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{{publication  

{{publication  

  | image  = Nov2002.jpg

  | image  = Nov2002.jpg

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  | series  = ''AAPG Bulletin,'' November 2002

  | series  = ''AAPG Bulletin,'' November 2002

  | title  = Basin-centered gas systems

  | title  = Basin-centered gas systems

}}

}}

The global distribution of basin-centered gas accumulations (BCGAs) is poorly known, and knowledge of the stratigraphic distribution of BCGAs is incomplete. Even in North America, where most of the exploration activity for BCGAs has occurred, the geographic distribution is not well known. [[:file:BasinCenteredGasFig13.jpg|Figure 1]] shows the locations of known and suspected BCGAs in the United States. A tabulation of these areas, as well as areas outside North America, is shown in Table 1. The geographic distribution of BCGAs is probably best known in the Rocky Mountain region, where a considerable amount of research has occurred.

The global distribution of basin-centered gas accumulations (BCGAs) is poorly known, and knowledge of the stratigraphic distribution of BCGAs is incomplete. Even in North America, where most of the exploration activity for BCGAs has occurred, the geographic distribution is not well known. [[:file:BasinCenteredGasFig13.jpg|Figure 1]] shows the locations of known and suspected BCGAs in the United States. A tabulation of these areas, as well as areas outside North America, is shown in Table 1. The geographic distribution of BCGAs is probably best known in the Rocky Mountain region, where a considerable amount of research has occurred.

Worldwide, there are only a few references available alluding to the presence of BCGAs (Table 1). Many more areas undoubtedly contain BCGAs, but because of the poor understanding of the concepts of BCGSs in countries outside North America, the global distribution of BCGAs is poorly known. For example, in North America, many Rocky Mountain basins contain direct BCGAs. By analogy with Rocky Mountain basins, it is likely that many of the Andean foreland basins of South America also contain BCGAs. Several of the basins in the Middle East and North Africa probably contain indirect BCGAs similar to those in Jordan and Algeria.

Worldwide, there are only a few references available alluding to the presence of BCGAs (Table 1). Many more areas undoubtedly contain BCGAs, but because of the poor understanding of the concepts of BCGSs in countries outside North America, the global distribution of BCGAs is poorly known. For example, in North America, many Rocky Mountain basins contain direct BCGAs. By analogy with Rocky Mountain basins, it is likely that many of the Andean [[foreland basin]]s of South America also contain BCGAs. Several of the basins in the Middle East and North Africa probably contain indirect BCGAs similar to those in Jordan and Algeria.

{| class = "wikitable"

{| class = "wikitable"

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| Cook Inlet, Alaska || Low || pre-Tertiary || ? || Popov et al.<ref name=Popovetal_2001 />

| Cook Inlet, Alaska || Low || pre-Tertiary || ? || Popov et al.<ref name=Popovetal_2001 />

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|-

| Norton Basin, Alaska || High || [[Eocene]]/[[Paleocene]] || Direct || Smith<ref name=Smith_1994>Smith, J. T., 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0000/0025.htm Petroleum system logic as an exploration tool in a frontier setting], ''in'' L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, p. 25-49.</ref>

| Norton Basin, Alaska || High || [[Eocene]]/[[Paleocene]] || Direct || Smith<ref name=Smith_1994>Smith, J. T., 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0000/0025.htm Petroleum system logic as an exploration tool in a frontier setting], ''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. 25-49.</ref>

|-

|-

| Alberta basin, Canada || High || Cretaceous || Direct || Masters<ref name=Masters_1979>Masters, J. A., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0002/0150/0152.htm Deep basin gas trap, western Canada]: AAPG Bulletin, v. 63, p. 152-181.</ref> <ref name=Masters_1984>Masters, J. A., ed., 1984, [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/m38.htm Elmworth-case study of a deep basin gas field]: AAPG Memoir 38, 316 p.</ref>

| Alberta basin, Canada || High || Cretaceous || Direct || Masters<ref name=Masters_1979>Masters, J. A., 1979, [http://archives.datapages.com/data/bulletns/1977-79/data/pg/0063/0002/0150/0152.htm Deep basin gas trap, western Canada]: AAPG Bulletin, v. 63, p. 152-181.</ref> <ref name=Masters_1984>Masters, J. A., ed., 1984, [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/m38.htm Elmworth-case study of a deep basin gas field]: AAPG Memoir 38, 316 p.</ref>

