Changes

The stratigraphic distribution of BCGAs extends from the Cambrian through the Eocene (Table 1). However, there appear to be some differences in the stratigraphic distribution of direct and indirect BCGAs. For example, the preponderance of direct BCGAs occur in Cretaceous through Eocene rocks (Table 1), whereas indirect BCGAs more commonly occur in pre-Cretaceous rocks. Although some of the apparent difference in stratigraphic distribution may be attributable to the disproportionate number of studies in Cretaceous and younger rocks compared to numbers of studies in pre-Cretaceous rocks, the question of seal integrity in direct systems arises. As previously discussed, the effective life of capillary pressure seals in direct systems is not known; therefore, because of the perceptions of a leaky seal in direct systems, the occurrence of direct systems in pre-Cretaceous rocks may be less common than in Cretaceous and younger rocks. Some examples, however, of pre-Cretaceous direct BCGAs include Permian rocks in the Timan-Pechora basin, Russia<ref name=Lawetal_1996 /> and the Sichuan basin, China;<ref name=Dajunandyunho_1994 /> Pennsylvanian rocks in the Arkoma basin;<ref name=Meckeletal_1992 /> and Carboniferous rocks in the Dnieper-Donets basin, Ukraine<ref name=Lawetal_1998b /> (Table 1).

The stratigraphic distribution of BCGAs extends from the Cambrian through the Eocene (Table 1). However, there appear to be some differences in the stratigraphic distribution of direct and indirect BCGAs. For example, the preponderance of direct BCGAs occur in Cretaceous through Eocene rocks (Table 1), whereas indirect BCGAs more commonly occur in pre-Cretaceous rocks. Although some of the apparent difference in stratigraphic distribution may be attributable to the disproportionate number of studies in Cretaceous and younger rocks compared to numbers of studies in pre-Cretaceous rocks, the question of seal integrity in direct systems arises. As previously discussed, the effective life of capillary pressure seals in direct systems is not known; therefore, because of the perceptions of a leaky seal in direct systems, the occurrence of direct systems in pre-Cretaceous rocks may be less common than in Cretaceous and younger rocks. Some examples, however, of pre-Cretaceous direct BCGAs include Permian rocks in the Timan-Pechora basin, Russia<ref name=Lawetal_1996 /> and the Sichuan basin, China;<ref name=Dajunandyunho_1994 /> Pennsylvanian rocks in the Arkoma basin;<ref name=Meckeletal_1992 /> and Carboniferous rocks in the Dnieper-Donets basin, Ukraine<ref name=Lawetal_1998b /> (Table 1).

Indirect BCGAs occur in rocks ranging from Cambrian through Cretaceous. Examples include Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria, Ordovician reservoirs in Jordan,<ref name=Ahlbrandtetal_1997 /> Lower Silurian reservoirs in the Appalachian basin,<ref name=Davis_1984 /><ref name=LawandDickinson_1985>Law, B. E., and W. W. Dickinson, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0008/1250/1295.htm A conceptual model for the origin of abnormally pressured gas accumulations in low-permeability reservoirs]: AAPG Bulletin, v. 69, p. 1295-1304.</ref><ref name=Zagorski_1988>Zagorski, W. A., 1988, Exploration concepts and methodology for deep Medina sandstone reservoirs in northwestern Pennsylvania (abs.): AAPG Bulletin, v. 72, p. 976.</ref><ref name=Zagorski_1991>Zagorski, W. A., 1991, Model of local and regional hydrocarbon traps in the Lower Silurian Medina Sandstone Group, Cooperstown gas field, Crawford and Venango counties, Pennsylvania: M.S. thesis, University of Pittsburgh, Pennsylvania, 132 p.</ref><ref name=LawandSpencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> and Jurassic sandstone reservoirs in the Bossier Shale<ref name=MontgomeryandKarlewicz_2001 /><ref name=EmmeandStancil_2002 /> in the United States Gulf Coast (Table 1).

Indirect BCGAs occur in rocks ranging from Cambrian through Cretaceous. Examples include Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria, Ordovician reservoirs in Jordan,<ref name=Ahlbrandtetal_1997 /> Lower Silurian reservoirs in the Appalachian basin,<ref name=Davis_1984 /><ref name=LawandDickinson_1985>Law, B. E., and W. W. Dickinson, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0008/1250/1295.htm A conceptual model for the origin of abnormally pressured gas accumulations in low-permeability reservoirs]: AAPG Bulletin, v. 69, p. 1295-1304.</ref><ref name=Zagorski_1988>Zagorski, W. A., 1988, Exploration concepts and methodology for deep Medina sandstone reservoirs in northwestern Pennsylvania (abs.): AAPG Bulletin, v. 72, p. 976.</ref><ref name=Zagorski_1991>Zagorski, W. A., 1991, Model of local and regional hydrocarbon traps in the Lower Silurian Medina Sandstone Group, Cooperstown gas field, Crawford and Venango counties, Pennsylvania: M.S. thesis, University of Pittsburgh, Pennsylvania, 132 p.</ref><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: U.S. Geological Survey Professional Paper 1570, p. 233-252.</ref><ref name=Lawetal_1998a>Law, B. E., R. T. Ryder, V. F. Nuccio, and R. C. Burrus, 1998a, Pressure end-members in the Lower Silurian Clinton-Medina-Tuscarora basin-centered gas accumulation, Appalachian basin, USA, in A. Mitchell and D. Grauls, eds., Overpressures in petroleum exploration: Elf Aquitaine Memoir 22, p. 133-135.</ref><ref name=Ryderandzagorski_2003 /> and Jurassic sandstone reservoirs in the Bossier Shale<ref name=MontgomeryandKarlewicz_2001 /><ref name=EmmeandStancil_2002 /> in the United States Gulf Coast (Table 1).

==References==

==References==