Changes

===Reconnaissance phase===

===Reconnaissance phase===

The reconnaissance phase entails the identification of basins that may contain BCGAs. In direct systems, identification of source rocks is critical. For example, the identification of humic, gas-prone coal beds is the most obvious source rock for direct BCGAs; in nearly every country with coal reserves, there are some published data concerning geographic distribution, rank, and thickness. The rank of coal beds must be greater than high-volatile C (greater than vitrinite reflectance values of 0.6% Ro) to initiate thermal generation of gas.<ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2nd ed.: New York, W. H. Freeman and Co., 743 p.</ref>

The reconnaissance phase entails the identification of basins that may contain BCGAs. In direct systems, identification of source rocks is critical. For example, the identification of humic, gas-prone coal beds is the most obvious source rock for direct BCGAs; in nearly every country with coal reserves, there are some published data concerning geographic distribution, rank, and thickness. The rank of coal beds must be greater than high-volatile C (greater than vitrinite reflectance values of 0.6% R_o) to initiate thermal generation of gas.<ref name=Hunt_1996>Hunt, J. M., 1996, Petroleum geochemistry and geology, 2nd ed.: New York, W. H. Freeman and Co., 743 p.</ref>

The existence of reservoirs with appropriate quality is another important aspect to consider during the reconnaissance phase. In most cases, coal-bearing intervals are associated with interbedded sandstones that have low porosity and permeability, especially at diagenetic stages commensurate with thermal maturity levels greater than 0.6% Ro. Sandstones deposited in alluvial plain, coal-bearing environments typically have poor reservoir properties. High porosity and permeability in reservoirs are not desirable attributes for the development of a BCGA. In basins where some drilling activity has occurred, gas shows are also very helpful.

The existence of reservoirs with appropriate quality is another important aspect to consider during the reconnaissance phase. In most cases, coal-bearing intervals are associated with interbedded sandstones that have low porosity and permeability, especially at diagenetic stages commensurate with thermal maturity levels greater than 0.6% R_o. Sandstones deposited in alluvial plain, coal-bearing environments typically have poor reservoir properties. High porosity and permeability in reservoirs are not desirable attributes for the development of a BCGA. In basins where some drilling activity has occurred, gas shows are also very helpful.

===Confirmation phase===

===Confirmation phase===

The delineation phase entails mapping the vertical and areal distribution of the gas accumulation. The preferred way of accomplishing this phase is through the use of reliable pressure data. In most basins, however, pressure data are absent or of such low quality that reliable maps cannot be constructed; consequently, some indirect method may have to be used. The selected mapping parameter should be one that has been calibrated to well-documented pressure data. For example, thermal maturity values ranging from 0.7 to 0.9% R_o were determined to be coincident with the top of overpressuring in the Greater Green River basin.<ref name=Law_1984 /> In later work, 0.8% R_o was used to map the depth to the top of overpressuring in the basin.<ref name=Pawlewiczetal_1986>Pawlewicz, M. J., M. K. Lickus, B. E. Law, and W. W. Dickinson, 1986, Thermal maturity map showing depth to 0.8% vitrinite reflectance in the Greater Green River basin, Wyoming, Colorado, and Utah: U.S Geological Survey Miscellaneous Field Studies Map MF-1890, scale 1:500,000, 1 sheet.</ref> <ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and J. C. 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> Johnson et al.<ref name=Johnsonetal_1987>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.</ref> <ref name=Johnsonetal_1996>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.</ref> <ref name=Johnsonetal_1999>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.</ref> used a value of 0.73% R_o to map the top of the gas- and water-bearing transition zone above gas-saturated reservoirs in the Piceance basin of Colorado and the Wind River and Bighorn basins of Wyoming.

