Changes

==Trap formation==

==Trap formation==

The development of a trap in the conventional sense of a structural or stratigraphic trap is an important process in a petroleum system. In a direct BCGS system, however, it is of secondary importance, whereas in indirect systems it is very important. In direct systems, the top of gas accumulations cuts across structural and stratigraphic boundaries<ref name=Law_1984 /><ref name=Spencer_1985 /><ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, 1993, Gas in tight reservoirs-an emerging source of energy, ''in'' D. G. Howell, ed., The future of energy gases: [http://pubs.er.usgs.gov/publication/pp1570 U.S. Geological Survey Professional Paper 1570], p. 233-252.</ref> and is, therefore, not normally dependent on the development of structural or stratigraphic traps. The Jonah field of western Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 3]], [[:file:BasinCenteredGasFig4.jpg|Figure 4]]) is a good example of a direct BCGA in which structural and stratigraphic aspects are important. The lateral boundaries of the field are defined by faults.<ref name=Montgomeryandrobinson_1997>Montgomery, S. L., and J. W. Robinson, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1049/1049.htm Jonah field, Sublette County, Wyoming: Gas production from overpressured Upper Cretaceous Lance sandstones of the Green River basin]: AAPG Bulletin, v. 81, p. 1049-1062.</ref><ref name=Warner_1998 /><ref name=Warner_2000>Warner, E. M., 2000, Structural geology and pressure compartmentalization of Jonah field based on 3-D seismic data and subsurface geology, Sublette County, Wyoming: The Mountain Geologist, v. 37, no. 1, p. 15-30.</ref> The top of the accumulation is defined by a silty shale seal in the Upper Cretaceous Lance Formation.

The development of a trap in the conventional sense of a structural or stratigraphic trap is an important process in a petroleum system. In a direct BCGS system, however, it is of secondary importance, whereas in indirect systems it is very important. In direct systems, the top of gas accumulations cuts across structural and stratigraphic boundaries<ref name=Law_1984 /><ref name=Spencer_1985 /><ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, 1993, Gas in tight reservoirs-an emerging source of energy, ''in'' D. G. Howell, ed., The future of energy gases: [http://pubs.er.usgs.gov/publication/pp1570 U.S. Geological Survey Professional Paper 1570], p. 233-252.</ref> and is, therefore, not normally dependent on the development of structural or stratigraphic traps. The Jonah field of western Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 3]], [[:file:BasinCenteredGasFig4.jpg|Figure 4]]) is a good example of a direct BCGA in which structural and stratigraphic aspects are important. The [[lateral]] boundaries of the field are defined by faults.<ref name=Montgomeryandrobinson_1997>Montgomery, S. L., and J. W. Robinson, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1049/1049.htm Jonah field, Sublette County, Wyoming: Gas production from overpressured Upper Cretaceous Lance sandstones of the Green River basin]: AAPG Bulletin, v. 81, p. 1049-1062.</ref><ref name=Warner_1998 /><ref name=Warner_2000>Warner, E. M., 2000, Structural geology and pressure compartmentalization of Jonah field based on 3-D seismic data and subsurface geology, Sublette County, Wyoming: The Mountain Geologist, v. 37, no. 1, p. 15-30.</ref> The top of the accumulation is defined by a silty shale seal in the Upper Cretaceous Lance Formation.

In the development of indirect BCGAs, a conventional structural or stratigraphic trap is necessary for the accumulation of oil and gas, much the same way as oil and gas accumulate in conventional, buoyancy-driven accumulations. The development of indirect BCGAs occurs at a later burial stage than direct systems, when conventionally accumulated oil is thermally cracked to gas, accompanied by a significant increase in pore fluid volume and pore pressure ([[:file:BasinCenteredGasFig1.jpg|Figure 2]]). Oil, however, does not always accumulate in discrete accumulations and may be disseminated throughout a reservoir. In such cases, the amount of oil in the accumulation may not be present in sufficient quantity to develop pore pressures high enough to form a BCGA during the thermal conversion of oil to gas. Thus, the formation of a suitable trap and the temporal relationships among trap formation and gas generation, expulsion, migration, and entrapment are critical processes in indirect systems.

In the development of indirect BCGAs, a conventional structural or stratigraphic trap is necessary for the accumulation of oil and gas, much the same way as oil and gas accumulate in conventional, buoyancy-driven accumulations. The development of indirect BCGAs occurs at a later burial stage than direct systems, when conventionally accumulated oil is thermally cracked to gas, accompanied by a significant increase in pore fluid volume and pore pressure ([[:file:BasinCenteredGasFig1.jpg|Figure 2]]). Oil, however, does not always accumulate in discrete accumulations and may be disseminated throughout a reservoir. In such cases, the amount of oil in the accumulation may not be present in sufficient quantity to develop pore pressures high enough to form a BCGA during the thermal conversion of oil to gas. Thus, the formation of a suitable trap and the temporal relationships among trap formation and gas generation, expulsion, migration, and entrapment are critical processes in indirect systems.