Changes

}}

}}

==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: [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.

[[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.]]

[[file:BasinCenteredGasFig3.jpg|thumb|400px|{{figure number|3}}Map of the Greater Green River basin, showing major structural elements and the locations of the Jonah field, the Belco 3-28 Merna and El Paso Natural Gas 1 Wagon Wheel wells, and cross section BB'.]]

[[file:BasinCenteredGasFig3.jpg|thumb|400px|{{figure number|3}}Map of the Greater Green River basin, showing major structural elements and the locations of the Jonah field, the Belco 3-28 Merna and El Paso Natural Gas 1 Wagon Wheel wells, and cross section BB'.]]

The level of thermal maturity marking the transformation of oil to gas in indirect systems (initiation of phase II on [[:file:BasinCenteredGasFig1.jpg|Figure 2]]) is uncertain. Conventional wisdom indicates that thermal cracking of oil to gas occurs at about 1.35% R_o.<ref name=Tissotandwelte_1984 /><ref name=Hunt_1996 /> Price<ref name=Price_1997>Price, L. C., 1997, Minimum thermal stability levels and controlling parameters of methane as determined by C₁₅₊ hydrocarbon thermal stabilities, ''in'' T. S. Dyman, D. D. Rice, and P. A. Westcott, eds., Geologic controls of deep natural gas resources in the United States: [http://pubs.usgs.gov/bul/b2146/b2146.pdf U.S. Geological Survey Bulletin 2146], p. 139-176.</ref> questioned this value and concluded that the transformation of oil to gas occurred at much higher levels of thermal maturity. More recent kinetic studies by Tsuzuki et al.<ref name=Tsuzukietal_1999>Tsuzukie, N., N. Takeda, M. Suzuki, and K. Yokoi, 1999, The kinetic modeling of oil cracking by hydrothermal pyrolysis experiments: International Journal of Coal Geology, v. 39, p. 227-250.</ref> using hydrous pyrolysis experiments also suggest that oil is stable over higher levels of thermal maturity than previously thought. Applying these kinetic parameters to burial history curves in the United States Gulf Coast indicates that oil cracking to gas starts at vitrinite reflectance values of 1.75% R_o (M. D. Lewan, 2002, personal communication).

The level of thermal maturity marking the transformation of oil to gas in indirect systems (initiation of phase II on [[:file:BasinCenteredGasFig1.jpg|Figure 2]]) is uncertain. Conventional wisdom indicates that thermal [[cracking]] of oil to gas occurs at about 1.35% R_o.<ref name=Tissotandwelte_1984 /><ref name=Hunt_1996 /> Price<ref name=Price_1997>Price, L. C., 1997, Minimum thermal stability levels and controlling parameters of methane as determined by C₁₅₊ hydrocarbon thermal stabilities, ''in'' T. S. Dyman, D. D. Rice, and P. A. Westcott, eds., Geologic controls of deep natural gas resources in the United States: [http://pubs.usgs.gov/bul/b2146/b2146.pdf U.S. Geological Survey Bulletin 2146], p. 139-176.</ref> questioned this value and concluded that the transformation of oil to gas occurred at much higher levels of thermal maturity. More recent kinetic studies by Tsuzuki et al.<ref name=Tsuzukietal_1999>Tsuzukie, N., N. Takeda, M. Suzuki, and K. Yokoi, 1999, The kinetic modeling of oil cracking by hydrothermal pyrolysis experiments: International Journal of Coal Geology, v. 39, p. 227-250.</ref> using hydrous pyrolysis experiments also suggest that oil is stable over higher levels of thermal maturity than previously thought. Applying these kinetic parameters to burial history curves in the United States Gulf Coast indicates that oil cracking to gas starts at vitrinite reflectance values of 1.75% R_o (M. D. Lewan, 2002, personal communication).

In general, hydrocarbon migration distances in direct BCGSs are short, perhaps on the order of a few hundred feet or less. The exception to short hydrocarbon migration distances may occur in cases where the regional top of a BCGA has been ruptured, facilitating vertical migration of gas along faults and fractures for distances far greater than a few hundred feet, such as in the Jonah field of western Wyoming, discussed below in the Trap formation section.

In general, hydrocarbon migration distances in direct BCGSs are short, perhaps on the order of a few hundred feet or less. The exception to short hydrocarbon migration distances may occur in cases where the regional top of a BCGA has been ruptured, facilitating vertical migration of gas along faults and fractures for distances far greater than a few hundred feet, such as in the Jonah field of western Wyoming, discussed below in the Trap formation section.