The delineation phase entails mapping the vertical and areal distribution of the gas accumulation. The preferred way of accomplishing this phase is through the use of reliable pressure data. In most basins, however, pressure data are absent or of such low quality that reliable maps cannot be constructed; consequently, some indirect method may have to be used. The selected mapping parameter should be one that has been calibrated to well-documented pressure data. For example, thermal maturity values ranging from 0.7 to 0.9% R_o were determined to be coincident with the top of overpressuring in the Greater Green River basin.<ref name=Law_1984 /> In later work, 0.8% R_o was used to map the depth to the top of overpressuring in the basin.<ref name=Pawlewiczetal_1986>Pawlewicz, M. J., M. K. Lickus, B. E. Law, and W. W. Dickinson, 1986, Thermal maturity map showing depth to 0.8% vitrinite reflectance in the Greater Green River basin, Wyoming, Colorado, and Utah: U.S Geological Survey Miscellaneous Field Studies Map MF-1890, scale 1:500,000, 1 sheet.</ref> <ref name=Lawetal_1989>Law, B. E., C. W. Spencer, R. R. Charpentier, R. A. Crovelli, R. F. Mast, G. L. Dolton, and J. C. 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> Johnson et al.<ref name=Johnsonetal_1987>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.</ref> <ref name=Johnsonetal_1996>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.</ref> <ref name=Johnsonetal_1999>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.</ref> used a value of 0.73% R_o to map the top of the gas- and water-bearing transition zone above gas-saturated reservoirs in the Piceance basin of Colorado and the Wind River and Bighorn basins of Wyoming.

To determine an accurate, reliable mapping method, a detailed study of a small area within the basin is recommended rather than a broad-based regional study. For the detailed study, a small representative area with relatively complete, high-quality data should be chosen. Comprehensive, multidiscipline investigations including stratigraphic, structural, source rock, reservoir rock, pressure, thermal history, petrophysical, and well log analyses should then be conducted within the selected area. The objective of this comprehensive investigation is to establish a type area or analog for the entire basin to which incomplete or fragmentary data from other parts of the basin can be compared. From such analog studies, indirect mapping tools, such as levels of thermal maturity, present-day temperature, and log responses, may be determined. Examples of such analog studies include the Pacific Creek area in the Greater Green River basin (Law et al., 1979, 1980), the Wagon Wheel well in the Greater Green River basin (Law and Spencer, 1989), and the Multiwell Experiment site in the Piceance basin, Colorado (Northrop et al., 1984; Spencer and Keighin, 1984; Law and Spencer, 1989). Regional mapping using some of these indirect parameters can then be used not only to determine the stratigraphic and areal distribution of the BCGA but also to help identify areas of enhanced reservoir quality, or sweet spots.

To determine an accurate, reliable mapping method, a detailed study of a small area within the basin is recommended rather than a broad-based regional study. For the detailed study, a small representative area with relatively complete, high-quality data should be chosen. Comprehensive, multidiscipline investigations including stratigraphic, structural, source rock, reservoir rock, pressure, thermal history, petrophysical, and well log analyses should then be conducted within the selected area. The objective of this comprehensive investigation is to establish a type area or analog for the entire basin to which incomplete or fragmentary data from other parts of the basin can be compared. From such analog studies, indirect mapping tools, such as levels of thermal maturity, present-day temperature, and log responses, may be determined. Examples of such analog studies include the Pacific Creek area in the Greater Green River basin,<ref name=Lawetal_1979 /> <ref name=Lawetal_1980 /> the Wagon Wheel well in the Greater Green River basin,<ref name=Lawandspencer_1989>Law, B. E., and C. W. Spencer, eds., 1989, Geology of tight gas reservoirs in the Pinedale anticline area, Wyoming and at the Multiwell Experiment site, Colorado: U.S. Geological Survey Bulletin 1886, p. 39-61.</ref> and the Multiwell Experiment site in the Piceance basin, Colorado.<ref name=Northropetal_1984>Northrop, D. A., A. R. Sattler, R. L. Mann, and K. H. Frohne, 1984, Current status of the Multiwell Experiment: Proceedings of the Society of Petroleum Engineers/Gas Research Institute/Department of Energy Unconventional Gas Recovery Symposium, p. 351-358.</ref> <ref name=Spencerandkeighin_1984>Spencer, C. W., and C. W. Keighin, eds., 1984, Geologic studies in support of the U.S. Department of Energy Multiwell Experiment, Garfield County, Colorado: U.S. Geological Survey Open-File Report 84-757, 134 p.</ref> <ref name=Lawandspencer_1989 /> Regional mapping using some of these indirect parameters can then be used not only to determine the stratigraphic and areal distribution of the BCGA but also to help identify areas of enhanced reservoir quality, or sweet spots.