| Wind River basn, Wyoming || High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_1996>Johnson, R. C., T. M. Finn, R. A. Crovelli, and R, H. Balay, 1996, An assessment of in-place gas resources in low-permeability Upper Cretaceous and lower Tertiary sandstone reservoirs, Wind River basin, Wyoming: U.S. Geological Survey Open-File Report 96-264, 67 p.</ref>

| Wind River basn, Wyoming || High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_1996>Johnson, R. C., T. M. Finn, R. A. Crovelli, and R, H. Balay, 1996, An assessment of in-place gas resources in low-permeability Upper Cretaceous and lower Tertiary sandstone reservoirs, Wind River basin, Wyoming: U.S. Geological Survey Open-File Report 96-264, 67 p.</ref>

|-

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| Greater Green River basin, Wyoming || High || Lower Tertiary/Cretaceous || Direct || Law et al.,<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostic, 1979, Preliminary results of organic maturation, temperature, and pressure studies in the Pacific Creek area, Sublette County, Wyoming, ''in'' 5th Department of Energy symposium on enhanced oil and gas recovery and improved drilling methods, v. 3-oil and gas recovery: Tulsa, Oklahoma, Petroleum Publishing, p. K-2/1-K-2/13.</ref> Law et al.,<ref name=Lawetal_1980>Law, B. E., C. W. Spencer, and N. H. Bostick, 1980, Evaluation of organic maturation, subsurface temperature, and pressure with regard to gas generation in low-permeability Upper Cretaceous and lower Tertiary strata in the Pacific Creek area, Sublette County, Wyoming: Mountain Geologist, v. 17, no. 2, p. 23-35.</ref> McPeek,<ref name=Mcpeek_1981>McPeek, L. A., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1050/1078.htm Eastern Green River basin-a developing giant gas supply from deep, overpressured Upper Cretaceous sandstones]: AAPG Bulletin, v. 65, p. 1078-1098.</ref> Law,<ref name=Law_1984>Law, B. E., 1984, Relationships of source rocks, thermal maturity, and overpressuring to gas generation and occurrence in low-permeability Upper Cretaceous and lower Tertiary rocks, Greater Green River basin, Wyoming, Colorado, and Utah, in J. Woodward, F. F. Meissner, and J. L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists Guidebook, p. 469-490.</ref> Law et al.<ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and C. J. Wandrey, 1989, Estimates of gas resources in overpressured low-permeability Cretaceous and Tertiary sandstone reservoirs, Greater Green River basin, Wyoming, Colorado, and Utah: 40th Annual Field Conference, Wyoming Geological Association Guidebook, p. 39-61.</ref>

| Greater Green River basin, Wyoming || High || Lower Tertiary/Cretaceous || Direct || Law et al.,<ref name=Lawetal_1979>Law, B. E., C. W. Spencer, and N. H. Bostic, 1979, Preliminary results of organic maturation, temperature, and pressure studies in the Pacific Creek area, Sublette County, Wyoming, ''in'' 5th Department of Energy symposium on enhanced oil and gas recovery and improved drilling methods, v. 3-oil and gas recovery: Tulsa, Oklahoma, Petroleum Publishing, p. K-2/1-K-2/13.</ref><ref name=Lawetal_1980>Law, B. E., C. W. Spencer, and N. H. Bostick, 1980, Evaluation of organic maturation, subsurface temperature, and pressure with regard to gas generation in low-permeability Upper Cretaceous and lower Tertiary strata in the Pacific Creek area, Sublette County, Wyoming: Mountain Geologist, v. 17, no. 2, p. 23-35.</ref><ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and C. J. Wandrey, 1989, Estimates of gas resources in overpressured low-permeability Cretaceous and Tertiary sandstone reservoirs, Greater Green River basin, Wyoming, Colorado, and Utah: 40th Annual Field Conference, Wyoming Geological Association Guidebook, p. 39-61.</ref> McPeek,<ref name=Mcpeek_1981>McPeek, L. A., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0006/1050/1078.htm Eastern Green River basin-a developing giant gas supply from deep, overpressured Upper Cretaceous sandstones]: AAPG Bulletin, v. 65, p. 1078-1098.</ref> Law,<ref name=Law_1984>Law, B. E., 1984, Relationships of source rocks, thermal maturity, and overpressuring to gas generation and occurrence in low-permeability Upper Cretaceous and lower Tertiary rocks, Greater Green River basin, Wyoming, Colorado, and Utah, in J. Woodward, F. F. Meissner, and J. L. Clayton, eds., Hydrocarbon source rocks of the greater Rocky Mountain region: Rocky Mountain Association of Geologists Guidebook, p. 469-490.</ref>

