Difference between revisions of "Basin-centered gas systems: development"

	Basin-centered gas systems
Series	AAPG Bulletin, November 2002
Author	Ben E. Law
Link	Web page

Revision as of 22:00, 13 January 2015

The developmental history of a basin-centered gas system (BCGS) may be viewed as four reservoir pressure cycles. As a consequence of the dynamic nature of geologic processes and the response to those processes, the phases discussed here are geologically ephemeral. Figure 1 is a diagrammatic representation showing these pressure phases and the development of direct and indirect BCGSs. Meissner^[1] and Law and Dickinson^[2] discussed these phase changes for gas accumulations in low-permeability reservoirs.

Figure 1 Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.

Phase I

Direct and indirect systems

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 (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^[3] 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^[4] 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.

Phase II

Direct systems

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 (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,^[5] Wind River,^[6] Big Horn,^[7] and Piceance basins^[8] in the Rocky Mountain region of the United States and the Taranaki Basin in New Zealand (B. E. Law, 2000, unpublished data) (Table 1).

**Table 1** Selected areas or basins containing known or suspected basin-centered gas systems
Area	Level of certainty	Age	Type of system	Reference
NORTH AMERICA
Colville basin, Alaska	High	Cretaceous	Direct ?	Popov et al.^[9]
Central Alaska basins	Low/Moderate	?	?	Popov et al.^[9]
Cook Inlet, Alaska	Low	pre-Tertiary	?	Popov et al.^[9]
Norton Basin, Alaska	High	Eocene/Paleocene	Direct	Smith^[10]
Alberta basin, Canada	High	Cretaceous	Direct	Masters^[11] ^[12]
Charlotte-Georgia Basin, Canada	Low/Moderate	Tertiary/Cretaceous	Direct ?
Willamette-Puget Sound Trough, Washington and Oregon	Moderate/High	Tertiary	Direct ?	Law,^[13] Popov et al.^[9]
Columbia basin, Washington	High	Tertiary	Direct	Law et al.,^[14] Law^[13]
Modoc Plateau, California	Low/Moderate	Cretaceous	Direct ?	Popov et al.^[9]
Sacramento/San Joaquin basins, California	Low/Moderate	Cretaceous	?	Popov et al.^[9]
Great Basin, Nevada	Low	Tertiary ?	?	Popov et al.^[9]
Snake River Plain, Idaho	Low/Moderate	Tertiary ?	?	Popov et al.^[9]
Big Horn basin, Wyoming	High	Lower Tertiary/Cretaceous	Direct	Johnson et al.^[7]
Wind River basn, Wyoming	High	Cretaceous	Direct	Johnson et al.^[6]
Greater Green River basin, Wyoming	High	Lower Tertiary/Cretaceous	Direct	Law et al.,^[15] Law et al.,^[16] McPeek,^[17] Law,^[5] Law et al.^[18]
Hanna basin, Wyoming	High	Cretaceous	Direct	Popov et al.,^[9] Wilson et al.^[19]
Powder River basin, Wyoming	High	Cretaceous	?	Surdam et al.,^[20] Maucione et al.^[21]
Wasatch Plateau, Utah	Moderate/High	Cretaceous	Direct	Popov et al.^[9]
Uinta basin, Utah	High	Lower Tertiary/Cretaceous	Direct	Fouch et al.,^[22] Fouch and Schmoker,^[23] Popov et al.^[9]
Piceance basin, Colorado	High	Cretaceous	Direct	Johnson et al.,^[8] Spencer,^[24] Spencer^[25]
South Park basin, Colorado	Moderate/High	Cretaceous	Direct/Indirect	Popov et al.^[9]
Raton basin, New Mexico and Colorado	High	Tertiary/Cretaceous	Direct/Indirect	Johnson and Finn,^[26] Popov et al.^[9]
Denver basin, Colorado	High	Cretaceous	Direct/Indirect	Higley et al.,^[27] Popov et al.^[9]
San Juan basin, New Mexico and Colorado	High	Cretaceous	Direct	Silver,^[28] Masters,^[11] Huffman^[29]
Permian basin, New Mexico	High	Permian	Indirect/Direct	Broadhead,^[30] Popov et al.^[9]
Albuquerque basin, New Mexico	Moderate/High	Cretaceous	Direct	Johnson et al.^[31] Popov et al.^[9]
Anadarko basin, Oklahoma	High	Pennsylvanian	Indirect	Al-Shaieb et al.,^[32] Popov et al.^[9]
Midcontinent Rift, Minnesota and Iowa	Low/Moderate	Precambrian	Indirect/Direct	Popov et al.^[9]
Arkoma basin, Arkansas and Oklahoma	High	Pennsylvanian	Direct	Meckel et al.,^[33] Popov et al.^[9]
Gulf Coast, United States	High	Cretaceous	Indirect	Popov et al.^[9]
East Texas basin, Texas	High	Jurassic	Indirect ?	Montgomery and Karlewiz,^[34] Emme and Stancil^[35]
Black Warrior basin, Alabama and Mississippi	Moderate/High	Pennsylvanian	Direct	Popov et al.^[9]
Michigan basin, Michigan	Low/Moderate	Ordovician	?	Popov et al.^[9]
Appalachian basin, eastern United States	High	Silurian/Devonian	Indirect	Davis,^[36] Law and Spencer,^[37] Law and Spencer,^[3] Popov et al.,^[9] Ryder and Zagorski^[38]
SOUTH AMERICA
Chaco basin, Bolivia	Moderate	Devonian	?	Williams et al.^[39]
Neuquen basin, Argentina	High	?	?	Fernandez-Sevesco and Surdam^[40]
EUROPE
Timan-Pechora basin, Russa	High	Permian	Direct	Law et al.^[41]
Dnieper-Donets basin, Ukraine	High	Carboniferous	Direct	Law et al.^[42]
West Netherlands basin, Netherlands	Indeterminate	?	?
Vlieland basin, Netherlands	Indeterminate	?	?
Polish basin, Poland	Indeterminate	?	?
Upper Silesian basin, Poland	Indeterminate	?	?
Bekes basin, Hungary	Moderate/High	Miocene	?	Spencer et al.^[4]
German basin, Germany	Indeterminate	?	?
Ruhr basin, Germany	Indeterminate	?	?
Thuringian basin, Germany	Indeterminate	?	?
Subhercynian basin, Germany	Indeterminate	?	?
Lower Saxony basin, Germany	Indeterminate	?	?
Saar-Nahe basin, Germany and France	Indeterminate	?	?
Rhine graben, Germany and France	Indeterminate	?	?
Nord-pas-de-Calais basin, France	Indeterminate	?	?
Lorraine basin, France	Indeterminate	?	?
Bresse basin, France	Indeterminate	?	?
Southeast basin, France	Indeterminate	?	?
Vienna basin, Austria and Slovakia	Indeterminate	?	?
Alpine Foreland basin, Switzerland	High	Permian/Carboniferous	Direct	Schegg et al.^[43]
ASIA-PACIFIC
Sichuan basin, China	High	Permian/Triassic	Direct ?	Da-jun and Yun-ho,^[44] Ryder et al.^[45]
Ordos Basin, China	High	Permian	?
Jungar basin, China	High	Permian	?	Zha et al.^[46]
Taranaki Basin, New Zealand	High	Eocene	Direct
Gippsland Basin, Australia	Moderate	Lower Tertiary/Cretaceous	Direct	Stainforth^[47]
Barrow Subbasin, Australia	High	Jurassic	?	He and Middleton^[48]
Perth basin (onshore), Australia	Moderate	Jurassic	?	Crostella^[49]
Carnarvon Basin, Australia	Low/Moderate	Permian	?	Crostella^[50]
Khorat Plateau basin, Thailand-Laos	Low	Triassic/Jurassic	?	Smith and stokes^[51]
SOUTH ASIA
Vendian basin, India	Low/Moderate	Precambrian	?
Suliaman range foreland, Pakistan	Low	Cretaceous	Direct ?
MIDDLE EAST
Risha area, Jordan	High	Ordovician	Indirect	Ahlbrandt et al.^[52]
AFRICA
Ahnet basin, Algeria	High	Cambrian/Ordovician	Indirect
Benue trough, Nigeria	Moderate/High	Cretaceous	Direct	Obaje and Abaa^[53]

