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 (Hunt, 1996).

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 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% 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.

===Confirmation phase===

===Confirmation phase===

Once a basin containing a potential BCGA has been identified, the task becomes one of confirmation. Because all BCGAs are abnormally pressured, the principal task during this phase is the determination of reservoir pressure and the mechanism of abnormal pressure. Most basins do not have sufficient quantity or quality of pressure data for this determination. Therefore, a combination of attributes listed on Table 1 can provide compelling evidence for the presence of abnormal pressure and a BCGA. Pore pressure indicators such as pore fluid composition (gas with little or no producible water) in conjunction with porosity (<13%), permeability (<0.1 md), thermal maturation (>0.7% Ro) data, and sustained gas shows are very useful. In some cases, sonic velocity data have been used to indicate the presence of abnormal pressures (Surdam et al., 1997, 2000, 2001; Surdam, 1997).

Once a basin containing a potential BCGA has been identified, the task becomes one of confirmation. Because all BCGAs are abnormally pressured, the principal task during this phase is the determination of reservoir pressure and the mechanism of abnormal pressure. Most basins do not have sufficient quantity or quality of pressure data for this determination. Therefore, a combination of attributes listed on Table 1 can provide compelling evidence for the presence of abnormal pressure and a BCGA. Pore pressure indicators such as pore fluid composition (gas with little or no producible water) in conjunction with porosity (<13%), permeability (<0.1 md), thermal maturation (>0.7% Ro) data, and sustained gas shows are very useful. In some cases, sonic velocity data have been used to indicate the presence of abnormal pressures.<ref name=Surdam_1997>Surdam, R. C., 1997, [http://archives.datapages.com/data/specpubs/mem67/ch17/ch17.htm A new paradigm for gas exploration in anomalously pressured "tight gas sands" in the Rocky Mountain Laramide basins], ''in'' R. C. Surdam, ed., Seals, traps, and the petroleum system: AAPG Memoir 67, p. 283-298.</ref> <ref name=Surdametal_1997>Surdam, R. C., Z. S. Jiao, and H. P. Heasler, 1997, [http://archives.datapages.com/data/specpubs/mem67/ch12/ch12.htm Anomalously pressured gas compartments in Cretaceous rocks of the Laramide basins of Wyoming: A new class of hydrocarbon accumulation], ''in'' R. C. Surdam, ed., Seals, traps, and the petroleum systems: AAPG Memoir 67, p. 199-222.</ref> <ref name=Surdametal_2000>Surdam, R. C., Z. S. Jiao, and N. K. Boyd III, 2000, Delineation of anomalous pressured gas accumulations in the Riverton Dome area, Wind River basin, Wyoming, ''in'' G. Winter, ed., Fifty-First Field Conference, Wyoming Geological Association, p. 121-148.</ref> <ref name=Surdametal_2001>Surdam, R. C., J. Robinson, Z. S. Jiao, and N. K. Boyd III, 2001, Delineation of Jonah field using seismic and sonic velocity interpretations, ''in'' D. S. Anderson, J. W. Robinson, J. E. Estes-Jackson, and E. B. Coalson, eds., Gas in the Rockies: Rocky Mountain Association of Geologists, p. 189-208.</ref>

{| class = "wikitable"

{| class = "wikitable"

|}

|}

Although the determination of abnormal pressure is important, it is equally important to determine the mechanism of abnormal pressure. For direct BCGAs, the pressure mechanism is hydrocarbon generation (Spencer, 1987), and for indirect BCGAs, the pressure mechanism is thermal cracking of liquid hydrocarbons to gas (Law, 2000). A useful criteria for determining the pressure mechanism is through a knowledge of the composition of pore fluids: pore fluids in direct and indirect systems are composed of gas with little or no producible water (Spencer, 1987; Law and Spencer, 1993), whereas in abnormally pressured reservoirs, where the composition of pore fluid is mainly water, the pressure mechanism may be one of several other mechanisms, thereby precluding a hydrocarbon-generation mechanism and presence of a BCGA.

Although the determination of abnormal pressure is important, it is equally important to determine the mechanism of abnormal pressure. For direct BCGAs, the pressure mechanism is hydrocarbon generation,<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> and for indirect BCGAs, the pressure mechanism is thermal cracking of liquid hydrocarbons to gas.<ref name=Law_2000>Law, B. E., 2000, What is a basin-centered gas system?" 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.</ref> A useful criteria for determining the pressure mechanism is through a knowledge of the composition of pore fluids: pore fluids in direct and indirect systems are composed of gas with little or no producible water,<ref name=Spencer_1987 /> <ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, eds., 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.</ref> whereas in abnormally pressured reservoirs, where the composition of pore fluid is mainly water, the pressure mechanism may be one of several other mechanisms, thereby precluding a hydrocarbon-generation mechanism and presence of a BCGA.

Formation resistivity and spontaneous potential curves measured on geophysical well logs also have been used to indicate the presence of a BCGA. In Upper Cretaceous rocks in the San Juan basin and Mesozoic rocks in the Alberta basin, resistivities greater than 20 W were reported to be gas saturated (Masters, 1979). Zagorski (1988, 1991) noted that the boundary between conventional and BCGA reservoirs in northwestern Pennsylvania could be distinguished at 80 W; reservoirs with high water saturation were defined by resistivities <80 W"m, and reservoirs within the BCGA have resistivities >80 W"m. In Upper Cretaceous rocks in the Greater Green River basin, spontaneous potential curves are commonly reversed in abnormally pressured BCGAs (Law et al., 1979, 1980; Law, 1984).

Formation resistivity and spontaneous potential curves measured on geophysical well logs also have been used to indicate the presence of a BCGA. In Upper Cretaceous rocks in the San Juan basin and Mesozoic rocks in the Alberta basin, resistivities greater than 20 Ω were reported to be gas saturated (Masters, 1979). Zagorski (1988, 1991) noted that the boundary between conventional and BCGA reservoirs in northwestern Pennsylvania could be distinguished at 80 Ω; reservoirs with high water saturation were defined by resistivities <80 Ω•m, and reservoirs within the BCGA have resistivities >80 Ω•m. In Upper Cretaceous rocks in the Greater Green River basin, spontaneous potential curves are commonly reversed in abnormally pressured BCGAs.<ref name=Law et al_1979>Law, B. E., C. W. Spencer, and N. H. Bostick, 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> <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> <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>

===Delineation===

===Delineation===

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% Ro were determined to be coincident with the top of overpressuring in the Greater Green River basin (Law, 1984). In later work, 0.8% Ro was used to map the depth to the top of overpressuring in the basin (Pawlewicz et al., 1986; Law et al., 1989). Johnson et al. (1987, 1996, 1999) used a value of 0.73% Ro 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 (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.