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The developmental history of a basin-centered gas system (BCGS) may be viewed as four reservoir pressure cycles. As a consequence of the dynamic nature of geologic processes and the response to those processes, the phases discussed here are geologically ephemeral. [[:file:BasinCenteredGasFig1.jpg|Figure 1]] is a diagrammatic representation showing these pressure phases and the development of direct and indirect BCGSs. Meissner<ref name=Meissner_1978>Meissner, F. Fm 1978, Patterns of source-rock maturity in non-marine source rocks of some typical western interior basins in non-marine Tertiary and Upper Cretaceous source rocks and the occurrence of oil and gas in the west central U.S.:Roky Mountain Association of Geologists Continuing Education Notes, unpaginated.</ref> and 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> discussed these phase changes for gas accumulations in low-permeability reservoirs.

The developmental history of a basin-centered gas system (BCGS) may be viewed as four reservoir pressure cycles. As a consequence of the dynamic nature of geologic processes and the response to those processes, the phases discussed here are geologically ephemeral. [[:file:BasinCenteredGasFig1.jpg|Figure 1]] is a diagrammatic representation showing these pressure phases and the development of direct and indirect BCGSs. Meissner<ref name=Meissner_1978>Meissner, F. Fm 1978, Patterns of source-rock maturity in non-marine source rocks of some typical western interior basins in non-marine Tertiary and Upper Cretaceous source rocks and the occurrence of oil and gas in the west central U.S.:Rocky Mountain Association of Geologists Continuing Education Notes, unpaginated.</ref> and 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> discussed these phase changes for gas accumulations in low-permeability reservoirs.

[[file:BasinCenteredGasFig1.jpg|thumb|400px|{{figure number|1}}Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.]]

[[file:BasinCenteredGasFig1.jpg|thumb|400px|{{figure number|1}}Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.]]

===Indirect systems===

===Indirect systems===

In contrast to direct systems, indirect systems require a liquid-prone source rock ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Reservoir quality in indirect systems is assumed to have been better than in direct systems. In this case, oil and gas are generated and expelled and migrate to reservoirs where they accumulate in structural and stratigraphic traps as discrete, buoyant accumulations with downdip water contacts. With subsequent burial and exposure to higher temperatures, the accumulated oil undergoes thermal cracking to gas, accompanied by a significant increase of fluid volume and pressures (Barker<ref name=Barker_1990>Barker, C., 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0008/0000/1254.htm Calculated volume and pressure change during the thermal cracking of oil to gas in reservoirs]: AAPG Bulletin, v. 74, p. 1254-1261.</ref>). The level of thermal maturity at which oil is transformed to gas is commonly thought to be about 1.35% vitrinite reflectance (Ro) (Tissot and Welte,<ref name=Tissotandwelte_1984>Tissot, B. P., and D. H. Welte, 1984, Petroleum formation and occurrence, 2d rev. ed.: Berlin, Springer-Verlag, 699 p.</ref> Hunt<ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2d ed.: New York, W. H. Freeman and co., 743 p.</ref>); however, some evidence, discussed in a following section, indicates that the transformation may occur at higher levels of thermal maturity. Alternatively, gas derived from thermally cracked oil within a source rock may subsequently be expelled and migrate to low-permeability reservoirs (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 oabnormally 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> Hunt<ref name=Hunt_1996 />). Under these conditions of changing fluid volume and pressure, the capillary pressure of the water-wet pore system is exceeded, and, like pore pressures in direct systems, the high pressures forcibly expel mobile, free water from the pore system, replacing water with gas, and the development of an overpressured BCGA ensues. An additionally important aspect of this phase is the necessity for the presence of an effective lithologic top seal in reservoirs formerly occupied by discrete oil accumulations.

In contrast to direct systems, indirect systems require a liquid-prone source rock ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Reservoir quality in indirect systems is assumed to have been better than in direct systems. In this case, oil and gas are generated and expelled and migrate to reservoirs where they accumulate in structural and stratigraphic traps as discrete, buoyant accumulations with downdip water contacts. With subsequent burial and exposure to higher temperatures, the accumulated oil undergoes thermal cracking to gas, accompanied by a significant increase of fluid volume and pressures.<ref name=Barker_1990>Barker, C., 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0008/0000/1254.htm Calculated volume and pressure change during the thermal cracking of oil to gas in reservoirs]: AAPG Bulletin, v. 74, p. 1254-1261.</ref> The level of thermal maturity at which oil is transformed to gas is commonly thought to be about 1.35% vitrinite reflectance (R_o);<ref name=Tissotandwelte_1984>Tissot, B. P., and D. H. Welte, 1984, Petroleum formation and occurrence, 2d rev. ed.: Berlin, Springer-Verlag, 699 p.</ref><ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2d ed.: New York, W. H. Freeman and co., 743 p.</ref> however, some evidence, discussed in a following section, indicates that the transformation may occur at higher levels of thermal maturity. Alternatively, gas derived from thermally cracked oil within a source rock may subsequently be expelled and migrate to low-permeability reservoirs.<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><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><ref name=Hunt_1996 /> Under these conditions of changing fluid volume and pressure, the capillary pressure of the water-wet pore system is exceeded, and, like pore pressures in direct systems, the high pressures forcibly expel mobile, free water from the pore system, replacing water with gas, and the development of an overpressured BCGA ensues. An additionally important aspect of this phase is the necessity for the presence of an effective lithologic top seal in reservoirs formerly occupied by discrete oil accumulations.