Indirect systems

In contrast to direct systems, indirect systems require a liquid-prone source rock (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 (Barker^[54]). The level of thermal maturity at which oil is transformed to gas is commonly thought to be about 1.35% vitrinite reflectance (Ro) (Tissot and Welte,^[55] Hunt^[56]); however, some evidence, discussed in a following section, 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 (Garcia-Gonzales et al.,^[57]^[58] MacGowan et al.,^[59] Hunt^[56]). 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

At the point where direct and indirect systems are in the overpressured phase (phase II), the processes involved in the transition to phase III are identical for both systems (Figure 1). Phase III occurs when the overpressured phase of direct and indirect systems evolves into underpressured conditions. Both systems, subsequent to the phase II history of overpressure, may experience a period of uplift and erosional unloading and/or heat flow perturbations. During, or subsequent to, these burial and thermal history disruptions, some gas is lost from the accumulation, and the overpressured gas reservoirs are subjected to reduced temperatures. The loss of gas, in conjunction with reduced temperatures, effectively results in the development of an underpressured BCGA (Meissner,^[1] Law and Dickinson^[2]). During this pressure transition, Meissner^[60] emphasized the importance of gas loss over temperature reduction as the dominant process.

Conjectural evidence concerning the integrity of seals in direct vs. indirect systems implies that gas is lost more easily from direct BCGAs than from indirect BCGAs. Johnson et al.^[61] have shown that gas in conventionally trapped accumulations in several Rocky Mountain basins originated from BCGAs, demonstrating that loss of gas through relative permeability, capillary pressure seals does occur. Examples of underpressured, phase III direct systems include Cretaceous rocks in the San Juan, Raton, and Denver basins, and examples of underpressured, phase III indirect systems include Lower Silurian reservoirs in the Appalachian basin, Ordovician reservoirs in the Risha area of eastern Jordan, and Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria (Table 1).

Phase IV

Phase IV is theoretical and may be more applicable to direct systems because of the perceived, relatively better quality of seals in indirect systems than seals in direct systems. During phase IV, continued loss of gas from capillary pressure seals in BCGAs is accompanied by water slowly reentering underpressured, gas-bearing reservoirs. Under these conditions, Meissner^[1] and Law and Dickinson^[2] hypothesized that the underpressured, gas-bearing reservoirs would eventually evolve into normally pressured, water-bearing reservoirs, thus completing the pressure cycle.

References

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↑ ^2.0 ^2.1 ^2.2 Law, B. E., and W. W. Dickinson, 1985, A conceptual model for the origin of abnormally pressured gas accumulations in low-permeability reservoirs: AAPG Bulletin, v. 69, p. 1295-1304.
↑ ^3.0 ^3.1 Law, B. E., and C. W. Spencer, 1998, Abnormal pressure in hydrocarbon environments, in B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: AAPG Memoir 70, p. 1-11.
↑ ^4.0 ^4.1 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.
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↑ 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 oabnormally 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.
↑ Meissner, F. F., 2000, Causes of anomalous deep basin fluid pressures in Rocky Mountain basins and their relation to regional gas accumulation, in 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 11 p.
↑ Johnson, R. C., D. D. Rice, and T. D. Fouch, 1994, Evidence for gas migration from Cretaceous basin-centered accumulations into lower Tertiary reservoirs in Rocky Mountain basins: Proceedings of the Rocky Mountain Association of Geologists and Colorado Association First Biennial Conference, Natural Gas in the Western United States, 8 p.

