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Basin-centered gas systems: development (view source)
Revision as of 23:48, 8 January 2015
, 23:48, 8 January 2015Created page with "{{publication | image = Nov2002.jpg | width = 120px | series = ''AAPG Bulletin,'' November 2002 | title = Basin-centered gas systems | part = | chapter = |..."
{{publication
| image = Nov2002.jpg
| width = 120px
| series = ''AAPG Bulletin,'' November 2002
| title = Basin-centered gas systems
| part =
| chapter =
| frompg = 1891
| topg = 1919
| author = Ben E. Law
| link = http://archives.datapages.com/data/bulletns/2002/11nov/1891/1891.htm
| pdf =
| store =
| 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.
[[file:BasinCenteredGasFig1.jpg|thumb|300px|{{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.]]
==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 ([[: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: 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.
==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).
{| class = "wikitable"
|-
|+ {{table number|1}}Selected areas or basins containing known or suspected basin-centered gas systems
|-
! Area || Level of certainty || Age || Type of system || Reference
|-
| colspan=5 | <div style="text-align: center;">'''NORTH AMERICA'''</div>
|-
| Colville basin, Alaska || High || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Central Alaska basins || Low/Moderate || ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| 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 />
|-
| Alberta basin, Canada || High || Cretaceous || Direct || Masters<ref name=Masters_1979 /><ref name=Masters_1984 />
|-
| 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 />
|-
| Columbia basin, Washington || High || Tertiary || Direct || Law et al.,<ref name=Lawetal_1994 /> Law<ref name=Law_1996 />
|-
| Modoc Plateau, California || Low/Moderate || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Sacramento/San Joaquin basins, California || Low/Moderate || Cretaceous || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Great Basin, Nevada || Low || Tertiary ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Snake River Plain, Idaho || Low/Moderate || Tertiary ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Big Horn basin, Wyoming || High || Lower Tertiary/Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_1999 />
|-
| asdf
|}
===Indirect systems===
asdf
==Phase III==
asdf
==Phase IV==
asdf
==References==
{{reflist}}
==See also==
* [[Basin-centered gas]]
* [[Basin-centered gas systems: historical development and classification]]
* [[Basin-centered gas systems]]
* [[Basin-centered gas systems: elements and processes]]
* [[Basin-centered gas systems: examples]]
* [[Basin-centered gas systems: gas resources]]
* [[Basin-centered gas systems: global distribution]]
* [[Basin-centered gas systems: evaluation and exploration strategies]]
* [[Tight gas reservoirs: evaluation]]
==External links==
{{search}}
| image = Nov2002.jpg
| width = 120px
| series = ''AAPG Bulletin,'' November 2002
| title = Basin-centered gas systems
| part =
| chapter =
| frompg = 1891
| topg = 1919
| author = Ben E. Law
| link = http://archives.datapages.com/data/bulletns/2002/11nov/1891/1891.htm
| pdf =
| store =
| 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.
[[file:BasinCenteredGasFig1.jpg|thumb|300px|{{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.]]
==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 ([[: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: 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.
==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).
{| class = "wikitable"
|-
|+ {{table number|1}}Selected areas or basins containing known or suspected basin-centered gas systems
|-
! Area || Level of certainty || Age || Type of system || Reference
|-
| colspan=5 | <div style="text-align: center;">'''NORTH AMERICA'''</div>
|-
| Colville basin, Alaska || High || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Central Alaska basins || Low/Moderate || ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| 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 />
|-
| Alberta basin, Canada || High || Cretaceous || Direct || Masters<ref name=Masters_1979 /><ref name=Masters_1984 />
|-
| 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 />
|-
| Columbia basin, Washington || High || Tertiary || Direct || Law et al.,<ref name=Lawetal_1994 /> Law<ref name=Law_1996 />
|-
| Modoc Plateau, California || Low/Moderate || Cretaceous || Direct ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Sacramento/San Joaquin basins, California || Low/Moderate || Cretaceous || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Great Basin, Nevada || Low || Tertiary ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Snake River Plain, Idaho || Low/Moderate || Tertiary ? || ? || Popov et al.<ref name=Popovetal_2001 />
|-
| Big Horn basin, Wyoming || High || Lower Tertiary/Cretaceous || Direct || Johnson et al.<ref name=Johnsonetal_1999 />
|-
| asdf
|}
===Indirect systems===
asdf
==Phase III==
asdf
==Phase IV==
asdf
==References==
{{reflist}}
==See also==
* [[Basin-centered gas]]
* [[Basin-centered gas systems: historical development and classification]]
* [[Basin-centered gas systems]]
* [[Basin-centered gas systems: elements and processes]]
* [[Basin-centered gas systems: examples]]
* [[Basin-centered gas systems: gas resources]]
* [[Basin-centered gas systems: global distribution]]
* [[Basin-centered gas systems: evaluation and exploration strategies]]
* [[Tight gas reservoirs: evaluation]]
==External links==
{{search}}