Changes

[[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 [[Kinetics|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 chart|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.