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Basin-centered gas systems: examples (view source)
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{{publication
| image = Nov2002.jpg
| width = 120px
| series = ''AAPG Bulletin,'' November 2002
| title = Basin-centered gas systems
| part =
| chapter =
| frompg = 1891
| topg = 1919
| author = Ben E. Law
| link = http://archives.datapages.com/data/bulletns/2002/11nov/1891/1891.htm
| pdf =
| store =
| isbn =
}}
To illustrate the elements and processes of direct and indirect basin-centered gas systems (BCGSs), an example of each system is included in the following discussion. Additional examples are provided in Table 1 of the [[Basin-centered gas systems: development]] article.
[[File:BasinCenteredGasFig3.jpg|thumb|400px|{{figure number|1}}Map of the Greater Green River basin, showing major structural elements and the locations of the Jonah field, the Belco 3-28 Merna and El Paso Natural Gas 1 Wagon Wheel wells, and cross section BB'. ]]
==Direct type: Greater Green River basin==
The Greater Green River basin, located in southwestern Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 1]]), is one of several [[foreland basin]]s in the Rocky Mountain region containing BCGSs. The stratigraphic interval containing the BCGS includes all of the [[Cretaceous]] sequence, locally extending into lower [[Tertiary]] rocks. Stratigraphic correlations of lower Tertiary and Cretaceous rocks in the Greater Green River basin are shown in [[:file:BasinCenteredGasFig5.jpg|Figure 2]]. For a comprehensive discussion of the stratigraphy and structure of the basin see Ryder.<ref name=Ryder_1988>Ryder, R. T., 1998, Greater Green River basin, ''in'' L. L. Sloss, ed., Sedimentary cover-North America craton: The Geology of North America, v. D-2, p. 154-165.</ref> Estimates of in-place gas resources contained in the basin-centered gas accumulation (BCGA) within Cretaceous and Tertiary rocks are as large as 5063 tcf,<ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and C. J. Wandrey, 1989, Estimates of gas resources in overpressured low-permeability Cretaceous and Tertiary sandstone reservoirs, Greater Green River basin, Wyoming, Colorado, and Utah: 40th Annual Field Conference, Wyoming Geological Association Guidebook, p. 39-61.</ref> and the mean estimate of recoverable gas is 119.3 tcf.<ref name=Law_1996>Law, B. E., 1996, Southwestern Wyoming province (037), ''in'' D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds., [http://pubs.usgs.gov/dds/dds-030/ 1995 national assessment of United States oil and gas resources-results, methodology, and supporting data]: U.S. Geological Survey Digital Data Series DDS-30, Release 2, 1 CD-ROM.</ref> Additional references to a BCGS in the Greater Green River basin include publications by Law et al.,<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostic, 1979, Preliminary results of organic maturation, temperature, and pressure studies in the Pacific Creek area, Sublette County, Wyoming, ''in'' 5th Department of Energy symposium on enhanced oil and gas recovery and improved drilling methods, v. 3-oil and gas recovery: Tulsa, Oklahoma, Petroleum Publishing, p. K-2/1-K-2/13.</ref><ref name=Lawetal_1980>Law, B. E., C. W. Spencer, and N. H. Bostick, 1980, Evaluation of organic maturation, subsurface temperature, and pressure with regard to gas generation in low-permeability Upper Cretaceous and lower Tertiary strata in the Pacific Creek area, Sublette County, Wyoming: Mountain Geologist, v. 17, no. 2, p. 23-35.</ref> McPeek,<ref name=McPeek_1981>McPeek, L. A., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1050/1078.htm Eastern Green River basin-a developing giant gas supply from deep, overpressured Upper Cretaceous sandstones]: AAPG Bulletin, v. 65, p. 1978-1098.</ref> Davis,<ref name=Davis_1984>Davis, T. B., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0150/0189.htm Subsurface pressure profiles in gas saturated basins], ''in'' J. A. Masters, ed., Elmworth-case study of a deep basin gas field: AAPG Memoir 38, p. 189-203.</ref> Law,<ref name=Law_1984>Law, B. E., 1984, Relationships of source rocks, thermal maturity, and overpressuring to gas generation and occurrence in low-permeability Upper Cretaceous and lower Tertiary rocks, Greater Green River basin, Wyoming, Colorado, and Utah, ''in'' J. Woodward, F. F. Meissner, and J. L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists Guidebook, P. 469-490.