External links

find literature about
Basin-centered gas systems: development

[Meissner_1978-1] 1.0 ^1.1 ^1.2 Meissner, F. Fm 1978, Patterns of source-rock maturity in non-marine source rocks of some typical western interior basins in non-marine Tertiary and Upper Cretaceous source rocks and the occurrence of oil and gas in the west central U.S.:Roky Mountain Association of Geologists Continuing Education Notes, unpaginated.

[Lawanddickinson_1985-2] 2.0 ^2.1 ^2.2 Law, B. E., and W. W. Dickinson, 1985, A conceptual model for the origin of abnormally pressured gas accumulations in low-permeability reservoirs: AAPG Bulletin, v. 69, p. 1295-1304.

[Lawandspencer_1998-3] 3.0 ^3.1 Law, B. E., and C. W. Spencer, 1998, Abnormal pressure in hydrocarbon environments, in B. E. Law, G. F. Ulmishek, and V. I. Slavin, eds., Abnormal pressures in hydrocarbon environments: AAPG Memoir 70, p. 1-11.

[Spenceretal_1994-4] 4.0 ^4.1 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.

[Law_1984-5] 5.0 ^5.1 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.

[Johnsonetal_1996-6] 6.0 ^6.1 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.

[Johnsonetal_1999-7] 7.0 ^7.1 Johnson, R. C., R. A. Crovelli, B. G. Lowell, and T. M. Finn, 1999, 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.

[Johnsonetal_1987-8] 8.0 ^8.1 Johnson, R. C., R. A. Crovelli, C. W. Spencer, and R. F. Mast, 1987, 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.

[Popovetal_2001-9] 9.00 ^9.01 ^9.02 ^9.03 ^9.04 ^9.05 ^9.06 ^9.07 ^9.08 ^9.09 ^9.10 ^9.11 ^9.12 ^9.13 ^9.14 ^9.15 ^9.16 ^9.17 ^9.18 ^9.19 ^9.20 ^9.21 ^9.22 Popov, M. A., V. F. Nuccio, T. S. Dyman, T. A. Gognat, R. C. Johnson, J. W. Schmoker, M. S. Wilson, and C. Bartberger, 2001, Basin-centered gas systems of the U.S.:U.S. Geological Survey Open-File Report 01-135, Version 1.0, 1 CD-ROM.

[Smith_1994-10] Smith, J. T., 1994, 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.

[Masters_1979-11] 11.0 ^11.1 Masters, J. A., 1979, Deep basin gas trap, western Canada: AAPG Bulletin, v. 63, p. 152-181.

[Masters_1984-12] Masters, J. A., ed., 1984, Elmworth-case study of a deep basin gas field: AAPG Memoir 38, 316 p.

[Law_1996-13] 13.0 ^13.1 Law, B. E., 1996, Southwestern Wyoming province (037), in D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds., 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.

[Lawetal_1994-14] Law, B. E., M. E. Tennyson, and S. Y. Johnson 1994, Basin-centered gas accumulations in the Pacific Northwest-a potentially large source of energy (abs.): AAPG Annual Meeting Program, v. 3, p. 194.

[Lawetal_1979-15] 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.

[Lawetal_1980-16] 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.

[Mcpeek_1981-17] McPeek, L. A., 1981, Eastern Green River basin-a developing giant gas supply from deep, overpressured Upper Cretaceous sandstones: AAPG Bulletin, v. 65, p. 1978-1098.

[Lawetal_1989-18] 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.

[Wilsonetal_2001-19] Wilson, M. S., T. S. Dyman, and V. F. Nuccio, 2001, Potential for deep basin-centered gas accumulations in Hanna basin, Wyoming: U.S. Geological Survey Bulletin 2184-A, 12 p.

[Surdametal_1994-20] Surdam, R. C., Z. S. Jiao, and R. S. Martinsen, 1994, 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.

[Maucioneetal_1994-21] Maucion, D., V. Serebryakov, P. Valasek, Y. Wang, and S. Smithson, 1994, 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.

[Fouchetal_1992-22] Fouch, T. D., V. F. Nuccio, J. C. Osmond, L. MacMillan, W. B. Cachion, and C. J. Wandrey, 1992, Oil and gas in uppermost Cretaceous and Tertiary rock, Uinta basin, Utah, in T. D. Fouch, V. F. Nuccio, and T. C. Chidsey Jr., eds., Hydrocarbon and mineral resources of the Uinta basin, Utah and Colorado: Utah Geological Association Guidebook 20, p. 9-47.

[Fouchandschmoker_1996-23] Fouch, T. D., and J. W. Schmoker, 1996, Tight gas plays of the Uinta basin, in D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds, 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.

[Spencer_1987-24] Spencer, C. W., 1987, Hydrocarbon generation as a mechanism for overpressuring in Rocky Mountain region: AAPG Bulletin, v. 71, p. 368-388.

[Spencer_1989a-25] Spencer, C. W., 1989, Review of characteristics of low-permeability gas reservoirs in western United States: AAPG Bulletin, v. 73, p. 613-629.

[Johnsonandfinn_2001-26] Johnson, R. C., and T. M. Finn, 2001, Potential for basin-centered gas accumulation in the Raton basin, Colorado and New Mexico: U.S. Geological Survey Bulletin 2184-B, 14 p.

[Higleyetal_1992-27] Higley, D. K., D. L. Gautier, and M. J. Pawlewicz, 1992, Influence of regional head flow variation on thermal maturity of the Lower Cretaceous Muddy ("J") Sandstone, Denver basin, Colorado, in The petroleum system-status of research and methods, 1992: U.S. Geological Survey Bulletin 2007, p. 66-69.

