Changes

==Phase II==

==Phase II==

===Direct systems===

===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>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).

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"

{| class = "wikitable"