</ref> Keighin et al.,<ref name=Keighinetal_1989>Keighin, C. W. B. E. Law, and R. M. Pollastro, 1989, Petrology and reservoir characteristics of the Almond Formation, Greater Green River basin, Wyoming, ''in'' E. B. Coalson, S. S. Kaplan, C. W. Keighin, C. A. Oglesby, and J. W. Robinson, eds., Petrogenesis and petrophysics of selected sandstone reservoirs of the Rocky Mountain region: Rocky Mountain Association of Geologists, p. 281-294.</ref> Law and Spencer,<ref name=Lawandspencer_1989>Law, B. E., and C. W. Spencer, eds., 1989, Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming and at the Multiwell Experiment site, Colorado: U.S. Geological Survey Bulletin 1886, p. 39-61.</ref> Spencer,<ref name=Spencer_1989b>Spencer, C. W., 1989, Comparison of overpressuring at the Pinedale anticline area, Wyoming, and the Multiwell Experiment site, Colorado, ''in'' B. E. Law and C. W. Spencer, eds., Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming and at the Multiwell Experiment site, Colorado: U.S. Geological Survey Bulletin 1886, 16 p.</ref> Surdam,<ref name=Surdam_1992>Surdam, R. C., 1992, Sandstone geometries, petrophysical characteristics, and pressure regimes, Mesaverde Group, Green River basin, Wyoming, ''in'' C. E. Mullen, ed. Rediscover the Rockies: Wyoming Geological Association Forty-third Field Conference, p. 167-169.</ref> Garcia-Gonzales et al.,<ref name=Garciagonzalesetal_1993a>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Coal as a source rock of petroleum and gas-a comparison between natural and artificial maturation of the Almond Formation coals, Greater Green River basin in Wyoming, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 405-437.</ref><ref name=Garciagonzalesetal_1993b>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Mechanisms of petroleum generation from coal, as evidenced from petrographic and geochemical studies: Examples from Almond Formation coals in the Greater Green River basin, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 311-323.</ref> MacGowan et al.,<ref name=Macgowanetal_1993>MacGowan, D. B., M. Garcia-Gonzales, D. R. Britton, and R. C. Surdam, 1993, Timing of hydrocarbon generation, organic-inorganic diagenesis, and the formation of abnormally pressured gas compartments in the Cretaceous of the Greater Green River basin: A geochemical model, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 325-357.</ref> and Surdam et al.<ref name=Surdametal_2001>Surdam, R. C., J. Robinson, Z. S. Jiao, and N. K. Boyd III, 2001, Delineation of Jonah field using seismic and sonic velocity interpretations, ''in'' D. S. Anderson, J. W. Robinson, J. E. Estes-Jackson, and E. B. Coalson, eds., Gas in the Rockies: Rocky Mountain Association of Geologists, p. 189-208.</ref>
===General characteristics of the Greater Green River basin BCGS===
* Area: 19,700 mi<sup>2</sup> (51,000 km<sup>2</sup>)
* Source rocks: Upper Cretaceous and lower Tertiary coal beds and carbonaceous shales in the Fort Union, Lance, Almond, and Rock Springs formations. Organic matter is largely gas-prone type III kerogen<ref name=Law_1984 /> with additional contribution from thermally cracked oils sourced from sapropelic coal beds.<ref name=Garciagonzalesetal_1993a /><ref name=Garciagonzalesetal_1993b /><ref name=Macgowanetal_1993 /><ref name=Surdametal_1997 />
* Generation-expulsion-migration: late Eocene-late Oligocene (40-25 Ma)
* Reservoir rocks: Cretaceous to lower Tertiary sandstones. Multiple, stacked reservoirs occur in rock intervals as thick as 14,000 ft (4267 m) ([[:file:BasinCenteredGasFig6.jpg|Figure 3]]). Individual reservoirs range in thickness from 15 to 125 ft (4.6-38 m). Gas reservoirs are saturated and contain water at irreducible levels. The gas-bearing interval does not commonly contain interbedded, water-bearing reservoirs.
* Porosity: <13%
* Permeability: <0.1 md (in-situ)
* Environments of deposition: mainly fluvial dominated and, to a lesser degree, marginal marine deltaic and barrier bar
* Reservoir pressure: overpressured, with gradients ranging from 0.5 to 0.9 psi/ft ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]])<ref name=Lawetal_1979 /><ref name=Lawetal_1980 /><ref name=Mcpeek_1981 /><ref name=Davis_1984 /><ref name=Law_1984 /><ref name=Spencer_1987 /><ref name=Spencer_1989b /><ref name=Surdametal_1997 />
* Seals: Regional seals are capillary pressure seals. Locally, structural and stratigraphic seals are important.