[Silver_1950-28] 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.

[Huffman_1996-29] Huffman, A. C., 1996, San Juan basin province, in D. L. Gautier, G. L. Dolton, K. I. Takahashi, and K. L. Varnes, eds, 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.

[Broadhead_1984-30] 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.

[Johnsonetal_2001-31] Johnson, R. C., T. M. Finn, and V. F. Nuccio, 2001, Potential for basin-centered gas accumulation in the Albuquerque basin, New Mexico: U.S. Geological Survey Bulletin 2184-C, 21 p.

[Alshaiebetal_1994-32] Al-Shaieb, Z., J. Puckette, A. Abdalla, and P. B. Ely, 1994, 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.

[Meckeletal_1992-33] Meckel, L. D., D. G. Smith, and L. A. Wells, 1992, 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.

[Montgomeryandkarlewicz_2001-34] Montgomery, S. L., and R. Karlewicz, 2001, Bossier play has room to grow: Oil & Gas Journal, v. 99, no. 5, p. 36-42.

[Emmeandstancil_2002-35] Emme, J., and B. Stancil, 2002, Anadarko's Bossier gas play-a sleeping giant in a mature basin (abs.): AAPG Annual Meeting Program, v. 11, p. A50.

[Davis_1984-36] Davis, T. B., 1984, 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.

[Lawandspencer_1993-37] 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.

[Ryderandzagorski_2003-38] Ryder, R. T., and W. A. Zagorski, 2003, 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.

[Williamsetal_1995-39] Williams, K. E., B. J. Radovich, and J. W. Brett, 1995, Exploration for deep gas in the Devonian Chaco basin of southern Bolivia: Sequence stratigraphy, prediction, and well results (abs.): AAPG Annual Convention, Abstracts with Program, v. 4, p. A104.

[Fernandezsevescoandsurdam_1997-40] Fernandez-Sevesco, F., and R. C. Surdam, 1997, A new exploration approach in the search for anomalously pressured hydrocarbon accumulations in the Neuquen Basin, Argentina (abs.): AAPG International Conference and Exhibition Program, p. A16.

[Lawetal_1996-41] Law, B. E., V. I. Bogatsky, S. . Danilevsky, L. V. Galkina, G. F. Ulmishek, and C. W. Spencer, 1996, Basin-centered gas accumulations in the Timan-Pechora basin, Russia (abs.): AAPG Annual Convention Program, v. 5, p. A81.

[Lawetal_1998b-42] Law, B. E., G. F. Ulmishek, B. P. Kabyshev, N. T. Pashova, and V. A. Krivosheya, 1998, Basin-centered gas evaluated in Dnieper-Donets basin, Donbas foldbelt, Ukraine: Oil & Gas Journal, v. 100, no. 30, p. 74-78.

[Scheggetal_1997-43] 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.

[Dajunandyunho_1994-44] Da-jun, P., and L. Yun-ho, 1994, Genetic mechanism of abnormal pressure, pressure seals, and natural gas accumulation in carbonate reservoirs, Sichuan basin: Proceedings of the AAPG Research Conference Abnormal Pressures in Hydrocarbon Environments, unpaginated.

[Ryderetal_1994-45] Ryder, R. T., D. D. Rice, Z. Sun, Y. Zhanz, Y. Qui, and Z. Guo, 1994, Petroleum geology of the Sichuan basin China-report on U.S. Geological Survey and Chinese Ministry of Geology and Mineral Resources, field investigation and meeting, October 1991: U.S. Geological Survey Open-File Report 94-426, 67 p.

[Zhaetal_1999-46] Zha, M., W. Zhang, and Q. Jiangxiu, 1999, Overpressured compartments in Junggar basin, northwest of China: Mechanisms and hydrocarbon distribution (abs.): AAPG Annual Convention, Program with Abstracts, v. 8, p. A158-159.

[Stainforth_1984-47] Stainforth, J.G., 1984, Gippsland hydrocarbons-a perspective from the basin edge: Australian Petroleum Exploration Association Journal, v. 24, p. 91-100.

[Heandmiddleton_2002-48] He, S., and M. Middleton, 2002, Pressure seal and modeling of the Jurassic overpressure in the Barrow Sub-basin, northwest shelf of Australia (abs.): AAPG Annual Meeting Program, v. 11, p. A74.

[Crostella_1995a-49] Crostella, A., 1995, An evaluation of the hydrocarbon potential of the onshore Northern Perth basin, western Australia: Geological Survey of Western Australia Report 43, 67 p.

[Crostella_1995b-50] Crostella, A., 1995, Structural evolution and hydrocarbon potential of the Merlinliegh and Byro sub-basins, Canarvon basin, western Australia: Geological Survey of Western Australia Report 45, 35 p.

[Smithandstokes_1997-51] Smith, P. F. L., and R. B. Stokes, 1997, Geology and petroleum potential of the Khorat Plateau Basin in the Vientiane area of Lao P.D.R.: Journal of Petroleum Geology, v. 20, p. 27-50.

[Ahlbrandtetal_1997-52] Ahlbrandt, T. S., O. A. Okasheh, and M. Lewan, 1997, A Middle East basin center hydrocarbon accumulation in Paleozoic rocks, eastern Jordan, western Iraq, and surrounding areas (abs.): AAPG International Conference and Exhibition, p. A1-A2.

[Obajeandabaa_1996-53] 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.

[Barker_1990-54] Barker, C., 1990, Calculated volume and pressure change during the thermal cracking of oil to gas in reservoirs: AAPG Bulletin, v. 74, p. 1254-1261.

[Tissotandwelte_1984-55] Tissot, B. P., and D. H. Welte, 1984, Petroleum formation and occurrence, 2d rev. ed.: Berlin, Springer-Verlag, 699 p.

