Changes

Where detailed work has been conducted in direct BCGAs, gas-saturated reservoirs grade vertically, across stratigraphic boundaries, as well as [[updip]] into transitional, water- and gas-bearing zones that, in turn, grade into normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 1]]). In indirect BCGAs, gas-saturated reservoirs grade updip into transitional, water- and gas-bearing zones; however, vertical transitional zones across bed boundaries do not occur, and there is an abrupt, distinct boundary between the abnormally pressured BCGA and normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 1]]). The nature of these fluid boundaries is related to seal integrity. Seals in BCGAs range from lithologic to relative permeability, or water-block, seals, referred to in this article as [[capillary pressure]] seals. Capillary pressure seals generally occur in reservoirs that have very small pore throats and two or more fluid phases. Under these conditions, the permeability to each fluid phase is effectively reduced.

Where detailed work has been conducted in direct BCGAs, gas-saturated reservoirs grade vertically, across stratigraphic boundaries, as well as [[updip]] into transitional, water- and gas-bearing zones that, in turn, grade into normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 1]]). In indirect BCGAs, gas-saturated reservoirs grade updip into transitional, water- and gas-bearing zones; however, vertical transitional zones across bed boundaries do not occur, and there is an abrupt, distinct boundary between the abnormally pressured BCGA and normally pressured, water-bearing reservoirs ([[:file:BasinCenteredGasFig2.jpg|Figure 1]]). The nature of these fluid boundaries is related to seal integrity. Seals in BCGAs range from lithologic to relative permeability, or water-block, seals, referred to in this article as [[capillary pressure]] seals. Capillary pressure seals generally occur in reservoirs that have very small pore throats and two or more fluid phases. Under these conditions, the permeability to each fluid phase is effectively reduced.

Because of the nature of seals in direct BCGAs, a question arises concerning the integrity of the seal. Based on burial and thermal history reconstructions, capillary pressure seals in Cretaceous and [[Tertiary]] BCGAs in the Rocky Mountain region are effective for periods of time ranging from 25 to 40 m.y., the lapsed time since formation of most BCGAs in the region. However, as a consequence of the nature of these seals, there is a perception that the seals are leaky and, given sufficient time, will degenerate and become ineffective. If the perception of a leaky seal over significantly long periods of time is correct, then one might expect to see a predominance of direct BCGAs in rocks that have experienced the formation of a BCGA within a few tens of million years. Also, in a more general sense, one would expect to observe a higher frequency of direct BCGAs in younger rocks than in older rocks. Observations of known BCGAs are skewed toward Cretaceous systems, largely because most of the work conducted on BCGAs has been in Cretaceous and Tertiary rocks. There are no detailed studies of seal integrity in pre-Cretaceous BCGAs, although Ryder and Zagorski<ref name=Ryderandzagorski_2003>Ryder, R. T., and W. A. Zagorski, 2003, [http://archives.datapages.com/data/bulletns/2003/05may/0847/0847.HTM Nature, origin, and production characteristics of the Lower Silurian regional oil and gas accumulation, central Appalachian basin, United States]: AAPG Bulletin, v. 87, no. 5, p. 847-872.</ref> have concluded that the updip seal in the Lower Silurian Clinton-Medina BCGA in the Appalachian basin is a water block. In indirect systems, it is important to distinguish between vertical seals, top seals, and updip seals; in the Clinton-Medina reservoir, there is an apparently effective updip, capillary pressure seal,<ref name=Ryderandzagorski_2003 /> whereas the upper, top seal is a lithologic seal composed of evaporite, shale, and carbonate.<ref name=Drozdandcole_1994>Drozd, R. J., and G. A. Cole, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0350/0387.htm Point Pleasant-Brassfield petroleum system, Appalachian basin, USA], ''in'' L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 387-398.</ref>

Because of the nature of seals in direct BCGAs, a question arises concerning the integrity of the seal. Based on burial and thermal history reconstructions, capillary pressure seals in Cretaceous and [[Tertiary]] BCGAs in the Rocky Mountain region are effective for periods of time ranging from 25 to 40 m.y., the lapsed time since formation of most BCGAs in the region. However, as a consequence of the nature of these seals, there is a perception that the seals are leaky and, given sufficient time, will degenerate and become ineffective. If the perception of a leaky seal over significantly long periods of time is correct, then one might expect to see a predominance of direct BCGAs in rocks that have experienced the formation of a BCGA within a few tens of million years. Also, in a more general sense, one would expect to observe a higher frequency of direct BCGAs in younger rocks than in older rocks. Observations of known BCGAs are skewed toward Cretaceous systems, largely because most of the work conducted on BCGAs has been in Cretaceous and Tertiary rocks. There are no detailed studies of seal integrity in pre-Cretaceous BCGAs, although Ryder and Zagorski<ref name=Ryderandzagorski_2003>Ryder, R. T., and W. A. Zagorski, 2003, [http://archives.datapages.com/data/bulletns/2003/05may/0847/0847.HTM Nature, origin, and production characteristics of the Lower Silurian regional oil and gas accumulation, central Appalachian basin, United States]: AAPG Bulletin, v. 87, no. 5, p. 847-872.</ref> have concluded that the updip seal in the Lower Silurian Clinton-Medina BCGA in the Appalachian basin is a water block. In indirect systems, it is important to distinguish between vertical seals, top seals, and updip seals; in the Clinton-Medina reservoir, there is an apparently effective updip, capillary pressure seal,<ref name=Ryderandzagorski_2003 /> whereas the upper, top seal is a lithologic seal composed of [[evaporite]], shale, and carbonate.<ref name=Drozdandcole_1994>Drozd, R. J., and G. A. Cole, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0350/0387.htm Point Pleasant-Brassfield petroleum system, Appalachian basin, USA], ''in'' L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 387-398.</ref>

[[file:BasinCenteredGasFig1.jpg|thumb|400px|{{figure number|2}}Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.]]

[[file:BasinCenteredGasFig1.jpg|thumb|400px|{{figure number|2}}Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.]]