* Gas accumulations: downdip from normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 6]]);<ref name=Law_1984 /><ref name=Spencer_1985 /> lacks a downdip water contact.<ref name=Law_1984 /> The level of thermal maturity at top of accumulation ranges from 0.7 to 0.9% R<sub>o</sub><ref name=Law_1984 /> ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]]), commonly 0.8% R<sub>o</sub>.<ref name=Law_1984 />
* Depth to accumulation: ranges from 8000 to 11,500 ft (2438-3505 m)
* Gas quality: Gas is of a thermal origin and generally composed of >90% methane, <5% ethane and higher homologs, <5% carbon dioxide, and negligible nitrogen. Condensate ranges from <5 to 70 bbl/mmcf gas.
* Sweet spots: structural and stratigraphic
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig5.jpg|{{figure number|2}}Generalized stratigraphic correlation chart of Cretaceous and lower Tertiary rocks in the Greater Green River basin, Wyoming and Colorado (modified from Law et al.<ref name=Lawetal_1989 />)
file:BasinCenteredGasFig6.jpg|{{figure number|3}}Cross section BB' showing spatial distribution of BCGA superimposed on structure through the Washakie basin (modified from Law et al.<ref name=Lawetal_1989 />). Shaded pattern shows overpressured, gas-saturated BCGA. Location of cross section shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]].
file:BasinCenteredGasFig7.jpg|{{figure number|4}}(A) Pressure and temperature and (B) vitrinite reflectance gradients for the Belco 3-28 Merna well, northern Green River basin, Wyoming (from Law,<ref name=Law_1984 /> reprinted by permission of the Rocky Mountain Association of Geologists). Location of well shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]]. Pressure gradient interpreted by C. W. Spencer.
</gallery>
==Indirect type: Lower Silurian Clinton-Medina-Tuscarora, Appalachian Basin==
The Lower Silurian Clinton-Medina-Tuscarora BCGS, located in the Appalachian basin ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig10.jpg|Figure 8]]), is one of the better documented examples of an indirect BCGS. Estimates of recoverable resources range from 8.0 to 30.3 tcf.<ref name=Gautieretal_1996 /><ref name=Mccormacetal_1996 /> For additional discussions of the Clinton-Medina-Tuscarora, refer to investigations by Davis,<ref name=Davis_1984 />, Law and Dickinson,<ref name=Lawanddickinson_1985 /> Laughrey and Harper,<ref name=Laughreyandharper_1986 /> Zagorski,<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /> Law et al.,<ref name=Lawetal_1998a /> Ryder,<ref name=Ryder_1998 /> and Ryder and Zagorski.<ref name=Ryderandzagorski_2003 />
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig8.jpg|{{figure number|5}} (A) Pressure and temperature and (B) vitrinite reflectance gradients in the El Paso Natural Gas 1 Wagon Wheel well, northern Green River basin, Wyoming (from Law,<ref name=Law_1984 /> reprinted by permission of the Rocky Mountain Association of Geologists). Location of well shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]]. Pressure gradient interpreted by C. W. Spencer.
file:BasinCenteredGasFig2.jpg|{{figure number|6}}Diagrammatic illustrations showing normal pressured/water-bearing zones, transitional water- and gas-bearing zones, and abnormally pressured/gas-bearing zones for (A) direct and (B) indirect BCGAs.
file:BasinCenteredGasFig9.jpg|{{figure number|7}}Location of Clinton-Medina-Tuscarora basin-centered gas system in the Appalachian basin, showing the normally pressured, underpressured, and overpressured parts of the system, as well as areas of conventional, hybrid, and BCGA production. Cross section CC' shown on [[:file:BasinCenteredGasFig11.jpg|Figure 9]]. Modified from Ryder and Zagorski.<ref name=Ryderandzagorski_2003 />
</gallery>
===General characteristics of the Lower Silurian Clinton-Medina-Tuscarora, Appalachian Basin BCGS===
* Area: Clinton-Medina part is 45,000 mi<sup>2</sup> (116,550 km<sup>2</sup>); Tuscarora part is 30,000 mi<sup>2</sup> (77,700 km<sup>2</sup>).
* Source rock: Ordovician Utica Shale.<ref name=Coleetal_1987 /><ref name=Drozdandcole_1994 /><ref name=Burrussandryder_1998 /><ref name=Ryderetal_1998 /> The Utica Shale contains type II kerogen and is thermally overmature (>1.3% R<sub>o</sub>).
