturation and production characteristics across the field have
been altered by the structural history that includes at least two directions of
regional tilting subsequent to deposition. At the time of hydrocarbon generation
and migration the dip was to the east, and WCHF was on or near the crest of a
broad arch. The Hunton Group in this paleo-trap likely was entirely filled with
hydrocarbons by mid–Mesozoic time and constituted a giant gas and oil field.
Tilting to the southwest and associated faulting (along reactivated faults in
the Nemaha system), probably in Jurassic time, would have allowed hydrocarbons
to escape and water to enter the reservoir. Finally, another complexity is the
presence of the Woodford Shale source rock immediately overlying the Hunton. It
is clear that pore geometries at the interface between the source and reservoir
rocks are critical to hydrocarbon emplacement.
This study is valuable because excellent core, wireline-log, and paleontologic control document the compartmentalization previously misunderstood, using simple mapping of gross lithologic units and productivity tests of exploration wells. Simulation models for field performance are being modified after carefully studying the cross-calibrated information.
Lessons to be learned from this study are that WCHF are:
1. A Subtle Trap – there is no structural closure, and no obvious facies change to create a stratigraphic trap.
2. Thermally Submature – the Woodford source rock in the area of the field is not considerably beyond the threshold for the “oil window” of thermal maturity.
3. A By-Passed Field – first developed in Hunton in 1996, superimposed on a much older Ordovician “2nd Wilcox” (Simpson) field.
4. Easily Overlooked – initial production is typically 100% water!
5. Beneath, not above, the Principal Source Rock.
6. Example of Differential Entrapment – Hydrocarbons are trapped in low-permeabilty oil-wet matrix pore system; high permeability system is water-filled.
7. Complex and heterogeneous in both sedimentary facies and porosity systems.
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Click here for sequence of phases of depositional history.
IntroductionThe West Carney Hunton Field (WCHF) is located in Logan and Lincoln counties in north-central Oklahoma in T14N-T16N, R1E-R3E (Figure 1). The field is in an area generally described as the Central Oklahoma Platform or Eastern Oklahoma Shelf; a structural element bounded by the Nemaha Range immediately to the west, the Ozark Uplift to the east and northeast, the Hunton-Pauls Valley Uplift to the south, and the Arbuckle Uplift and Arkoma Basin to the south and east (Figure 2). This location was on the northeast flank of the Oklahoma Basin (Figure 3) during deposition of the Hunton but was separated from the deeper part of the basin by the Nemaha Uplift during Pennsylvanian time.
The field produces oil and gas from the Hunton
Group, which is a major target for petroleum exploration in the southern
Midcontinent. Stratigraphically, the Hunton Group lies between the
subjacent Sylvan Shale and the superjacent Woodford Shale (Figure
4).
The West Carney Hunton Field is located about 6 miles southwest of the
truncated edge of the Hunton beneath the Woodford Formation (Figure
2).
The Hunton crops out only in eastern Oklahoma, on the flanks of the
Ozark Uplift, and in southern Oklahoma, in the Arbuckle Mountain
complex. Because of its limited exposure and its distance from the study
area, understanding the Hunton Group in north-central Oklahoma must be
obtained from study of core data and wireline logs, Development of West Carney Hunton Field began in 1996, when Altex Resources placed the Decker # 1 (NE Sec.1, T15N, R2E) on a large beam pump and soon realized that large quantities of oil and gas could be produced by moving larger amounts of water. Four companies operate a majority of the wells in the field: Altex Resources, New Dominion, Craig Elder, and Marjo Operating Company. The field, which covers nearly 30,000 acres, currently has more than 230 producing wells and 16 saltwater disposal wells. The field produces an average of 6000 barrels of oil, 55,000 MCF gas, and 86,000 barrels of water daily. In 1999, the U.S. Department of Energy, National Petroleum Technology Office awarded Contract DE-FC26-00BC15125 to the University of Tulsa Department of Petroleum Engineering, Dr. Mohan Kelkar, Project Director, for a study of the “Exploitation and Optimization of Reservoir Performance in Hunton Formation, Oklahoma”. Participating in the study are Marjo Operating Company, Inc. and Joe Podpechan, as the operating company, Dr. Kishore Mohanty, University of Houston, who performs special core analyses, graduate students in the Department of Petroleum Engineering, University of Tulsa, and Dr. James R. Derby, Consultant. Dan Ferguson, U.S. DOE National Petroleum Technology Office, Tulsa, Oklahoma, is the Project Manager. This report focuses on a geological description of the reservoir based on core descriptions by Derby and Ramakrishna, and regional correlation and mapping by Podpechan. Podpechan and Derby have cut most of samples for paleoanalysis and thin-sections. Derby and Andrews wrote this report, and Andrews prepared most of the illustrations.
