Changes

===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.<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 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.

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 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==