==Phase III==

==Phase III==

At the point where direct and indirect systems are in the overpressured phase (phase II), the processes involved in the transition to phase III are identical for both systems ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Phase III occurs when the overpressured phase of direct and indirect systems evolves into underpressured conditions. Both systems, subsequent to the phase II history of overpressure, may experience a period of uplift and erosional unloading and/or heat flow perturbations. During, or subsequent to, these burial and thermal history disruptions, some gas is lost from the accumulation, and the overpressured gas reservoirs are subjected to reduced temperatures. The loss of gas, in conjunction with reduced temperatures, effectively results in the development of an underpressured BCGA (Meissner,<ref name=Meissner_1978 /> Law and Dickinson<ref name=Lawanddickinson_1985 />). During this pressure transition, Meissner<ref name=Meissner_2000>Meissner, F. F., 2000, Causes of anomalous deep basin fluid pressures in Rocky Mountain basins and their relation to regional gas accumulation, ''in'' 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 11 p.</ref> emphasized the importance of gas loss over temperature reduction as the dominant process.

At the point where direct and indirect systems are in the overpressured phase (phase II), the processes involved in the transition to phase III are identical for both systems ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Phase III occurs when the overpressured phase of direct and indirect systems evolves into underpressured conditions. Both systems, subsequent to the phase II history of overpressure, may experience a period of uplift and erosional unloading and/or heat flow perturbations. During, or subsequent to, these burial and thermal history disruptions, some gas is lost from the accumulation, and the overpressured gas reservoirs are subjected to reduced temperatures. The loss of gas, in conjunction with reduced temperatures, effectively results in the development of an underpressured BCGA.<ref name=Meissner_1978 /><ref name=Lawanddickinson_1985 /> During this pressure transition, Meissner<ref name=Meissner_2000>Meissner, F. F., 2000, Causes of anomalous deep basin fluid pressures in Rocky Mountain basins and their relation to regional gas accumulation, ''in'' 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 11 p.</ref> emphasized the importance of gas loss over temperature reduction as the dominant process.

Conjectural evidence concerning the integrity of seals in direct vs. indirect systems implies that gas is lost more easily from direct BCGAs than from indirect BCGAs. Johnson et al.<ref name=Johnsonetal_1994>Johnson, R. C., D. D. Rice, and T. D. Fouch, 1994, Evidence for gas migration from Cretaceous basin-centered accumulations into lower Tertiary reservoirs in Rocky Mountain basins: Proceedings of the Rocky Mountain Association of Geologists and Colorado Association First Biennial Conference, Natural Gas in the Western United States, 8 p.</ref> have shown that gas in conventionally trapped accumulations in several Rocky Mountain basins originated from BCGAs, demonstrating that loss of gas through relative permeability, capillary pressure seals does occur. Examples of underpressured, phase III direct systems include Cretaceous rocks in the San Juan, Raton, and Denver basins, and examples of underpressured, phase III indirect systems include Lower Silurian reservoirs in the Appalachian basin, Ordovician reservoirs in the Risha area of eastern Jordan, and Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria (Table 1).

Conjectural evidence concerning the integrity of seals in direct vs. indirect systems implies that gas is lost more easily from direct BCGAs than from indirect BCGAs. Johnson et al.<ref name=Johnsonetal_1994>Johnson, R. C., D. D. Rice, and T. D. Fouch, 1994, Evidence for gas migration from Cretaceous basin-centered accumulations into lower Tertiary reservoirs in Rocky Mountain basins: Proceedings of the Rocky Mountain Association of Geologists and Colorado Association First Biennial Conference, Natural Gas in the Western United States, 8 p.</ref> have shown that gas in conventionally trapped accumulations in several Rocky Mountain basins originated from BCGAs, demonstrating that loss of gas through relative permeability, capillary pressure seals does occur. Examples of underpressured, phase III direct systems include Cretaceous rocks in the San Juan, Raton, and Denver basins, and examples of underpressured, phase III indirect systems include Lower Silurian reservoirs in the Appalachian basin, Ordovician reservoirs in the Risha area of eastern Jordan, and Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria (Table 1).