This study is unusual in the abundance of data
available. Over 500 wells have penetrated the Hunton in the field; many
were drilled to the underlying Ordovician “Wilcox” (Simpson Group) in an
earlier, deeper play. Twenty-seven cores are currently available for
study. Marjo cores every Although production techniques and engineering are part of this overall study, those topics will not be discussed in detail here. David Chernicky and Scott Schad of New Dominion described production techniques in an Oklahoma Geological Survey workshop in May, 2002. Vineet Marwah, University of Tulsa Department of Petroleum Engineering, recently presented an interpretation of the primary production mechanism of part of the field (in his M.S. thesis). Engineers in this research project presented two papers on the Hunton reservoir and production at the SPE/DOE Symposium on Improved Oil Recovery, April 13-17, 2002, in Tulsa.
This study and the high level of interest
concerning it were prompted, at least in part, by the unique production
characteristics of the field. When initially completed, wells in the
field produce large amounts of water with a relatively low oil-and-gas
cut. As the water within the reservoir is pumped, the gas volume slowly
begins to increase, followed by an increase in oil cut (Figure
5).
Within a few days to a few months, depending on several factors, the
typical
The purpose of this work is to develop a
geological understanding of the Hunton Group in the West Carney Hunton
Field, especially in relation to optimizing reservoir performance and
exploitation of the Hunton reservoir. When faced with large initial
water rates associated with producing from this type of reservoir, most
oil and gas companies would cease operation before significant oil and
gas production is realized, resulting in a field of this nature being
bypassed. Alternatively, it is important to identify and define the
unique characteristics of this type of reservoir in order to avoid the
economic disaster caused by operators completing and pumping every new
StratigraphyThe stratigraphic section in the West Carney area is shown in Figure 4. Units both above and below the Hunton Group are expressed in terms of general lithology, age, and thickness. Permian strata crop out within the study area. Depth to the Hunton Group in the field averages about 5000 ft. Numerous formations in the Pennsylvanian produce oil in the area. The Ordovician Bromide sand (“Second Wilcox”) also is a major petroleum producer in the area and the target of most wells that fully penetrate the Hunton strata. Although the Arbuckle dolomite does not produce oil and gas in the area, it is an excellent zone for the disposal of salt water. The interval of specific interest in the West Carney Hunton Field is as follows: Woodford Formation--a black shale and rich source rock. The Woodford is reported to be the source of 70% of the oil produced in Oklahoma (Comer and Hinch, 1987). Hunton Group--a shallow-shelf carbonate of latest Ordovician through Middle Devonian age. Detailed subdivisions of the Hunton Group are shown in Figure 6. Only the Lower Silurian portion has been recognized in WCHF. Sylvan formation--a gray-green marine shale, commonly containing graptolites, suggesting that it was deposited in relatively deep water, below storm wave base. The litho- and time-stratigraphic diagram of the Hunton Group in Oklahoma prepared by Stanley (2001) is shown in Figure 6. This diagram shows the biostratigraphic correlation of global conodont biozones and North American brachiopod biozones with the fauna in the rock units of the Hunton Group in Oklahoma. The Hunton Group ranges in age from latest Ordovician (Hirnantian substage, Ashgillian stage ), about 440 Ma, to Middle Devonian, 380 Ma; a time span of approximately 60 million years. The Hunton Group in Oklahoma is generally in conformable contact with the subjacent Sylvan shale, and in unconformable contact with the Woodford Shale above. As determined from conodont studies, the entire West Carney field is composed of the Cochrane and Clarita formations of the Chimney Hill Subgroup; representing an estimated 10 million years, or only 16% of total “Hunton time”. There is no present evidence for the presence of the basal Keel Formation or for the Henryhouse, Bois d’Arc, or Frisco formations above the Chimney Hill. Therefore, within the West Carney field, the Hunton Group is in unconformable relationship with both the underlying Sylvan Shale and the overlying Woodford Shale. The hiatus above the Clarita and/or Cochrane Formations in the field, accounts for some 50 million years of time during which sediment was either never deposited, or if deposited, subsequently eroded. Structural SettingWCHF lies along the northern flanks of the Paleozoic Oklahoma Basin (Figure 3). The strata deposited in most parts of the