===Sweet spot identification===

===Sweet spot identification===

A good example of a thermal maturity anomaly associated with a sweet spot is the Lower Cretaceous Muddy ("J") Sandstone in the Denver basin of Colorado. Regional thermal maturity mapping in the Denver basin of Colorado (Higley et al., 1992) shows the presence of an anomaly associated with a BCGA (Figure 14). The anomaly, defined by reflectance values greater than 0.9% Ro, is nearly coincident with the field boundaries of production from the Muddy Sandstone in the Wattenburg field. The anomaly is located north of the structurally deepest part of the basin and is coincident with the northeast projection of the Colorado Mineral Belt. The field is also coincident with a temperature anomaly mapped by Meyer and McGee (1985).

A good example of a thermal maturity anomaly associated with a sweet spot is the Lower Cretaceous Muddy ("J") Sandstone in the Denver basin of Colorado. Regional thermal maturity mapping in the Denver basin of Colorado (Higley et al., 1992) shows the presence of an anomaly associated with a BCGA (Figure 14). The anomaly, defined by reflectance values greater than 0.9% Ro, is nearly coincident with the field boundaries of production from the Muddy Sandstone in the Wattenburg field. The anomaly is located north of the structurally deepest part of the basin and is coincident with the northeast projection of the Colorado Mineral Belt. The field is also coincident with a temperature anomaly mapped by Meyer and McGee (1985).

Because the top of a BCGA is determined, in part, by permeability variations and the ease with which gas may move through reservoirs, measured levels of thermal maturity at the top of a BCGA may provide indirect evidence of the presence of a sweet spot; relatively low values of thermal maturity (<0.8% Ro) at the top of an overpressured BCGA are indicative of a potential sweet spot, whereas relatively high values of thermal maturity (>0.8% Ro) are indicative of very low permeability in an overpressured BCGA. Based on vitrinite reflectance profiles from two wells within the Jonah field (Warner, 1998), the level of thermal maturity at the top of overpressured, gas-saturated reservoirs is less than 0.7% Ro, compared to 0.8% Ro outside the field. Thermal maturity indices, however, cannot be used to identify potential sweet spots in underpressured BCGAs. The level of thermal maturity at the top of an underpressured BCGA most likely is higher than the level of thermal maturity at the top of an overpressured BCGA because the dimensions, or size, of a BCGA are reduced during the transition from overpressure to underpressure. Consequently, the level of thermal maturity at the top of an underpressured BCGA reflects that size constriction.

Because the top of a BCGA is determined, in part, by permeability variations and the ease with which gas may move through reservoirs, measured levels of thermal maturity at the top of a BCGA may provide indirect evidence of the presence of a sweet spot; relatively low values of thermal maturity (<0.8% R_o) at the top of an overpressured BCGA are indicative of a potential sweet spot, whereas relatively high values of thermal maturity (>0.8% R_o) are indicative of very low permeability in an overpressured BCGA. Based on vitrinite reflectance profiles from two wells within the Jonah field (Warner, 1998), the level of thermal maturity at the top of overpressured, gas-saturated reservoirs is less than 0.7% R_o, compared to 0.8% R_o outside the field. Thermal maturity indices, however, cannot be used to identify potential sweet spots in underpressured BCGAs. The level of thermal maturity at the top of an underpressured BCGA most likely is higher than the level of thermal maturity at the top of an overpressured BCGA because the dimensions, or size, of a BCGA are reduced during the transition from overpressure to underpressure. Consequently, the level of thermal maturity at the top of an underpressured BCGA reflects that size constriction.

Stratigraphic sweet spots are more difficult to discern than structural sweet spots because detailed facies mapping requires close-spaced to moderately spaced subsurface data. An example of a stratigraphic sweet spot includes the Upper Cretaceous Almond Formation in the Washakie basin of southwest Wyoming, where reservoirs in the upper, marginal marine part of the formation are typically much more productive than reservoirs in the lower, fluvial-dominated part of the formation. Additional stratigraphic sweet spots include sandstones within the Upper Cretaceous Lewis Shale in the Great Divide basin and the Frontier Formation along the structural crest of the Moxa arch in the Green River basin.

Stratigraphic sweet spots are more difficult to discern than structural sweet spots because detailed facies mapping requires close-spaced to moderately spaced subsurface data. An example of a stratigraphic sweet spot includes the Upper Cretaceous Almond Formation in the Washakie basin of southwest Wyoming, where reservoirs in the upper, marginal marine part of the formation are typically much more productive than reservoirs in the lower, fluvial-dominated part of the formation. Additional stratigraphic sweet spots include sandstones within the Upper Cretaceous Lewis Shale in the Great Divide basin and the Frontier Formation along the structural crest of the Moxa arch in the Green River basin.