[Hunt_1996-56] 56.0 ^56.1 Hunt, J. M., 1996, Petroleum geochemistry and geology, 2d ed.: New York, W. H. Freeman and co., 743 p.

[Garciagonzalesetal_1993a-57] 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., The future of energy gases: U.S. Geological Survey Professional Paper 1570, p. 405-437.

[Garciagonzalesetal_1993b-58] 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.

[Macgowanetal_1993-59] 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 oabnormally 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.

[Meissner_2000-60] Meissner, F. F., 2000, Causes of anomalous deep basin fluid pressures in Rocky Mountain basins and their relation to regional gas accumulation, in 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 11 p.

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@@ Line 14: / Line 14: @@
   | isbn    =
 }}
-The developmental history of a basin-centered gas system (BCGS) may be viewed as four reservoir pressure cycles. As a consequence of the dynamic nature of geologic processes and the response to those processes, the phases discussed here are geologically ephemeral. [[:file:BasinCenteredGasFig1.jpg|Figure 1]] is a diagrammatic representation showing these pressure phases and the development of direct and indirect BCGSs. Meissner<ref name=Meissner_1978>Meissner, F. Fm 1978, Patterns of source-rock maturity in non-marine source rocks of some typical western interior basins in non-marine Tertiary and Upper Cretaceous source rocks and the occurrence of oil and gas in the west central U.S.:Roky Mountain Association of Geologists Continuing Education Notes, unpaginated.</ref> and Law and Dickinson<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> discussed these phase changes for gas accumulations in low-permeability reservoirs.
+The developmental history of a basin-centered gas system (BCGS) may be viewed as four reservoir pressure cycles. As a consequence of the dynamic nature of geologic processes and the response to those processes, the phases discussed here are geologically ephemeral. [[:file:BasinCenteredGasFig1.jpg|Figure 1]] is a diagrammatic representation showing these pressure phases and the development of direct and indirect BCGSs. Meissner<ref name=Meissner_1978>Meissner, F. Fm 1978, Patterns of source-rock maturity in non-marine source rocks of some typical western interior basins in non-marine Tertiary and Upper [[Cretaceous]] source rocks and the occurrence of oil and gas in the west central U.S.:Roky Mountain Association of Geologists Continuing Education Notes, unpaginated.</ref> and Law and Dickinson<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> discussed these phase changes for gas accumulations in low-permeability reservoirs.
 [[file:BasinCenteredGasFig1.jpg|thumb|400px|{{figure number|1}}Schematic diagram showing evolution of direct and indirect basin-centered gas systems. Evolutionary phases are shown along the side of each system.]]
@@ Line 26: / Line 26: @@
 ==Phase II==
 ===Direct systems===
-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 /> Wind River,<ref name=Johnsonetal_1996 /> Big Horn,<ref name=Johnsonetal_1999 /> and Piceance basins<ref name=Johnsonetal_1987 /> in the Rocky Mountain region of the United States and the Taranaki Basin in New Zealand (B. E. Law, 2000, unpublished data) (Table 1).
+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"
@@ Line 36: / Line 36: @@
 | colspan=5 | <div style="text-align: center;">'''NORTH AMERICA'''</div>
 |-
-| Colville basin, Alaska || High || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
+| Colville basin, Alaska || High || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001>Popov, M. A., V. F. Nuccio, T. S. Dyman, T. A. Gognat, R. C. Johnson, J. W. Schmoker, M. S. Wilson, and C. Bartberger, 2001, Basin-centered gas systems of the U.S.:U.S. Geological Survey Open-File Report 01-135, Version 1.0, 1 CD-ROM.</ref>
 |-
 | Central Alaska basins || Low/Moderate || ? || ? || Popov et al.<ref name=Popovetal_2001 />
@@ Line 42: / Line 42: @@
 | Cook Inlet, Alaska || Low || pre-Tertiary || ? || Popov et al.<ref name=Popovetal_2001 />
 |-
-| Norton Basin, Alaska || High || Eocene/Paleocene || Direct || Smith<ref name=Smith_1994 />
+| 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>
 |-
-| Alberta basin, Canada || High || Cretaceous || Direct || Masters<ref name=Masters_1979 /><ref name=Masters_1984 />
+| 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>
 |-
 | Charlotte-Georgia Basin, Canada || Low/Moderate || Tertiary/Cretaceous || Direct ? ||
 |-
-| Willamette-Puget Sound Trough, Washington and Oregon || Moderate/High || Tertiary || Direct ? || Law,<ref name=Law_1996 /> Popov et al.<ref name=Popovetal_2001 />
+| Willamette-Puget Sound Trough, Washington and Oregon || Moderate/High || Tertiary || Direct ? || Law,<ref name=Law_1996>Law, B. E., 1996, Southwestern Wyoming province (037), ''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> Popov et al.<ref name=Popovetal_2001 />
 |-
-| Columbia basin, Washington || High || Tertiary || Direct || Law et al.,<ref name=Lawetal_1994 /> Law<ref name=Law_1996 />
+| Columbia basin, Washington || High || Tertiary || Direct || Law et al.,<ref name=Lawetal_1994>Law, B. E., M. E. Tennyson, and S. Y. Johnson 1994, Basin-centered gas accumulations in the Pacific Northwest-a potentially large source of energy (abs.): AAPG Annual Meeting Program, v. 3, p. 194.</ref> Law<ref name=Law_1996 />
 |-
