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==Source rocks==

==Source rocks==

[[Source rock]] quality is the fundamentally most important element distinguishing direct from indirect basin-centered gas systems (BCGSs) and sets the stage for all subsequent differences between the two systems. The source rocks for direct BCGSs are most commonly [[humic]]-type [[coal bed]]s and [[carbonaceous]] [[shale]], such as occur in [[Cretaceous]] rocks in most Rocky Mountain basins or Carboniferous rocks in Europe. Source rocks for indirect BCGSs are hydrogen-rich shales such as those in the [[Ordovician]] shale in the Appalachian basin or in [[Silurian]] shales in the Middle East and North Africa. 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_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> concluded that some of the coal beds in the Greater Green River basin of Wyoming (Upper Cretaceous Almond coal beds) generated liquid hydrocarbons that were subsequently thermally cracked to gas, while still in the coal beds. They further speculated that, because of the increased fluid volume associated with the oil to gas transformation, high pressures created fractures within the coal beds, facilitating the expulsion of gas. The gas then migrated and accumulated in low-permeability reservoirs. 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> concluded that all, or most, of the gas in low-permeability reservoirs in the Greater Green River basin was sourced from humic, type III organic matter contained in coal beds and carbonaceous shale in several coal-bearing Upper Cretaceous intervals. The relative contribution of gas to basin-centered gas accumulation (BCGA) reservoirs from these different processes is not known. In the Greater Green River basin BCGA, the gas likely is dominantly sourced directly from gas-prone, humic coal beds in the Lance, Almond, and Rock Springs formations, with a minor contribution from the cracking of oil to gas in Almond Formation coal beds in the very deepest part of the Great Divide and Washakie basins.

[[Source rock]] quality is the fundamentally most important element distinguishing direct from indirect basin-centered gas systems (BCGSs) and sets the stage for all subsequent differences between the two systems. The source rocks for direct BCGSs are most commonly [[humic]]-type [[coal bed]]s and [[carbonaceous]] [[shale]], such as occur in [[Cretaceous]] rocks in most Rocky Mountain basins or Carboniferous rocks in Europe. Source rocks for indirect BCGSs are hydrogen-rich shales such as those in the [[Ordovician]] shale in the Appalachian basin or in [[Silurian]] shales in the Middle East and North Africa. 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_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: [http://store.aapg.org/detail.aspx?id=749 AAPG Memoir 67], p. 199-222.</ref> concluded that some of the coal beds in the Greater Green River basin of Wyoming (Upper Cretaceous Almond coal beds) generated liquid hydrocarbons that were subsequently thermally cracked to gas, while still in the coal beds. They further speculated that, because of the increased fluid volume associated with the oil to gas transformation, high pressures created fractures within the coal beds, facilitating the expulsion of gas. The gas then migrated and accumulated in low-permeability reservoirs. 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> concluded that all, or most, of the gas in low-permeability reservoirs in the Greater Green River basin was sourced from humic, type III organic matter contained in coal beds and carbonaceous shale in several coal-bearing Upper Cretaceous intervals. The relative contribution of gas to basin-centered gas accumulation (BCGA) reservoirs from these different processes is not known. In the Greater Green River basin BCGA, the gas likely is dominantly sourced directly from gas-prone, humic coal beds in the Lance, Almond, and Rock Springs formations, with a minor contribution from the cracking of oil to gas in Almond Formation coal beds in the very deepest part of the Great Divide and Washakie basins.

[[file:BasinCenteredGasFig2.jpg|thumb|400px|{{figure number|1}}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:BasinCenteredGasFig2.jpg|thumb|400px|{{figure number|1}}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.]]