* Generation-migration-accumulation: Late Devonian-Early Mississippian (370-320 Ma)<ref name=Drozdandcole_1994 /><ref name=Laughreyandharper_1996 /><ref name=Nuccioetal_1997 /><ref name=Ryderetal_1998 /><ref name=Ryderandzagorski_2003 />
* Reservoir rocks: Lower Silurian Clinton-Medina in eastern Ohio and western Pennsylvania and Tuscarora Sandstone in central Pennsylvania. The reservoir interval ranges in thickness from 100 to 600 ft (30-183 m).<ref name=Ryderandzagorski_2003 /> The thermal maturity of the reservoir ranges from 1.1 to 2.0% R<sub>o</sub>.<ref name=Wandreyetal_1997 />
* Porosity: 5-10%<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />
* Permeability: <0.1 md<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />
* Environments of deposition: Fluvial, estuarine, and inner marine shelf in the eastern part to outer marine shelf and tidal in the western part<ref name=Cotter_1983 /><ref name=Brettetal_1990 /><ref name=Castle_1998 />
* Reservoir pressure: Reservoirs are normally pressured in the updip part of the Clinton-Medina in eastern Ohio, producing oil, gas, and water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). In western Pennsylvania reservoirs are underpressured and produce mainly gas with very small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients of 0.39-0.25 psi/ft in the underpressured part of the system. In central Pennsylvania, the Tuscarora Sandstone, equivalent to the Clinton-Medina, is overpressured and produces gas with small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients ranging from 0.50 to 0.60 psi/ft in the overpressured Tuscarora Sandstone in central Pennsylvania. The variable pressure gradients within the stratigraphic interval are shown in [[:file:BasinCenteredGasFig12.jpg|Figure 10]].
* Seals: The top seal is interpreted to be the shales, carbonates, and evaporites in the overlying Upper Silurian.<ref name=Drozdandcole_1994 /> The updip seal has been identified as a water block.<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /><ref name=Ryderandzagorski_2003 />
* Gas accumulations: Downdip from normally pressured, water-bearing reservoirs; lacks downdip water contact ([[:file:BasinCenteredGasFig11.jpg|Figure 9]])
* Depth to accumulation: 6500 ft (1981 m) in western Pennsylvania to 12,000 ft (3658 m) in central Pennsylvania<ref name=Ryderandzagorski_2003 />
* Gas quality: Gas is interpreted to be a product of thermally cracked oil.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Gas in the Clinton-Medina sandstone is generally composed of 79-94% methane; 3-12% ethane, propane, and C<sub>4+</sub> hydrocarbon; and 3-9% nitrogen and carbon dioxide.<ref name=Burrussandryder_1998 /><ref name=Ryderandzagorski_2003 /> In the Tuscarora Sandstone, gas is commonly dry (C1/C1-5 = 0.98-0.99), with nitrogen and carbon dioxide contents of 4-22% and <1-83%, respectively.<ref name=Ryderandzagorski_2003 />
* Sweet spots: structural and stratigraphic
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig10.jpg|{{figure number|8}}Geologic column of OrdovicianñDevonian rocks in eastern Ohio and western Pennsylvania, Appalachian basin (modified from Law and Spencer<ref name=Lawandspencer_1993 />).
file:BasinCenteredGasFig11.jpg|{{figure number|9}}Generalized cross section CC' showing normally pressured, underpressured, and overpressured parts of the Clinton-Medina-Tuscarora interval (modified from Ryder and Zagorski<ref name=Ryderandzagorski_2003 />). The underpressured and overpressured areas of the Clinton-Medina-Tuscarora represent the indirect BCGA part of the interval where gas and minor amounts of water are produced, and the normally pressured area represents the conventional part of the interval where oil, gas, and water are produced. The hybrid, underpressured area represents transition from oil, gas, and water production in eastern Ohio to gas and minor water production in the BCGA.
file:BasinCenteredGasFig12.jpg|{{figure number|10}}Composite pressure gradient showing pressure end members (normal, underpressuring, and overpressuring relationships) within the Clinton-Medina-Tuscarora interval (modified from Law et al.<ref name=Lawetal_1998a />).
The underpressured reservoirs in the Clinton-Medina are interpreted to have undergone an earlier overpressured phase caused by the thermal transformation of oil to gas.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Later, during a period of regional uplift accompanied by loss of gas and reservoir cooling, the overpressured, gas-bearing Clinton-Medina underwent a transition to an underpressuring phase. The overpressured, gas-bearing Tuscarora Sandstone reservoirs in central Pennsylvania are interpreted to be pressure remnants of the earlier overpressured phase in the Clinton-Medina.<ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 />.
==References==
{{reflist}}
==See also==
* [[Basin-centered gas]]
* [[Basin-centered gas systems: historical development and classification]]
* [[Basin-centered gas systems]]
* [[Basin-centered gas systems: development]]
* [[Basin-centered gas systems: elements and processes]]
* [[Basin-centered gas systems: gas resources]]
* [[Basin-centered gas systems: global distribution]]
* [[Basin-centered gas systems: evaluation and exploration strategies]]
* [[Tight gas reservoirs: evaluation]]
==External links==
{{search}}
| image = Nov2002.jpg
| width = 120px
| series = ''AAPG Bulletin,'' November 2002
| title = Basin-centered gas systems
| part =
| chapter =
| frompg = 1891
| topg = 1919
| author = Ben E. Law
| link = http://archives.datapages.com/data/bulletns/2002/11nov/1891/1891.htm
| pdf =
| store =
| isbn =
}}
To illustrate the elements and processes of direct and indirect basin-centered gas systems (BCGSs), an example of each system is included in the following discussion. Additional examples are provided in Table 1 of the [[Basin-centered gas systems: development]] article.
