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==Phase I==

===Direct and indirect systems===

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During the early burial and thermal histories of direct and indirect systems, the reservoirs are, for the most part, normally pressured, and the fluid phase in the pore system is 100% water saturated ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Compaction of framework grains during this phase is an important process. The defining processes for each system, however, are different. For direct systems, phase I terminates with the initiation of thermal gas generation, whereas the termination of phase I in indirect systems occurs with the initiation of thermal cracking of oil to gas. Reservoir quality in indirect systems during phase I is assumed to be relatively better than reservoir quality in direct systems because buoyant accumulations of oil require better porosity and permeability.

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During the early burial and thermal histories of direct and indirect systems, the reservoirs are, for the most part, normally pressured, and the fluid phase in the pore system is 100% water saturated ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Compaction of framework grains during this phase is an important process. The defining processes for each system, however, are different. For direct systems, phase I terminates with the initiation of thermal gas generation, whereas the termination of phase I in indirect systems occurs with the initiation of thermal [[cracking]] of oil to gas. Reservoir quality in indirect systems during phase I is assumed to be relatively better than reservoir quality in direct systems because buoyant accumulations of oil require better porosity and permeability.

During phase I there may be some cases in which reservoir pressures are overpressured. 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> suggested that in the early burial stages of a basin-centered gas accumulation (BCGA) sequence, prior to the development of a recognizable BCGA, and in some depositional settings of rapid sedimentation, compaction disequilibrium may have been the initial overpressuring mechanism. In this scenario, the pressuring fluid phase is water. However, as the sequence experiences further burial and hotter temperatures, the compaction disequilibrium pressure mechanism may be replaced by hydrocarbon generation and the development of abnormally high pressures characterized by pore fluids composed of gas and little or no water. A possible example of the transition of pressure mechanisms from compaction disequilibrium to hydrocarbon generation may be present in [[Miocene]] and [[Pliocene]] rocks in the Bekes basin<ref name=Spenceretal_1994>Spencer, C. W., A. Szalay, and E. Tatar, 1994, Abnormal pressure and hydrocarbon migration in the Bekes basin, ''in'' P. G. Teleki, R. E. Mattick, and J. Kokai, eds., Basin analysis in petroleum exploration: Dordrecht Netherlands, Kluwer Academic Publishers, p. 201-219.</ref> and the Mako trench (B. E. Law, 2000, unpublished data) of Hungary. In these areas, Miocene and Pliocene rocks are overpressured and possess many of the distinguishing characteristics of a BCGA. The overpressures in Miocene rocks appear to be caused by hydrocarbon generation, whereas overlying, overpressured Pliocene rocks appear to be in a transitional pressure phase between compaction disequilibrium and hydrocarbon generation. In this case, a knowledge of pore fluid composition (mainly gas or mainly water) in the Pliocene sequence would offer considerable insight in resolving the problem.

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==Phase II==

===Direct systems===

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Direct systems require gas-prone source rocks and low-permeability reservoirs in close proximity to each other. As the source and reservoir rocks undergo further burial and exposure to increasing temperatures, the source rocks begin to generate gas ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Concomitant with increased gas generation, expulsion, and migration, gas begins to enter adjacent, water-wet sandstones. Because these sandstones have low permeability, the rate at which gas is generated and accumulated in reservoirs is greater than the rate at which gas is lost. Eventually, as newly generated gas accumulates in the pore system, the capillary pressure of the water-wet pores is exceeded, and free, mobile water is expelled from the pore system, resulting in the development of an overpressured, gas-saturated reservoir with little or no free water. Examples of BCGA systems exhibiting this overpressured phase include the Greater Green River,<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> Wind River,<ref name=Johnsonetal_1996>Johnson, R. C., T. M. Finn, R. A. Crovelli, and R. H. Balay, 1996, [http://pubs.er.usgs.gov/publication/ofr96264 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> Big Horn,<ref name=Johnsonetal_1999>Johnson, R. C., R. A. Crovelli, B. G. Lowell, and T. M. Finn, 1999, [http://pubs.er.usgs.gov/publication/ofr99315A An assessment of in-place gas resources in the low-permeability basin-centered gas accumulation of the Big Horn basin, Wyoming and Montana]: U.S. Geological Survey Open-File Report 99-315A, 123 p.</ref> and Piceance basins<ref name=Johnsonetal_1987>Johnson, R. C., R. A. Crovelli, C. W. Spencer, and R. F. Mast, 1987, [http://pubs.er.usgs.gov/publication/ofr87357 An assessment of gas resources in low-permeability sandstones of the Upper Cretaceous Mesaverde Group, Piceance basin, Colorado]: U.S. Geological Survey Open-File Report 87-357, 165 p.</ref> in the Rocky Mountain region of the United States and the Taranaki Basin in New Zealand (B. E. Law, 2000, unpublished data) (Table 1).