Oklahoma Basin are widespread and laterally persistent, indicating the relative tectonic and orogenic stability of the region during Early Paleozoic. The Hunton Group was deposited in a broad, shallow epicontinental sea, with depositional slope toward the southwestward into the more rapidly subsiding Southern Oklahoma Aulacogen. This southwest dip was accentuated in Late Devonian (pre-Woodford) time with uplift of the broad Chautauqua Arch (Figure 3). Before deposition of Pennsylvanian beds, there was truncation of the Mississippian, Woodford, and Hunton in the WCHF area; based on subcrop patterns (Jordan, 1962), the general area was gently tilted east-southeastward, in response to the cumulative effects of the Nemaha Uplift to the west and the Arkoma Basin to the east and southeast. The eastward tilting apparently continued throughout the remainder of Paleozoic, as evidenced by the east-southeast thickening of about 10 ft. per mile of the Pennsylvanian sequence (cf. Levorsen, 1967, p. 543). The area was subsequently tilted southwestward during the Mesozoic (probably Jurassic), resulting in a modern structural dip of about 45 ft. per mile southwestward (Figure 7). This structural scenario is complicated by tectonic movements and selective erosion affecting the Carney area both prior to and following Hunton deposition. A regional thin in the Viola in WCHF area suggests that the area may have been affected by a slight paleotopographic high prior to and during Hunton time. Possibly the absence of Keel in the field is due to nondeposition, or deposition and subsequent erosion over this “high.” The distribution and thickness of the Clarita, relative to the Lower and Upper Cochrane (Figures 7 and 8), suggest some Early Silurian paleostructural influence. Post-Hunton structural movements in the area are evidenced by the presence of faults that did not affect Hunton thickness but did affect the thickness of the Mississippian (Figure 9). However, post-Hunton - pre-Woodford activity in the general area may have occurred, as evidenced by erosion of the Hunton along the Seminole Uplift and local areas of “zero Hunton” both southeast and northeast of WCHF as shown by Amsden (1975, pl. 9) (Figure 10). Depositional HistoryThe rock units of the Hunton Group in WCHF, and throughout all of Oklahoma, suggest a depositional history of episodic cycles of deposition and erosion (Figures 6, 11), related to worldwide sea-level, oceanic, and climatic events (Jeppsson, 1998; Barrick, 2001). The Chimney Hill Subgroup, in ascending order, is composed of the Keel, Cochrane, and Clarita formations (Figure 6). The generally oolitic limestone comprising the Keel, latest Ordovician (Ashgillian) to early Silurian (Llandoverian) in age, appears to be absent in WCHF. Therefore, the relationship between the top of the Sylvan Shale, and the base of the Cochrane Formation in the Carney area is unconformable. In WCHF the Cochrane Formation consists of a variety of fossiliferous, open-marine limestone facies. Conodont data, in combination with the relative stratigraphic position of the rock units, indicate that a widespread Lower Cochrane unit is unconformably overlain locally by an Upper Cochrane unit. Within WCHF, the Cochrane Formation is composed of a central fossiliferous limestone macrofacies, flanked by a nonporous mudstone facies (Figures 7, 11), the age of which has not yet been confirmed by paleontology. Deposition of the upper Cochrane Formation was followed by a fall in relative sea level, during which the Cochrane was eroded differentially. Seemingly, the fossiliferous limestone macrofacies in the center part of the field (Figure 7), was more resistive to erosion than the nonporous mudstone facies on the flanks of the field, with the result being a topographic high composed of the fossiliferous Cochrane limestone (Figure 11B). When relative sea level began to rise again, the Clarita Formation was deposited across the area (Figure 11C), and relatively thick sequences of Clarita, generally a shoal-water dolomite or dolomitized limestone, are present on the east and west sides of the field, where the Clarita was deposited in the post-Cochrane paleotopographically lower areas. The Clarita Formation is the youngest unit of the Hunton Group present in the West Carney Hunton Field. The hiatus between the Clarita and the Woodford represents approximately 50 million years. Although the depositional history represented by this hiatus is purely speculative, regional studies of the Hunton suggest that at least the Henryhouse Formation, if not the Haragan-Bois d’Arc and the apparently widespread but rarely preserved Frisco Formation, was deposited across the field (Figure 10). As explained by numerous authors, each of these formations is unconformity-bounded, and some contain numerous sequences that are also unconformity bounded. The