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| Hanna basin, Wyoming || High || Cretaceous || Direct || Popov et al.,<ref name=Popovetal_2001 /> Wilson et al.<ref name=Wilsonetal_2001>Wilson, M. S., T. S. Dyman, and V. F. Nuccio, 2001, [http://pubs.usgs.gov/bul/b2184-a/ Potential for deep basin-centered gas accumulations in Hanna basin, Wyoming]: U.S. Geological Survey Bulletin 2184-A, 12 p.</ref>

| Hanna basin, Wyoming || High || Cretaceous || Direct || Popov et al.,<ref name=Popovetal_2001 /> Wilson et al.<ref name=Wilsonetal_2001>Wilson, M. S., T. S. Dyman, and V. F. Nuccio, 2001, [http://pubs.usgs.gov/bul/b2184-a/ Potential for deep basin-centered gas accumulations in Hanna basin, Wyoming]: U.S. Geological Survey Bulletin 2184-A, 12 p.</ref>

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| Powder River basin, Wyoming || High || Cretaceous || ? || Surdam et al.,<ref name=Surdametal_1994>Surdam, R. C., Z. S. Jiao, and R. S. Martinsen, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch15/0213.htm The regional pressure regime in Cretaceous sandstones and shales in the Powder River basin], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: AAPG Memoir 61, p. 213-233.</ref> Maucione et al.<ref name=Maucioneetal_1994>Maucion, D., V. Serebryakov, P. Valasek, Y. Wang, and S. Smithson, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch22/0333.htm A sonic log study of abnormally pressured zones in the Powder River basin of Wyoming], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: AAPG Memoir 61, p. 333-348.</ref>

| Powder River basin, Wyoming || High || Cretaceous || ? || Surdam et al.,<ref name=Surdametal_1994>Surdam, R. C., Z. S. Jiao, and R. S. Martinsen, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch15/0213.htm The regional pressure regime in Cretaceous sandstones and shales in the Powder River basin], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: [http://store.aapg.org/detail.aspx?id=748 AAPG Memoir 61], p. 213-233.</ref> Maucione et al.<ref name=Maucioneetal_1994>Maucion, D., V. Serebryakov, P. Valasek, Y. Wang, and S. Smithson, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch22/0333.htm A sonic log study of abnormally pressured zones in the Powder River basin of Wyoming], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: [http://store.aapg.org/detail.aspx?id=748 AAPG Memoir 61], p. 333-348.</ref>

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| Wasatch Plateau, Utah || Moderate/High || Cretaceous || Direct || Popov et al.<ref name=Popovetal_2001 />

| Wasatch Plateau, Utah || Moderate/High || Cretaceous || Direct || Popov et al.<ref name=Popovetal_2001 />

| San Juan basin, New Mexico and Colorado || High || Cretaceous || Direct || Silver,<ref name=Silver_1950>Silver, C., 1950, The occurrence of gas in the Cretaceous rocks of the San Juan basin, New Mexico and Colorado: New Mexico Geological Society, First Field Conference, San Juan basin, p. 109-123.</ref> Masters,<ref name=Masters_1979 /> Huffman<ref name=Huffman_1996>Huffman, A. C., 1996, San Juan basin province, ''in'' D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds, [http://pubs.usgs.gov/dds/dds-030/ 1995 national assessment of United States oil and gas resources-results, methodology, and supporting data]: U.S. Geological Survey Digital Data Series DDS-30, Release 2, 1 CD-ROM.</ref>

| San Juan basin, New Mexico and Colorado || High || Cretaceous || Direct || Silver,<ref name=Silver_1950>Silver, C., 1950, The occurrence of gas in the Cretaceous rocks of the San Juan basin, New Mexico and Colorado: New Mexico Geological Society, First Field Conference, San Juan basin, p. 109-123.</ref> Masters,<ref name=Masters_1979 /> Huffman<ref name=Huffman_1996>Huffman, A. C., 1996, San Juan basin province, ''in'' D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds, [http://pubs.usgs.gov/dds/dds-030/ 1995 national assessment of United States oil and gas resources-results, methodology, and supporting data]: U.S. Geological Survey Digital Data Series DDS-30, Release 2, 1 CD-ROM.</ref>