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Direct systems require gas-prone source rocks and low-permeability reservoirs in close proximity to each other. As the source and reservoir rocks undergo further burial and exposure to increasing temperatures, the source rocks begin to generate gas ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Concomitant with increased gas generation, expulsion, and migration, gas begins to enter adjacent, water-wet sandstones. Because these sandstones have low permeability, the rate at which gas is generated and accumulated in reservoirs is greater than the rate at which gas is lost. Eventually, as newly generated gas accumulates in the pore system, the [[capillary pressure]] of the water-wet pores is exceeded, and free, mobile water is expelled from the pore system, resulting in the development of an overpressured, gas-saturated reservoir with little or no free water. Examples of BCGA systems exhibiting this overpressured phase include the Greater Green River,<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> Wind River,<ref name=Johnsonetal_1996>Johnson, R. C., T. M. Finn, R. A. Crovelli, and R. H. Balay, 1996, [http://pubs.er.usgs.gov/publication/ofr96264 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> Big Horn,<ref name=Johnsonetal_1999>Johnson, R. C., R. A. Crovelli, B. G. Lowell, and T. M. Finn, 1999, [http://pubs.er.usgs.gov/publication/ofr99315A An assessment of in-place gas resources in the low-permeability basin-centered gas accumulation of the Big Horn basin, Wyoming and Montana]: U.S. Geological Survey Open-File Report 99-315A, 123 p.</ref> and Piceance basins<ref name=Johnsonetal_1987>Johnson, R. C., R. A. Crovelli, C. W. Spencer, and R. F. Mast, 1987, [http://pubs.er.usgs.gov/publication/ofr87357 An assessment of gas resources in low-permeability sandstones of the Upper Cretaceous Mesaverde Group, Piceance basin, Colorado]: U.S. Geological Survey Open-File Report 87-357, 165 p.</ref> in the Rocky Mountain region of the United States and the Taranaki Basin in New Zealand (B. E. Law, 2000, unpublished data) (Table 1).

{| class = "wikitable"

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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>

|-

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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>

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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: [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>

|-

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

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| 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 />

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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 />

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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 />

|-

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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 />

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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: [http://store.aapg.org/detail.aspx?id=748 AAPG Memoir 61], p. 55-62.</ref> 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 />

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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: [http://store.aapg.org/detail.aspx?id=143 AAPG Memoir 55], p. 427-444.</ref> Popov et al.<ref name=Popovetal_2001 />

|-

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

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| 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 /> 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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| 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> Law and Spencer,<ref name=Lawandspencer_1998 /> 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>

|-

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

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| Vienna basin, Austria and Slovakia || Indeterminate || ? || ? ||

|-

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| Alpine Foreland basin, Switzerland || High || Permian/Carboniferous || Direct || Schegg et al.<ref name=Scheggetal_1997>Schegge, R., W. Leu, and E. Greber, 1997, New exploration concepts spark Swiss gas, oil prospects: Oil & Gas Journal, v. 95, no. 39, p. 102-106.</ref>

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| Alpine [[Foreland basin]], Switzerland || High || Permian/Carboniferous || Direct || Schegg et al.<ref name=Scheggetal_1997>Schegge, R., W. Leu, and E. Greber, 1997, New exploration concepts spark Swiss gas, oil prospects: Oil & Gas Journal, v. 95, no. 39, p. 102-106.</ref>