[[File:BasinCenteredGasFig3.jpg|thumb|400px|{{figure number|1}}Map of the Greater Green River basin, showing major structural elements and the locations of the Jonah field, the Belco 3-28 Merna and El Paso Natural Gas 1 Wagon Wheel wells, and cross section BB'. ]]
==Direct type: Greater Green River basin==
The Greater Green River basin, located in southwestern Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 1]]), is one of several [[foreland basin]]s in the Rocky Mountain region containing BCGSs. The stratigraphic interval containing the BCGS includes all of the [[Cretaceous]] sequence, locally extending into lower [[Tertiary]] rocks. Stratigraphic correlations of lower Tertiary and Cretaceous rocks in the Greater Green River basin are shown in [[:file:BasinCenteredGasFig5.jpg|Figure 2]]. For a comprehensive discussion of the stratigraphy and structure of the basin see Ryder.<ref name=Ryder_1988>Ryder, R. T., 1998, Greater Green River basin, ''in'' L. L. Sloss, ed., Sedimentary cover-North America craton: The Geology of North America, v. D-2, p. 154-165.</ref> Estimates of in-place gas resources contained in the basin-centered gas accumulation (BCGA) within Cretaceous and Tertiary rocks are as large as 5063 tcf,<ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and C. J. Wandrey, 1989, Estimates of gas resources in overpressured low-permeability Cretaceous and Tertiary sandstone reservoirs, Greater Green River basin, Wyoming, Colorado, and Utah: 40th Annual Field Conference, Wyoming Geological Association Guidebook, p. 39-61.</ref> and the mean estimate of recoverable gas is 119.3 tcf.<ref name=Law_1996>Law, B. E., 1996, Southwestern Wyoming province (037), ''in'' D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds., [http://pubs.usgs.gov/dds/dds-030/ 1995 national assessment of United States oil and gas resources-results, methodology, and supporting data]: U.S. Geological Survey Digital Data Series DDS-30, Release 2, 1 CD-ROM.</ref> Additional references to a BCGS in the Greater Green River basin include publications by Law et al.,<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostic, 1979, Preliminary results of organic maturation, temperature, and pressure studies in the Pacific Creek area, Sublette County, Wyoming, ''in'' 5th Department of Energy symposium on enhanced oil and gas recovery and improved drilling methods, v. 3-oil and gas recovery: Tulsa, Oklahoma, Petroleum Publishing, p. K-2/1-K-2/13.</ref><ref name=Lawetal_1980>Law, B. E., C. W. Spencer, and N. H. Bostick, 1980, Evaluation of organic maturation, subsurface temperature, and pressure with regard to gas generation in low-permeability Upper Cretaceous and lower Tertiary strata in the Pacific Creek area, Sublette County, Wyoming: Mountain Geologist, v. 17, no. 2, p. 23-35.</ref> McPeek,<ref name=McPeek_1981>McPeek, L. A., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1050/1078.htm Eastern Green River basin-a developing giant gas supply from deep, overpressured Upper Cretaceous sandstones]: AAPG Bulletin, v. 65, p. 1978-1098.</ref> Davis,<ref name=Davis_1984>Davis, T. B., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0150/0189.htm Subsurface pressure profiles in gas saturated basins], ''in'' J. A. Masters, ed., Elmworth-case study of a deep basin gas field: AAPG Memoir 38, p. 189-203.</ref> Law,<ref name=Law_1984>Law, B. E., 1984, Relationships of source rocks, thermal maturity, and overpressuring to gas generation and occurrence in low-permeability Upper Cretaceous and lower Tertiary rocks, Greater Green River basin, Wyoming, Colorado, and Utah, ''in'' J. Woodward, F. F. Meissner, and J. L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists Guidebook, P. 469-490.</ref> Keighin et al.,<ref name=Keighinetal_1989>Keighin, C. W. B. E. Law, and R. M. Pollastro, 1989, Petrology and reservoir characteristics of the Almond Formation, Greater Green River basin, Wyoming, ''in'' E. B. Coalson, S. S. Kaplan, C. W. Keighin, C. A. Oglesby, and J. W. Robinson, eds., Petrogenesis and petrophysics of selected sandstone reservoirs of the Rocky Mountain region: Rocky Mountain Association of Geologists, p. 281-294.</ref> Law and Spencer,<ref name=Lawandspencer_1989>Law, B. E., and C. W. Spencer, eds., 1989, Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming and at the Multiwell Experiment site, Colorado: U.S. Geological Survey Bulletin 1886, p. 39-61.