 | Modoc Plateau, California || Low/Moderate || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
@@ Line 64: / Line 64: @@
 | Wind River basn, Wyoming || High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_1996 />
 |-
-| Greater Green River basin, Wyoming || High || Lower Tertiary/Cretaceous || Direct || Law et al.,<ref name=Lawetal_1979 /> Law et al.,<ref name=Lawetal_1980 /> McPeek,<ref name=Mcpeek_1981 /> Law,<ref name=Law_1984 /> Law et al.<ref name=Lawetal_1989 />
+| 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. 1978-1098.</ref> Law,<ref name=Law_1984 /> 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>
 |-
-| Hanna basin, Wyoming || High || Cretaceous || Direct || Popov et al.,<ref name=Popovetal_2001 /> Wilson et al.<ref name=Wilsonetal_2001 />
+| 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>
 |-
-| Powder River basin, Wyoming || High || Cretaceous || ? || Surdam et al.,<ref name=Surdametal_1994 /> Maucione et al.<ref name=Maucioneetal_1994 />
+| 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>
 |-
 | Wasatch Plateau, Utah || Moderate/High || Cretaceous || Direct || Popov et al.<ref name=Popovetal_2001 />
 |-
-| Uinta basin, Utah || High || Lower Tertiary/Cretaceous || Direct || Fouch et al.,<ref name=Fouchetal_1992 /> Fouch and Schmoker,<ref name=Fouchandschmoker_1996 /> Popov et al.<ref name=Popovetal_2001 />
+| Uinta basin, Utah || High || Lower [[Tertiary]]/Cretaceous || Direct || Fouch et al.,<ref name=Fouchetal_1992>Fouch, T. D., V. F. Nuccio, J. C. Osmond, L. MacMillan, W. B. Cachion, and C. J. Wandrey, 1992, Oil and gas in uppermost Cretaceous and Tertiary rock, Uinta basin, Utah, ''in'' T. D. Fouch, V. F. Nuccio, and T. C. Chidsey Jr., eds., Hydrocarbon and mineral resources of the Uinta basin, Utah and Colorado: Utah Geological Association Guidebook 20, p. 9-47.</ref> Fouch and Schmoker,<ref name=Fouchandschmoker_1996>Fouch, T. D., and J. W. Schmoker, 1996, Tight gas plays of the Uinta basin, ''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> Popov et al.<ref name=Popovetal_2001 />
 |-
-| Piceance basin, Colorado || High || Cretaceous || Direct || Johnson et al.,<ref name=Johnsonetal_1987 /> Spencer,<ref name=Spencer_1987 /> Spencer<ref name=Spencer_1989a />
+| Piceance basin, Colorado || High || Cretaceous || Direct || Johnson et al.,<ref name=Johnsonetal_1987 /> Spencer,<ref name=Spencer_1987>Spencer, C. W., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0004/0350/0368.htm Hydrocarbon generation as a mechanism for overpressuring in Rocky Mountain region]: AAPG Bulletin, v. 71, p. 368-388.</ref> Spencer<ref name=Spencer_1989a>Spencer, C. W., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0005/0600/0613.htm Review of characteristics of low-permeability gas reservoirs in western United States]: AAPG Bulletin, v. 73, p. 613-629.</ref>
 |-
 | South Park basin, Colorado || Moderate/High || Cretaceous || Direct/Indirect || Popov et al.<ref name=Popovetal_2001 />
 |-
-| Raton basin, New Mexico and Colorado || High || Tertiary/Cretaceous || Direct/Indirect || Johnson and Finn,<ref name=Johnsonandfinn_2001 /> Popov et al.<ref name=Popovetal_2001 />
+| Raton basin, New Mexico and Colorado || High || Tertiary/Cretaceous || Direct/Indirect || Johnson and Finn,<ref name=Johnsonandfinn_2001>Johnson, R. C., and T. M. Finn, 2001, [http://pubs.usgs.gov/bul/b2184-b/ Potential for basin-centered gas accumulation in the Raton basin, Colorado and New Mexico]: U.S. Geological Survey Bulletin 2184-B, 14 p.</ref> Popov et al.<ref name=Popovetal_2001 />
 |-
-| Denver basin, Colorado || High || Cretaceous || Direct/Indirect || Higley et al.,<ref name=Higleyetal_1992 /> Popov et al.<ref name=Popovetal_2001 />
+| Denver basin, Colorado || High || Cretaceous || Direct/Indirect || Higley et al.,<ref name=Higleyetal_1992>Higley, D. K., D. L. Gautier, and M. J. Pawlewicz, 1992, Influence of regional head flow variation on thermal maturity of the Lower Cretaceous Muddy ("J") Sandstone, Denver basin, Colorado, ''in'' [http://pubs.usgs.gov/bul/2007/report.pdf The petroleum system-status of research and methods, 1992]: U.S. Geological Survey Bulletin 2007, p. 66-69.</ref> Popov et al.<ref name=Popovetal_2001 />
 |-
-| San Juan basin, New Mexico and Colorado || High || Cretaceous || Direct || Silver,<ref name=Silver_1950 /> Masters,<ref name=Masters_1979 /> Huffman<ref name=Huffman_1996 />
+| 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>
 |-
-| Permian basin, New Mexico || High || Permian || Indirect/Direct || Broadhead,<ref name=Broadhead_1984 /> 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 />
 |-
-| Albuquerque basin, New Mexico || Moderate/High || Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_2001 /> 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 />
 |-
-| Anadarko basin, Oklahoma || High || Pennsylvanian || Indirect || Al-Shaieb et al.,<ref name=Alshaiebetal_1994 /> 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: 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 />
+| Midcontinent Rift, Minnesota and Iowa || Low/Moderate || [[Precambrian]] || Indirect/Direct || Popov et al.<ref name=Popovetal_2001 />
 |-
-| Arkoma basin, Arkansas and Oklahoma || High || Pennsylvanian || Direct || Meckel et al.,<ref name=Meckeletal_1992 /> 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: 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 />
 |-
-| East Texas basin, Texas || High || Jurassic || Indirect ? || Montgomery and Karlewiz,<ref name=Montgomeryandkarlewicz_2001 /> Emme and Stancil<ref name=Emmeandstancil_2002 />