|-

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

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

|-

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

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

|-

| 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>

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===Indirect systems===

−

In contrast to direct systems, indirect systems require a liquid-prone source rock ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Reservoir quality in indirect systems is assumed to have been better than in direct systems. In this case, oil and gas are generated and expelled and migrate to reservoirs where they accumulate in structural and stratigraphic traps as discrete, buoyant accumulations with downdip water contacts. With subsequent burial and exposure to higher temperatures, the accumulated oil undergoes thermal cracking to gas, accompanied by a significant increase of fluid volume and pressures.<ref name=Barker_1990>Barker, C., 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0008/0000/1254.htm Calculated volume and pressure change during the thermal cracking of oil to gas in reservoirs]: AAPG Bulletin, v. 74, p. 1254-1261.</ref> The level of thermal maturity at which oil is transformed to gas is commonly thought to be about 1.35% vitrinite reflectance (R<sub>o</sub>);<ref name=Tissotandwelte_1984>Tissot, B. P., and D. H. Welte, 1984, Petroleum formation and occurrence, 2d rev. ed.: Berlin, Springer-Verlag, 699 p.</ref><ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2d ed.: New York, W. H. Freeman and co., 743 p.</ref> however, some evidence indicates that the transformation may occur at higher levels of thermal maturity. Alternatively, gas derived from thermally cracked oil within a source rock may subsequently be expelled and migrate to low-permeability reservoirs.<ref name=Garciagonzalesetal_1993a>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Coal as a source rock of petroleum and gas-a comparison between natural and artificial maturation of the Almond Formation coals, Greater Green River basin in Wyoming, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 405-437.</ref><ref name=Garciagonzalesetal_1993b>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Mechanisms of petroleum generation from coal, as evidenced from petrographic and geochemical studies: Examples from Almond Formation coals in the Greater Green River basin, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 311-323.</ref><ref name=Macgowanetal_1993>MacGowan, D. B., M. Garcia-Gonzales, D. R. Britton, and R. C. Surdam, 1993, Timing of hydrocarbon generation, organic-inorganic diagenesis, and the formation of abnormally pressured gas compartments in the Cretaceous of the Greater Green River basin: A geochemical model, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 325-357.</ref><ref name=Hunt_1996 /> Under these conditions of changing fluid volume and pressure, the capillary pressure of the water-wet pore system is exceeded, and, like pore pressures in direct systems, the high pressures forcibly expel mobile, free water from the pore system, replacing water with gas, and the development of an overpressured BCGA ensues. An additionally important aspect of this phase is the necessity for the presence of an effective lithologic top seal in reservoirs formerly occupied by discrete oil accumulations.

+

In contrast to direct systems, indirect systems require a liquid-prone source rock ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Reservoir quality in indirect systems is assumed to have been better than in direct systems. In this case, oil and gas are generated and expelled and migrate to reservoirs where they accumulate in structural and stratigraphic traps as discrete, buoyant accumulations with downdip water contacts. With subsequent burial and exposure to higher temperatures, the accumulated oil undergoes thermal cracking to gas, accompanied by a significant increase of fluid volume and pressures.<ref name=Barker_1990>Barker, C., 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0008/0000/1254.htm Calculated volume and pressure change during the thermal cracking of oil to gas in reservoirs]: AAPG Bulletin, v. 74, p. 1254-1261.</ref> The level of thermal maturity at which oil is transformed to gas is commonly thought to be about 1.35% [[vitrinite reflectance]] (R<sub>o</sub>);<ref name=Tissotandwelte_1984>Tissot, B. P., and D. H. Welte, 1984, Petroleum formation and occurrence, 2d rev. ed.: Berlin, Springer-Verlag, 699 p.</ref><ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2d ed.: New York, W. H. Freeman and co., 743 p.</ref> however, some evidence indicates that the transformation may occur at higher levels of thermal maturity. Alternatively, gas derived from thermally cracked oil within a source rock may subsequently be expelled and migrate to low-permeability reservoirs.<ref name=Garciagonzalesetal_1993a>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Coal as a source rock of petroleum and gas-a comparison between natural and artificial maturation of the Almond Formation coals, Greater Green River basin in Wyoming, ''in'' D. G. Howell, ed., [http://pubs.er.usgs.gov/publication/pp1570 The future of energy gases]: U.S. Geological Survey Professional Paper 1570, p. 405-437.</ref><ref name=Garciagonzalesetal_1993b>Garcia-Gonzales, M., D. B. MacGowan, and R. C. Surdam, 1993, Mechanisms of petroleum generation from coal, as evidenced from petrographic and geochemical studies: Examples from Almond Formation coals in the Greater Green River basin, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 311-323.</ref><ref name=Macgowanetal_1993>MacGowan, D. B., M. Garcia-Gonzales, D. R. Britton, and R. C. Surdam, 1993, Timing of hydrocarbon generation, organic-inorganic diagenesis, and the formation of abnormally pressured gas compartments in the Cretaceous of the Greater Green River basin: A geochemical model, ''in'' B. Strook and S. Andrew, eds., Wyoming Geological Association Jubilee Anniversary Field Conference Guidebook, p. 325-357.</ref><ref name=Hunt_1996 /> Under these conditions of changing fluid volume and pressure, the capillary pressure of the water-wet pore system is exceeded, and, like pore pressures in direct systems, the high pressures forcibly expel mobile, free water from the pore system, replacing water with gas, and the development of an overpressured BCGA ensues. An additionally important aspect of this phase is the necessity for the presence of an effective lithologic top seal in reservoirs formerly occupied by discrete oil accumulations.

==Phase III==

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