</ref> Spencer,<ref name=Spencer_1989b>Spencer, C. W., 1989, Comparison of overpressuring at the Pinedale anticline area, Wyoming, and the Multiwell Experiment site, Colorado, ''in'' B. E. Law and C. W. Spencer, eds., Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming and at the Multiwell Experiment site, Colorado: U.S. Geological Survey Bulletin 1886, 16 p.</ref> Surdam,<ref name=Surdam_1992>Surdam, R. C., 1992, Sandstone geometries, petrophysical characteristics, and pressure regimes, Mesaverde Group, Green River basin, Wyoming, ''in'' C. E. Mullen, ed. Rediscover the Rockies: Wyoming Geological Association Forty-third Field Conference, p. 167-169.</ref> Garcia-Gonzales et al.,<ref name=Garciagonzalesetal_1993a>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Coal as a source rock of petroleum and gas-a comparison between natural and artificial maturation of the Almond Formation coals, Greater Green River basin in Wyoming, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 405-437.</ref><ref name=Garciagonzalesetal_1993b>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Mechanisms of petroleum generation from coal, as evidenced from petrographic and geochemical studies: Examples from Almond Formation coals in the Greater Green River basin, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 311-323.</ref> MacGowan et al.,<ref name=Macgowanetal_1993>MacGowan, D. B., M. Garcia-Gonzales, D. R. Britton, and R. C. Surdam, 1993, Timing of hydrocarbon generation, organic-inorganic diagenesis, and the formation of abnormally pressured gas compartments in the Cretaceous of the Greater Green River basin: A geochemical model, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 325-357.</ref> and Surdam et al.<ref name=Surdametal_2001>Surdam, R. C., J. Robinson, Z. S. Jiao, and N. K. Boyd III, 2001, Delineation of Jonah field using seismic and sonic velocity interpretations, ''in'' D. S. Anderson, J. W. Robinson, J. E. Estes-Jackson, and E. B. Coalson, eds., Gas in the Rockies: Rocky Mountain Association of Geologists, p. 189-208.</ref>
===General characteristics of the Greater Green River basin BCGS===
* Area: 19,700 mi<sup>2</sup> (51,000 km<sup>2</sup>)
* Source rocks: Upper Cretaceous and lower Tertiary coal beds and carbonaceous shales in the Fort Union, Lance, Almond, and Rock Springs formations. Organic matter is largely gas-prone type III kerogen<ref name=Law_1984 /> with additional contribution from thermally cracked oils sourced from sapropelic coal beds.<ref name=Garciagonzalesetal_1993a /><ref name=Garciagonzalesetal_1993b /><ref name=Macgowanetal_1993 /><ref name=Surdametal_1997 />
* Generation-expulsion-migration: late Eocene-late Oligocene (40-25 Ma)
* Reservoir rocks: Cretaceous to lower Tertiary sandstones. Multiple, stacked reservoirs occur in rock intervals as thick as 14,000 ft (4267 m) ([[:file:BasinCenteredGasFig6.jpg|Figure 3]]). Individual reservoirs range in thickness from 15 to 125 ft (4.6-38 m). Gas reservoirs are saturated and contain water at irreducible levels. The gas-bearing interval does not commonly contain interbedded, water-bearing reservoirs.
* Porosity: <13%
* Permeability: <0.1 md (in-situ)
* Environments of deposition: mainly fluvial dominated and, to a lesser degree, marginal marine deltaic and barrier bar
* Reservoir pressure: overpressured, with gradients ranging from 0.5 to 0.9 psi/ft ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]])<ref name=Lawetal_1979 /><ref name=Lawetal_1980 /><ref name=Mcpeek_1981 /><ref name=Davis_1984 /><ref name=Law_1984 /><ref name=Spencer_1987 /><ref name=Spencer_1989b /><ref name=Surdametal_1997 />
* Seals: Regional seals are capillary pressure seals. Locally, structural and stratigraphic seals are important.
* Gas accumulations: downdip from normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 6]]);<ref name=Law_1984 /><ref name=Spencer_1985 /> lacks a downdip water contact.<ref name=Law_1984 /> The level of thermal maturity at top of accumulation ranges from 0.7 to 0.9% R<sub>o</sub><ref name=Law_1984 /> ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]]), commonly 0.8% R<sub>o</sub>.<ref name=Law_1984 />
* Depth to accumulation: ranges from 8000 to 11,500 ft (2438-3505 m)
* Gas quality: Gas is of a thermal origin and generally composed of >90% methane, <5% ethane and higher homologs, <5% carbon dioxide, and negligible nitrogen. Condensate ranges from <5 to 70 bbl/mmcf gas.