+| East Texas basin, Texas || High || [[Jurassic]] || Indirect ? || Montgomery and Karlewiz,<ref name=Montgomeryandkarlewicz_2001>Montgomery, S. L., and R. Karlewicz, 2001, Bossier play has room to grow: Oil & Gas Journal, v. 99, no. 5, p. 36-42.</ref> Emme and Stancil<ref name=Emmeandstancil_2002>Emme, J., and B. Stancil, 2002, Anadarko's Bossier gas play-a sleeping giant in a mature basin (abs.): AAPG Annual Meeting Program, v. 11, p. A50.</ref>
 |-
 | Black Warrior basin, Alabama and Mississippi || Moderate/High || Pennsylvanian || Direct || 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 />
 |-
-| Appalachian basin, eastern United States || High || Silurian/Devonian || Indirect || Davis,<ref name=Davis_1984 /> Law and Spencer,<ref name=Lawandspencer_1993 /> 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>
+| 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>
 |-
 | colspan=5 | <div style="text-align: center;">'''SOUTH AMERICA'''</div>
 |-
-|ASDF
+|Chaco basin, Bolivia || Moderate || Devonian || ? || Williams et al.<ref name=Williamsetal_1995>Williams, K. E., B. J. Radovich, and J. W. Brett, 1995, Exploration for deep gas in the Devonian Chaco basin of southern Bolivia: Sequence stratigraphy, prediction, and well results (abs.): AAPG Annual Convention, Abstracts with Program, v. 4, p. A104.</ref>
+|-
+| Neuquen basin, Argentina || High || ? || ? || Fernandez-Sevesco and Surdam<ref name=Fernandezsevescoandsurdam_1997>Fernandez-Sevesco, F., and R. C. Surdam, 1997, A new exploration approach in the search for anomalously pressured hydrocarbon accumulations in the Neuquen Basin, Argentina (abs.): AAPG International Conference and Exhibition Program, p. A16.</ref>
+|-
+| colspan=5 | <div style="text-align: center;">'''EUROPE'''</div>
+|-
+| Timan-Pechora basin, Russa || High || Permian || Direct || Law et al.<ref name=Lawetal_1996>Law, B. E., V. I. Bogatsky, S. . Danilevsky, L. V. Galkina, G. F. Ulmishek, and C. W. Spencer, 1996, Basin-centered gas accumulations in the Timan-Pechora basin, Russia (abs.): AAPG Annual Convention Program, v. 5, p. A81.</ref>
+|-
+| Dnieper-Donets basin, Ukraine || High || [[Carboniferous]] || Direct || Law et al.<ref name=Lawetal_1998b>Law, B. E., G. F. Ulmishek, B. P. Kabyshev, N. T. Pashova, and V. A. Krivosheya, 1998, Basin-centered gas evaluated in Dnieper-Donets basin, Donbas foldbelt, Ukraine: Oil & Gas Journal, v. 100, no. 30, p. 74-78.</ref>
+|-
+| West Netherlands basin, Netherlands || Indeterminate || ? || ? ||
+|-
+| Vlieland basin, Netherlands || Indeterminate || ? || ? ||
+|-
+| Polish basin, Poland || Indeterminate || ? || ? ||
+|-
+| Upper Silesian basin, Poland || Indeterminate || ? || ? ||
+|-
+| Bekes basin, Hungary || Moderate/High || Miocene || ? || Spencer et al.<ref name=Spenceretal_1994 />
+|-
+| German basin, Germany || Indeterminate || ? || ? ||
+|-
+| Ruhr basin, Germany || Indeterminate || ? || ? ||
+|-
+| Thuringian basin, Germany || Indeterminate || ? || ? ||
+|-
+| Subhercynian basin, Germany || Indeterminate || ? || ? ||
+|-
+| Lower Saxony basin, Germany || Indeterminate || ? || ? ||
+|-
+| Saar-Nahe basin, Germany and France || Indeterminate || ? || ? ||
+|-
+| Rhine graben, Germany and France || Indeterminate || ? || ? ||
+|-
+| Nord-pas-de-Calais basin, France || Indeterminate || ? || ? ||
+|-
+| Lorraine basin, France || Indeterminate || ? || ? ||
+|-
+| Bresse basin, France || Indeterminate || ? || ? ||
+|-
+| Southeast basin, France || Indeterminate || ? || ? ||
+|-
+| Vienna basin, Austria and Slovakia || Indeterminate || ? || ? ||
+|-
+| 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>
+|-
+| Sichuan basin, China || High || Permian/[[Triassic]] || Direct ? || Da-jun and Yun-ho,<ref name=Dajunandyunho_1994>Da-jun, P., and L. Yun-ho, 1994, Genetic mechanism of abnormal pressure, pressure seals, and natural gas accumulation in carbonate reservoirs, Sichuan basin: Proceedings of the AAPG Research Conference Abnormal Pressures in Hydrocarbon Environments, unpaginated.</ref> Ryder et al.<ref name=Ryderetal_1994>Ryder, R. T., D. D. Rice, Z. Sun, Y. Zhanz, Y. Qui, and Z. Guo, 1994, Petroleum geology of the Sichuan basin China-report on U.S. Geological Survey and Chinese Ministry of Geology and Mineral Resources, field investigation and meeting, October 1991: U.S. Geological Survey Open-File Report 94-426, 67 p.</ref>
+|-
+| Ordos Basin, China || High || Permian || ? ||
+|-
+| Jungar basin, China || High || Permian || ? || Zha et al.<ref name=Zhaetal_1999>Zha, M., W. Zhang, and Q. Jiangxiu, 1999, Overpressured compartments in Junggar basin, northwest of China: Mechanisms and hydrocarbon distribution (abs.): AAPG Annual Convention, Program with Abstracts, v. 8, p. A158-159.</ref>
+|-
+| Taranaki Basin, New Zealand || High || Eocene || Direct ||
+|-
+| Gippsland Basin, Australia || Moderate || Lower Tertiary/Cretaceous || Direct || Stainforth<ref name=Stainforth_1984>Stainforth, J.G., 1984, Gippsland hydrocarbons-a perspective from the basin edge: Australian Petroleum Exploration Association Journal, v. 24, p. 91-100.</ref>
+|-
+| Barrow Subbasin, Australia || High || Jurassic || ? || He and Middleton<ref name=Heandmiddleton_2002>He, S., and M. Middleton, 2002, Pressure seal and modeling of the Jurassic overpressure in the Barrow Sub-basin, northwest shelf of Australia (abs.): AAPG Annual Meeting Program, v. 11, p. A74.</ref>
+|-
