Changes

===General characteristics of the Greater Green River basin BCGS===

===General characteristics of the Greater Green River basin BCGS===

* Area: 19,700 mi² (51,000 km²)

* Area: 19,700 mi² (51,000 km²)

* 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 />

* 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>Surdam, R. C., Z. S. Jiao, and H. P. Heasler, 1997, [http://archives.datapages.com/data/specpubs/mem67/ch12/ch12.htm Anomalously pressured gas compartments in Cretaceous rocks of the Laramide basins of Wyoming: A new class of hydrocarbon accumulation], ''in'' R. C. Surdam, ed., Seals, traps, and the petroleum system: AAPG Memoir 67, p. 199-222.</ref>

* Generation-expulsion-migration: late Eocene-late Oligocene (40-25 Ma)

* 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.

* 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.

* Permeability: <0.1 md (in-situ)

* Permeability: <0.1 md (in-situ)

* Environments of deposition: mainly fluvial dominated and, to a lesser degree, marginal marine deltaic and barrier bar

* 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 />  

* 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>Spencer, C. W., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0004/0350/0368.htm Hydrocarbon generation as a mechanism for overpressuring in Rocky Mountain region]: AAPG Bulletin, v. 71, p. 368-388.</ref><ref name=Spencer_1989b /><ref name=Surdametal_1997 />  

* Seals: Regional seals are capillary pressure seals. Locally, structural and stratigraphic seals are important.   

* 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_o<ref name=Law_1984 /> ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]]), commonly 0.8% R_o.<ref name=Law_1984 />  

* Gas accumulations: downdip from normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 6]]);<ref name=Law_1984 /><ref name=Spencer_1985>Spencer, C. W., 1985, Geologic aspects of tight gas reservoirs in the Rock Mountain region: Journal of Petroleum Geology, p. 1308-1314.</ref> 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_o<ref name=Law_1984 /> ([[:file:BasinCenteredGasFig7.jpg|Figure 4]], [[:file:BasinCenteredGasFig8.jpg|Figure 5]]), commonly 0.8% R_o.<ref name=Law_1984 />  

* Depth to accumulation: ranges from 8000 to 11,500 ft (2438-3505 m)  

* 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.  

* 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.  

==Indirect type: Lower Silurian Clinton-Medina-Tuscarora, Appalachian Basin==

==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 />

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>Gautier, D. L., G. L. Dolton, K. I Takahashi, and K. L. Varnes, eds., 1996, 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><ref name=Mccormacetal_1996>McCormac, M. P., G. O. Mychkovsky, S. T. Opritza, R. A. Riley, M. E. Wolfe, G. E. Larson, and M. T. Baronoski, 1996, Play Scm: Lower Silurian Cataract/Medina Group ("Clinton") sandstone play, ''in'' J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: West Virginia Geological and Economic Survey Publication V-25, p. 156-163.</ref> For additional discussions of the Clinton-Medina-Tuscarora, refer to investigations by Davis,<ref name=Davis_1984 />, Law and Dickinson,<ref name=Lawanddickinson_1985>Law, B. E., and W. W. Dickinson, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0008/1250/1295.htm A conceptual model for the origin of abnormally pressured gas accumulations in low-permeability reservoirs]: AAPG Bulletin, v. 69, p. 1295-1304.</ref> Laughrey and Harper,<ref name=Laughreyandharper_1986>Laughrey, C. D., and J. A. Harper, 1996, Play Obe: Upper Ordovician Bald Eagle Formation fractured play, ''in'' J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: West Virginia Geological and Economic Survey Publication V-25, p. 164-167.</ref> Zagorski,<ref name=Zagorski_1988>Zagorski, W. A., 1988, Exploration concepts and methodology for deep Medina sandstone reservoirs in northwestern Pennsylvania (abs.): AAPG Bulletin, v. 72, p. 976.</ref><ref name=Zagorski_1991>Zagorski, W. A., 1991, Model of local and regional hydrocarbon traps in the Lower Silurian Medina Sandstone Group, Cooperstown gas field, Crawford and Venango counties, Pennsylvania: M.S. thesis, University of Pittsburgh, Pennsylvania, 132 p.</ref> Law et al.,<ref name=Lawetal_1998a /> Ryder,<ref name=Ryder_1998 /> and Ryder and Zagorski.<ref name=Ryderandzagorski_2003  

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