Basin-centered gas systems: development

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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]
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Indirect systems

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Phase III

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Phase IV

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References

  1. ↑ 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.
  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. ↑ 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. ↑ 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.
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Law_1984
  6. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Johnsonetal_1996
  7. ↑ 7.0 7.1 Cite error: Invalid <ref> tag; no text was provided for refs named Johnsonetal_1999
  8. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Johnsonetal_1987
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 Cite error: Invalid <ref> tag; no text was provided for refs named Popovetal_2001
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Smith_1994
  11. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Masters_1979
  12. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Masters_1984
  13. ↑ 13.0 13.1 Cite error: Invalid <ref> tag; no text was provided for refs named Law_1996
  14. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Lawetal_1994

See also

External links

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