* Sweet spots: structural and stratigraphic
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig5.jpg|{{figure number|2}}Generalized stratigraphic correlation chart of Cretaceous and lower Tertiary rocks in the Greater Green River basin, Wyoming and Colorado (modified from Law et al.<ref name=Lawetal_1989 />)
file:BasinCenteredGasFig6.jpg|{{figure number|3}}Cross section BB' showing spatial distribution of BCGA superimposed on structure through the Washakie basin (modified from Law et al.<ref name=Lawetal_1989 />). Shaded pattern shows overpressured, gas-saturated BCGA. Location of cross section shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]].
file:BasinCenteredGasFig7.jpg|{{figure number|4}}(A) Pressure and temperature and (B) vitrinite reflectance gradients for the Belco 3-28 Merna well, northern Green River basin, Wyoming (from Law,<ref name=Law_1984 /> reprinted by permission of the Rocky Mountain Association of Geologists). Location of well shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]]. Pressure gradient interpreted by C. W. Spencer.
</gallery>
==Indirect type: Lower Silurian Clinton-Medina-Tuscarora, Appalachian Basin==
The Lower Silurian Clinton-Medina-Tuscarora BCGS, located in the Appalachian basin ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig10.jpg|Figure 8]]), is one of the better documented examples of an indirect BCGS. Estimates of recoverable resources range from 8.0 to 30.3 tcf.<ref name=Gautieretal_1996 /><ref name=Mccormacetal_1996 /> For additional discussions of the Clinton-Medina-Tuscarora, refer to investigations by Davis,<ref name=Davis_1984 />, Law and Dickinson,<ref name=Lawanddickinson_1985 /> Laughrey and Harper,<ref name=Laughreyandharper_1986 /> Zagorski,<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /> Law et al.,<ref name=Lawetal_1998a /> Ryder,<ref name=Ryder_1998 /> and Ryder and Zagorski.<ref name=Ryderandzagorski_2003 />
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig8.jpg|{{figure number|5}} (A) Pressure and temperature and (B) vitrinite reflectance gradients in the El Paso Natural Gas 1 Wagon Wheel well, northern Green River basin, Wyoming (from Law,<ref name=Law_1984 /> reprinted by permission of the Rocky Mountain Association of Geologists). Location of well shown on [[:file:BasinCenteredGasFig3.jpg|Figure 1]]. Pressure gradient interpreted by C. W. Spencer.
file:BasinCenteredGasFig2.jpg|{{figure number|6}}Diagrammatic illustrations showing normal pressured/water-bearing zones, transitional water- and gas-bearing zones, and abnormally pressured/gas-bearing zones for (A) direct and (B) indirect BCGAs.
file:BasinCenteredGasFig9.jpg|{{figure number|7}}Location of Clinton-Medina-Tuscarora basin-centered gas system in the Appalachian basin, showing the normally pressured, underpressured, and overpressured parts of the system, as well as areas of conventional, hybrid, and BCGA production. Cross section CC' shown on [[:file:BasinCenteredGasFig11.jpg|Figure 9]]. Modified from Ryder and Zagorski.<ref name=Ryderandzagorski_2003 />
</gallery>
===General characteristics of the Lower Silurian Clinton-Medina-Tuscarora, Appalachian Basin BCGS===
* Area: Clinton-Medina part is 45,000 mi<sup>2</sup> (116,550 km<sup>2</sup>); Tuscarora part is 30,000 mi<sup>2</sup> (77,700 km<sup>2</sup>).
* Source rock: Ordovician Utica Shale.<ref name=Coleetal_1987 /><ref name=Drozdandcole_1994 /><ref name=Burrussandryder_1998 /><ref name=Ryderetal_1998 /> The Utica Shale contains type II kerogen and is thermally overmature (>1.3% R<sub>o</sub>).