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| Permian basin, New Mexico || High || [[Permian]] || Indirect/Direct || Broadhead,<ref name=Broadhead_1984>Broadhead, R. F., 1984, Geology of gas production from tight Abo red beds, east central New Mexico: Oil & Gas Journal, v. 82, no. 24, p. 147-158.</ref> Popov et al.<ref name=Popovetal_2001 />

| [[Permian basin]], New Mexico || High || [[Permian]] || Indirect/Direct || Broadhead,<ref name=Broadhead_1984>Broadhead, R. F., 1984, Geology of gas production from tight Abo red beds, east central New Mexico: Oil & Gas Journal, v. 82, no. 24, p. 147-158.</ref> Popov et al.<ref name=Popovetal_2001 />

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|-

| Albuquerque basin, New Mexico || Moderate/High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_2001>Johnson, R. C., T. M. Finn, and V. F. Nuccio, 2001, [http://pubs.usgs.gov/bul/b2184-c/ Potential for basin-centered gas accumulation in the Albuquerque basin, New Mexico]: U.S. Geological Survey Bulletin 2184-C, 21 p.</ref> Popov et al.<ref name=Popovetal_2001 />

| Albuquerque basin, New Mexico || Moderate/High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_2001>Johnson, R. C., T. M. Finn, and V. F. Nuccio, 2001, [http://pubs.usgs.gov/bul/b2184-c/ Potential for basin-centered gas accumulation in the Albuquerque basin, New Mexico]: U.S. Geological Survey Bulletin 2184-C, 21 p.</ref> Popov et al.<ref name=Popovetal_2001 />

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| Anadarko basin, Oklahoma || High || [[Pennsylvanian]] || Indirect || Al-Shaieb et al.,<ref name=Alshaiebetal_1994>Al-Shaieb, Z., J. Puckette, A. Abdalla, and P. B. Ely, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch04/0055.htm Megacompartment complex in the Anadarko basin: A completely sealed overpressured phenomenon], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: AAPG Memoir 61, p. 55-62.</ref> Popov et al.<ref name=Popovetal_2001 />

| Anadarko basin, Oklahoma || High || [[Pennsylvanian]] || Indirect || Al-Shaieb et al.,<ref name=Alshaiebetal_1994>Al-Shaieb, Z., J. Puckette, A. Abdalla, and P. B. Ely, 1994, [http://archives.datapages.com/data/specpubs/memoir61/ch04/0055.htm Megacompartment complex in the Anadarko basin: A completely sealed overpressured phenomenon], ''in'' P. J. Ortoleva, ed., Basin compartments and seals: [http://store.aapg.org/detail.aspx?id=748 AAPG Memoir 61], p. 55-62.</ref> Popov et al.<ref name=Popovetal_2001 />

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| Midcontinent Rift, Minnesota and Iowa || Low/Moderate || [[Precambrian]] || Indirect/Direct || Popov et al.<ref name=Popovetal_2001 />

| Midcontinent Rift, Minnesota and Iowa || Low/Moderate || [[Precambrian]] || Indirect/Direct || Popov et al.<ref name=Popovetal_2001 />

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| Arkoma basin, Arkansas and Oklahoma || High || Pennsylvanian || Direct || Meckel et al.,<ref name=Meckeletal_1992>Meckel, L. D., D. G. Smith, and L. A. Wells, 1992, [http://archives.datapages.com/data/specpubs/basinar3/data/a136/a136/0001/0400/0427.htm Ouachita foredeep basins: Regional paleogeography and habitat of hydrocarbons], ''in'' R. W. Macqueen and D. A. Leckie, eds., Foreland basins and fold belts: AAPG Memoir 55, p. 427-444.</ref> Popov et al.<ref name=Popovetal_2001 />

| Arkoma basin, Arkansas and Oklahoma || High || Pennsylvanian || Direct || Meckel et al.,<ref name=Meckeletal_1992>Meckel, L. D., D. G. Smith, and L. A. Wells, 1992, [http://archives.datapages.com/data/specpubs/basinar3/data/a136/a136/0001/0400/0427.htm Ouachita foredeep basins: Regional paleogeography and habitat of hydrocarbons], ''in'' R. W. Macqueen and D. A. Leckie, eds., Foreland basins and fold belts: [http://store.aapg.org/detail.aspx?id=143 AAPG Memoir 55], p. 427-444.</ref> Popov et al.<ref name=Popovetal_2001 />