final episode (of the multiple episodes) of deposition and erosion that followed deposition of the Clarita Formation was a sea-level lowstand and a long period of erosion and subaerial exposure during the 10 million years between the deposition of the youngest Hunton and onset of Misener/Woodford deposition. Extensive karst development, including multiple generations of cross-cutting karst dissolution and sedimentation, is evident in nearly every core of the field, from the top to the base of the Hunton (in numerous cases), suggesting complete emergence during sea level lowstands. After the final episode of erosion and subaerial exposure during Hunton time, relative sea level rose again, resulting in deposition of the Woodford Shale across the region. In the West Carney Hunton Field, the Woodford was deposited uniformly across most of the field, but it is exceptionally thick where the subjacent Cochrane Formation has been incised (where Clarita is absent) (Figure 11E). The Woodford Shale is considered to be the primary source rock for oil and gas accumulations within the Hunton reservoir, with perhaps minor amounts of hydrocarbons derived from the Sylvan Shale. In WCHF, the Woodford Shale may have achieved the threshold depth of burial for oil generation by the end of Permian time. In fact, the depth of burial was likely greater, as an unknown amount of Permian and Mesozoic strata have been eroded from the region. The Woodford in the area attained an early-oil-generation stage, based on vitrinite reflectance (Figure 12 [Comer, 1992, Fig. 13]). Higher thermal maturity values are present eastward, reflecting the eastward thickening of post-Woodford strata. By mid-Mesozoic time oil and gas generated east of WCHF would have migrated updip to the west, possibly filling reservoirs in WCHF area. The subsequent southwest tilting would have altered the reservoir configuration, possibly partially breaching the seal, allowing water to invade a previously oil- and gas-filled reservoir, leading to the complex conditions observed today. ProductionThe Hunton Group in WCHF produces oil and gas from the Cochrane and Clarita formations of the Chimney Hill Subgroup. The reservoir ranges from 24 to 146 ft. thick throughout the area. A gentle homoclinal dip of approximately 40-45 ft. per mile to the southwest; little to no structural closure (Figure 7) suggest a stratigraphic mechanism of entrapment. The producing part of the field is currently thought to be approximately 30,000 acres. The field now has around 230 producing wells and 16 saltwater disposal wells. All saltwater disposal wells are open-hole completed in the Arbuckle dolomite, which is nearly 2000 ft. thick. Approximately 50 (22%) of Hunton wells are located in those parts of the field composed primarily of dolomite facies, whereas approximately 180 (78%) wells produce from the Hunton limestone macrofacies (Figure 7). Eight horizontal wells have been drilled and completed in the West Carney Hunton Field. Relative economic success of horizontal wells compared with “straight-hole” (vertical) wells has yet to be determined; however, early indications are encouraging.
The field currently produces 6000 barrels of
oil per day, 55,000 MCF gas per day, and 86,000 barrels of water per
day. An average
WCHF unique production characteristics include
the heterogeneous nature of the field, prohibiting the use of the term
“typical” in reference to any single The heterogeneity of the field may contribute to this behavior. The Hunton reservoir rock within WCHF is thought to have a dual permeability system: a higher permeability component consisting of “touching vugs” (Lucia, 1995) and solution-enhanced fractures, and a lower permeability component consisting of microporosity and intercrystalline porosity. At this time, the fluids are thought to move readily through the higher permeability component; however, more hydrocarbons are stored in the lower permeability component. As a result, wells when initially completed produce large quantities of water with a relatively low oil-and-gas cut, indicating that the higher permeability component of the dual porosity system is being “flushed.” Eventually enough of the fluid contained in the higher permeability component is removed to create a pressure differential between the low and high components of the dual permeability system. As a pressure differential develops, fluid contained within the microporosity of the low permeability system mobilizes and moves from an area of high pressure to an area of low pressure; thus, it “bleeds” into the high permeability component of the system. When the fluid reaches the higher permeability component, it becomes recoverable. Gas, because of its lower viscosity, is more readily moved than oil, as reflected by the increase in the production of gas prior to the production of oil.