* Generation-migration-accumulation: Late Devonian-Early Mississippian (370-320 Ma)<ref name=Drozdandcole_1994 /><ref name=Laughreyandharper_1996 /><ref name=Nuccioetal_1997 /><ref name=Ryderetal_1998 /><ref name=Ryderandzagorski_2003 />
* Reservoir rocks: Lower Silurian Clinton-Medina in eastern Ohio and western Pennsylvania and Tuscarora Sandstone in central Pennsylvania. The reservoir interval ranges in thickness from 100 to 600 ft (30-183 m).<ref name=Ryderandzagorski_2003 /> The thermal maturity of the reservoir ranges from 1.1 to 2.0% R<sub>o</sub>.<ref name=Wandreyetal_1997 />
* Porosity: 5-10%<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />
* Permeability: <0.1 md<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />
* Environments of deposition: Fluvial, estuarine, and inner marine shelf in the eastern part to outer marine shelf and tidal in the western part<ref name=Cotter_1983 /><ref name=Brettetal_1990 /><ref name=Castle_1998 />
* Reservoir pressure: Reservoirs are normally pressured in the updip part of the Clinton-Medina in eastern Ohio, producing oil, gas, and water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). In western Pennsylvania reservoirs are underpressured and produce mainly gas with very small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients of 0.39-0.25 psi/ft in the underpressured part of the system. In central Pennsylvania, the Tuscarora Sandstone, equivalent to the Clinton-Medina, is overpressured and produces gas with small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients ranging from 0.50 to 0.60 psi/ft in the overpressured Tuscarora Sandstone in central Pennsylvania. The variable pressure gradients within the stratigraphic interval are shown in [[:file:BasinCenteredGasFig12.jpg|Figure 10]].
* Seals: The top seal is interpreted to be the shales, carbonates, and evaporites in the overlying Upper Silurian.<ref name=Drozdandcole_1994 /> The updip seal has been identified as a water block.<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /><ref name=Ryderandzagorski_2003 />
* Gas accumulations: Downdip from normally pressured, water-bearing reservoirs; lacks downdip water contact ([[:file:BasinCenteredGasFig11.jpg|Figure 9]])
* Depth to accumulation: 6500 ft (1981 m) in western Pennsylvania to 12,000 ft (3658 m) in central Pennsylvania<ref name=Ryderandzagorski_2003 />
* Gas quality: Gas is interpreted to be a product of thermally cracked oil.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Gas in the Clinton-Medina sandstone is generally composed of 79-94% methane; 3-12% ethane, propane, and C<sub>4+</sub> hydrocarbon; and 3-9% nitrogen and carbon dioxide.<ref name=Burrussandryder_1998 /><ref name=Ryderandzagorski_2003 /> In the Tuscarora Sandstone, gas is commonly dry (C1/C1-5 = 0.98-0.99), with nitrogen and carbon dioxide contents of 4-22% and <1-83%, respectively.<ref name=Ryderandzagorski_2003 />
* Sweet spots: structural and stratigraphic
<gallery mode=packed heights=200px widths=200px>
file:BasinCenteredGasFig10.jpg|{{figure number|8}}Geologic column of OrdovicianñDevonian rocks in eastern Ohio and western Pennsylvania, Appalachian basin (modified from Law and Spencer<ref name=Lawandspencer_1993 />).
file:BasinCenteredGasFig11.jpg|{{figure number|9}}Generalized cross section CC' showing normally pressured, underpressured, and overpressured parts of the Clinton-Medina-Tuscarora interval (modified from Ryder and Zagorski<ref name=Ryderandzagorski_2003 />). The underpressured and overpressured areas of the Clinton-Medina-Tuscarora represent the indirect BCGA part of the interval where gas and minor amounts of water are produced, and the normally pressured area represents the conventional part of the interval where oil, gas, and water are produced. The hybrid, underpressured area represents transition from oil, gas, and water production in eastern Ohio to gas and minor water production in the BCGA.
file:BasinCenteredGasFig12.jpg|{{figure number|10}}Composite pressure gradient showing pressure end members (normal, underpressuring, and overpressuring relationships) within the Clinton-Medina-Tuscarora interval (modified from Law et al.<ref name=Lawetal_1998a />).
The underpressured reservoirs in the Clinton-Medina are interpreted to have undergone an earlier overpressured phase caused by the thermal transformation of oil to gas.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Later, during a period of regional uplift accompanied by loss of gas and reservoir cooling, the overpressured, gas-bearing Clinton-Medina underwent a transition to an underpressuring phase. The overpressured, gas-bearing Tuscarora Sandstone reservoirs in central Pennsylvania are interpreted to be pressure remnants of the earlier overpressured phase in the Clinton-Medina.<ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 />.
==References==
{{reflist}}
==See also==
* [[Basin-centered gas]]
* [[Basin-centered gas systems: historical development and classification]]
* [[Basin-centered gas systems]]
* [[Basin-centered gas systems: development]]
* [[Basin-centered gas systems: elements and processes]]
* [[Basin-centered gas systems: gas resources]]
* [[Basin-centered gas systems: global distribution]]
* [[Basin-centered gas systems: evaluation and exploration strategies]]
* [[Tight gas reservoirs: evaluation]]
==External links==
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