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| Gulf Coast, United States || High || Cretaceous || Indirect || Popov et al.<ref name=Popovetal_2001 />

| Gulf Coast, United States || High || Cretaceous || Indirect || Popov et al.<ref name=Popovetal_2001 />

| Michigan basin, Michigan || Low/Moderate || [[Ordovician]] || ? || Popov et al.<ref name=Popovetal_2001 />

| Michigan basin, Michigan || Low/Moderate || [[Ordovician]] || ? || Popov et al.<ref name=Popovetal_2001 />

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| Appalachian basin, eastern United States || High || [[Silurian]]/[[Devonian]] || Indirect || Davis,<ref name=Davis_1984>Davis, T. B., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0150/0189.htm Subsurface pressure profiles in gas saturated basins], ''in'' J. A. Masters, ed., Elmworth-case study of a deep basin gas field: AAPG Memoir 38, p. 189-203.</ref> Law and Spencer,<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> Law and Spencer,<ref name=Lawandspencer_1998>Law, B. E., and C. W. Spencer, 1998, [http://archives.datapages.com/data/specpubs/memoir70/m70ch01/m70ch01.htm Abnormal pressure in hydrocarbon environments], in B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: [http://store.aapg.org/detail.aspx?id=751 AAPG Memoir 70], p. 1-11.</ref> Popov et al.,<ref name=Popovetal_2001 /> 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>

| Appalachian basin, eastern United States || High || [[Silurian]]/[[Devonian]] || Indirect || Davis,<ref name=Davis_1984>Davis, T. B., 1984, [http://archives.datapages.com/data/specpubs/fieldst4/data/a013/a013/0001/0150/0189.htm Subsurface pressure profiles in gas saturated basins], ''in'' J. A. Masters, ed., Elmworth-case study of a deep basin gas field: [http://store.aapg.org/detail.aspx?id=67 AAPG Memoir 38], p. 189-203.</ref> Law and Spencer,<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><ref name=Lawandspencer_1998>Law, B. E., and C. W. Spencer, 1998, [http://archives.datapages.com/data/specpubs/memoir70/m70ch01/m70ch01.htm Abnormal pressure in hydrocarbon environments], in B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: [http://store.aapg.org/detail.aspx?id=751 AAPG Memoir 70], p. 1-11.</ref> Popov et al.,<ref name=Popovetal_2001 /> 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>

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| colspan=5 | <div style="text-align: center;">'''SOUTH AMERICA'''</div>

| colspan=5 | <div style="text-align: center;">'''SOUTH AMERICA'''</div>

| colspan=5 | <div style="text-align: center;">'''AFRICA'''</div>

| colspan=5 | <div style="text-align: center;">'''AFRICA'''</div>

|-

|-

| Ahnet basin, Algeria || High || Cambrian/Ordovician || Indirect ||  

| Ahnet basin, Algeria || High || [[Cambrian]]/Ordovician || Indirect ||  

|-

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| Benue trough, Nigeria || Moderate/High || Cretaceous || Direct || Obaje and Abaa<ref name=Obajeandabaa_1996>Obaje, N. G., and S. I. Abaa, 1996, Potential for col-derived gaseous hydrocarbons in the middle Benue Trough of Nigeria: Journal of Petroleum Geology, v. 19, p. 77-94.</ref>

| Benue trough, Nigeria || Moderate/High || Cretaceous || Direct || Obaje and Abaa<ref name=Obajeandabaa_1996>Obaje, N. G., and S. I. Abaa, 1996, Potential for col-derived gaseous hydrocarbons in the middle Benue Trough of Nigeria: Journal of Petroleum Geology, v. 19, p. 77-94.</ref>

|}

|}

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 /><ref name=Zagorski_1991 /><ref name=LawandSpencer_1993 /><ref name=Lawetal_1998a /><ref name=RyderandZagorski_forthcoming /> 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==