+| Perth basin (onshore), Australia || Moderate || Jurassic || ? || Crostella<ref name=Crostella_1995a>Crostella, A., 1995, An evaluation of the hydrocarbon potential of the onshore Northern Perth basin, western Australia: Geological Survey of Western Australia Report 43, 67 p.</ref>
+|-
+| Carnarvon Basin, Australia || Low/Moderate || Permian || ? || Crostella<ref name=Crostella_1995b>Crostella, A., 1995, Structural evolution and hydrocarbon potential of the Merlinliegh and Byro sub-basins, Canarvon basin, western Australia: Geological Survey of Western Australia Report 45, 35 p.</ref>
+|-
+| Khorat Plateau basin, Thailand-Laos || Low || Triassic/Jurassic || ? || Smith and stokes<ref name=Smithandstokes_1997>Smith, P. F. L., and R. B. Stokes, 1997, Geology and petroleum potential of the Khorat Plateau Basin in the Vientiane area of Lao P.D.R.: Journal of Petroleum Geology, v. 20, p. 27-50.</ref>
+|-
+| colspan=5 | <div style="text-align: center;">'''SOUTH ASIA'''</div>
+|-
+| Vendian basin, India || Low/Moderate || Precambrian || ? ||
+|-
+| Suliaman range foreland, Pakistan || Low || Cretaceous || Direct ? ||
+|-
+| colspan=5 | <div style="text-align: center;">'''MIDDLE EAST'''</div>
+|-
+| Risha area, Jordan || High || Ordovician || Indirect || Ahlbrandt et al.<ref name=Ahlbrandtetal_1997>Ahlbrandt, T. S., O. A. Okasheh, and M. Lewan, 1997, A Middle East basin center hydrocarbon accumulation in Paleozoic rocks, eastern Jordan, western Iraq, and surrounding areas (abs.): AAPG International Conference and Exhibition, p. A1-A2.</ref>
+|-
+| colspan=5 | <div style="text-align: center;">'''AFRICA'''</div>
+|-
+| 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>
 |}
 ===Indirect systems===
-asdf
+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 (Barker<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 (Ro) (Tissot and Welte,<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> Hunt<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, discussed in a following section, 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 (Garcia-Gonzales et al.,<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> MacGowan et al.,<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 oabnormally 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> Hunt<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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+At the point where direct and indirect systems are in the overpressured phase (phase II), the processes involved in the transition to phase III are identical for both systems ([[:file:BasinCenteredGasFig1.jpg|Figure 1]]). Phase III occurs when the overpressured phase of direct and indirect systems evolves into underpressured conditions. Both systems, subsequent to the phase II history of overpressure, may experience a period of uplift and erosional unloading and/or heat flow perturbations. During, or subsequent to, these burial and thermal history disruptions, some gas is lost from the accumulation, and the overpressured gas reservoirs are subjected to reduced temperatures. The loss of gas, in conjunction with reduced temperatures, effectively results in the development of an underpressured BCGA (Meissner,<ref name=Meissner_1978 /> Law and Dickinson<ref name=Lawanddickinson_1985 />). During this pressure transition, Meissner<ref name=Meissner_2000>Meissner, F. F., 2000, Causes of anomalous deep basin fluid pressures in Rocky Mountain basins and their relation to regional gas accumulation, ''in'' 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 11 p.</ref> emphasized the importance of gas loss over temperature reduction as the dominant process.
+Conjectural evidence concerning the integrity of seals in direct vs. indirect systems implies that gas is lost more easily from direct BCGAs than from indirect BCGAs. Johnson et al.<ref name=Johnsonetal_1994>Johnson, R. C., D. D. Rice, and T. D. Fouch, 1994, Evidence for gas migration from Cretaceous basin-centered accumulations into lower Tertiary reservoirs in Rocky Mountain basins: Proceedings of the Rocky Mountain Association of Geologists and Colorado Association First Biennial Conference, Natural Gas in the Western United States, 8 p.</ref> have shown that gas in conventionally trapped accumulations in several Rocky Mountain basins originated from BCGAs, demonstrating that loss of gas through relative permeability, capillary pressure seals does occur. Examples of underpressured, phase III direct systems include Cretaceous rocks in the San Juan, Raton, and Denver basins, and examples of underpressured, phase III indirect systems include Lower Silurian reservoirs in the Appalachian basin, Ordovician reservoirs in the Risha area of eastern Jordan, and Cambrian and Ordovician reservoirs in the Ahnet basin of Algeria (Table 1).
 ==Phase IV==
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+Phase IV is theoretical and may be more applicable to direct systems because of the perceived, relatively better quality of seals in indirect systems than seals in direct systems. During phase IV, continued loss of gas from capillary pressure seals in BCGAs is accompanied by water slowly reentering underpressured, gas-bearing reservoirs. Under these conditions, Meissner<ref name=Meissner_1978 /> and Law and Dickinson<ref name=Lawanddickinson_1985 /> hypothesized that the underpressured, gas-bearing reservoirs would eventually evolve into normally pressured, water-bearing reservoirs, thus completing the pressure cycle.
 ==References==