As the gas-and-oil cut slowly increases, the
Early in the project, the operating company,
Marjo Operating Company, Inc., determined that coring the Hunton in
every
These data and the
During a preliminary study of each core, samples are cut for
petrographic thin-sections, and for paleontologic (conodont) analysis.
Prior to detailed description of a core, it is desirable to have at
least a preliminary description of each sample in thin-section in order
to identify grain Facies
In the fourteen wells described to date, 11
distinct lithofacies have been identified in the Hunton , as Numeric codes for 14 identified lithofacies
Stratigraphic Correlation and Facies Interpretation Initial attempts at stratigraphic correlation of WCHF lithofacies yielded unsatisfactory results because of what appears to be very abrupt lateral changes in facies. The addition of paleontological data provided recognition that WCHF stratigraphy consists of three separate sequences, with major topographic relief at the unconformities. An east-west cross-section (Figure 13) shows over 100 ft. of deeply eroded Lower Cochrane overlain by an Upper Cochrane sequence, which in turn is deeply eroded and overlain by a Clarita sequence. Seven conodont zones have been recognized in Upper Ordovican and Silurian strata in WCHF. Conodont identifications and interpretations, provided by Dr. James Barrick, Texas Tech University, are in the Appendix of the DOE BP1Report (Kelkar, 2002). Lithofacies relations between wells, as shown in the 6-well cross-section (Figure 13), are interpreted, using the classification of Johnson et al. (1997) (Figure 14), in terms of Silurian benthic assemblages (BA) on an open-marine shelf. The big pentamerid brachiopod assemblage (facies 7) is diagnostic for benthic assemblage 3, with suggested water depths of 30 to 60 meters (in the middle part of the shallow shelf). This facies is locally more than 60 ft. thick and locally is an excellent reservoir rock, with large vugs between the equally large brachiopod shells. However, these vugs may be occluded with karst infill and diagenetic cements, resulting in a poor reservoir. The coral-, stromatoporoid-, and crinoid-dominated facies (4,5,6,8,9), also common in the Cochrane, suggest a BA 2, or upper shallow shelf position, in water depths of 10 to 30 meters. Therefore, almost all of the Cochrane units described can be interpreted as middle to upper-middle shallow-shelf depositional environments. This lithofacies and macrofossil environmental interpretation are supported by the conodont data (Kelkar, 2002). One of the interesting results from this study is that in contrast to the “layer-cake” stratigraphy characteristic of peritidal settings, the lateral relationships of WCHF facies, shown in Figure 13, demonstrate extensive heterogeneity. Large brachiopod biostromes (shell mound accumulations) grade laterally, in some cases in less than one mile, to crinoidal grainstones or to coral or stromatoporoid-dominated wackestones.
In contrast to the Cochrane described above,
the Clarita is dominated by dolomitized fine grainstones and some fine
mudstones with sedimentary features suggestive of shoal to
intermittently emergent conditions (BA 1). The geographically abrupt
lateral transition from BA 3 (in the Cochrane) to BA 1 (in the Clarita)
is extremely unlikely, and this study shows that they represent two
separate depositional events. Recognition of these two distinctly
different sequences allows the logs to be correlated and interpreted, as
illustrated in Figures 15 and 16. Confirmation of this interpretation is
based on conodont data from the Griffen Paleoenvironmental interpretation of the Clarita remains problematic. The mix of lithofacies, including fine to medium crinoidal and small-brachiopod wackestones, packstones, and grainstones, all suggest a very shallow-water, moderately high-energy environment. A few beds of mudstone with sub-horizontal mottles suggest sabkha conditions. The overprint of early karst and early dolomitization also suggests a shoal to emergent environment; namely, BA 1. In contrast, the contained conodont fauna is a deep-water open-marine fauna consistent with the fact that the Wenlockian transgression is one of strongest and most extensive Silurian sea level rises (Jeppsson, 1998). The apparent conflict between observed lithofacies and apparent biofacies probably reflects the allogenic nature of the packstones and grainstones in addition to sea-level change(s) during and after deposition of the Clarita. Thicker parts of the Lower and Upper Cochrane (Figures 13, 17) probably reflect an emergent island surrounded by shallow sea. Porosity
Porosity
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