| 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<sub>o</sub>.<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 C<sub>15+</sub> 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<sub>o</sub> (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<sub>o</sub>.<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 C<sub>15+</sub> 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<sub>o</sub> (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. |
| In indirect BCGSs, hydrocarbon migration distances are highly variable, similar to migration distances in conventional petroleum systems. Approximately 1000 ft (305 m) of vertical migration is proposed for the Clinton-Medina BCGA.<ref name=Ryderandzagorski_2003 /> In direct BCGSs, gas is the dominant migrating hydrocarbon phase, and in indirect BCGSs, oil and gas may be expected to be the migrating fluid phases. | | In indirect BCGSs, hydrocarbon migration distances are highly variable, similar to migration distances in conventional petroleum systems. Approximately 1000 ft (305 m) of vertical migration is proposed for the Clinton-Medina BCGA.<ref name=Ryderandzagorski_2003 /> In direct BCGSs, gas is the dominant migrating hydrocarbon phase, and in indirect BCGSs, oil and gas may be expected to be the migrating fluid phases. |