Changes

Although the determination of abnormal pressure is important, it is equally important to determine the mechanism of abnormal pressure. For direct BCGAs, the pressure mechanism is hydrocarbon generation,<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> and for indirect BCGAs, the pressure mechanism is thermal cracking of liquid hydrocarbons to gas.<ref name=Law_2000>Law, B. E., 2000, What is a basin-centered gas system?" 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.</ref> A useful criteria for determining the pressure mechanism is through a knowledge of the composition of pore fluids: pore fluids in direct and indirect systems are composed of gas with little or no producible water,<ref name=Spencer_1987 /> <ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, eds., 1993, Gas in tight reservoirs: An emerging source of energy, ''in'' D. G. Howell, ed., The future of energy gases: U.S. Geological Survey Professional Paper 1570, p. 233-252.</ref> whereas in abnormally pressured reservoirs, where the composition of pore fluid is mainly water, the pressure mechanism may be one of several other mechanisms, thereby precluding a hydrocarbon-generation mechanism and presence of a BCGA.

Although the determination of abnormal pressure is important, it is equally important to determine the mechanism of abnormal pressure. For direct BCGAs, the pressure mechanism is hydrocarbon generation,<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> and for indirect BCGAs, the pressure mechanism is thermal cracking of liquid hydrocarbons to gas.<ref name=Law_2000>Law, B. E., 2000, What is a basin-centered gas system?" 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.</ref> A useful criteria for determining the pressure mechanism is through a knowledge of the composition of pore fluids: pore fluids in direct and indirect systems are composed of gas with little or no producible water,<ref name=Spencer_1987 /> <ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, eds., 1993, Gas in tight reservoirs: An emerging source of energy, ''in'' D. G. Howell, ed., The future of energy gases: U.S. Geological Survey Professional Paper 1570, p. 233-252.</ref> whereas in abnormally pressured reservoirs, where the composition of pore fluid is mainly water, the pressure mechanism may be one of several other mechanisms, thereby precluding a hydrocarbon-generation mechanism and presence of a BCGA.

Formation resistivity and spontaneous potential curves measured on geophysical well logs also have been used to indicate the presence of a BCGA. In Upper Cretaceous rocks in the San Juan basin and Mesozoic rocks in the Alberta basin, resistivities greater than 20 Ω were reported to be gas saturated.<ref name=Masters_1979>Masters, J. A., 1979, Deep basin gas trap, western Canada: AAPG Bulletin, v. 63, p. 152-181.</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> noted that the boundary between conventional and BCGA reservoirs in northwestern Pennsylvania could be distinguished at 80 Ω; reservoirs with high water saturation were defined by resistivities <80 Ω•m, and reservoirs within the BCGA have resistivities >80 Ω•m. In Upper Cretaceous rocks in the Greater Green River basin, spontaneous potential curves are commonly reversed in abnormally pressured BCGAs.<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostick, 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> <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>

Formation resistivity and spontaneous potential curves measured on geophysical well logs also have been used to indicate the presence of a BCGA. In Upper Cretaceous rocks in the San Juan basin and Mesozoic rocks in the Alberta basin, resistivities greater than 20 Ω were reported to be gas saturated.<ref name=Masters_1979>Masters, J. A., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0002/0150/0152.htm Deep basin gas trap, western Canada]: AAPG Bulletin, v. 63, p. 152-181.</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> noted that the boundary between conventional and BCGA reservoirs in northwestern Pennsylvania could be distinguished at 80 Ω; reservoirs with high water saturation were defined by resistivities <80 Ω•m, and reservoirs within the BCGA have resistivities >80 Ω•m. In Upper Cretaceous rocks in the Greater Green River basin, spontaneous potential curves are commonly reversed in abnormally pressured BCGAs.<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostick, 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> <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>

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