Changes

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.

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 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=Lawetal_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>

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.<ref name=Masters_1979>Masters, J. A., 1979, Deep basin gas trap, western Canada: AAPG Bulletin, v. 63, p. 152-181.</ref> Zagorski<ref name=Zagorski_1988>Zagorski, W. A., 1988, Exploration concepts and methodology for deep Medina sandstone reservoirs in northwestern Pennsylvania (abs): AAPG Bulletin, v. 72, p. 976.</ref> <ref name=Zagorski_1991>Zagorski, W. A., 1991, Model of local and regional hydrocarbon traps in the Lower Silurian Medina Sandstone Group, Cooperstown gas field, Crawford and Venango counties, Pennsylvania: M.S. thesis, University of Pittsburgh, Pennsylvania, 132 p.</ref> 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=Lawetal_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===

Although a few BCGAs are commercially productive over their entire areal extent, such as the San Juan basin of Colorado and New Mexico, most BCGAs are not commercially productive over their entire area. Consequently, areas within the BCGA of enhanced reservoir quality (sweet spots) must be identified. These sweet spots may be structural or stratigraphic in nature and always occur within the abnormal pressure envelope. In addition, they most likely occur near the upper boundary of the BCGA.

Although a few BCGAs are commercially productive over their entire areal extent, such as the San Juan basin of Colorado and New Mexico, most BCGAs are not commercially productive over their entire area. Consequently, areas within the BCGA of enhanced reservoir quality (sweet spots) must be identified. These sweet spots may be structural or stratigraphic in nature and always occur within the abnormal pressure envelope. In addition, they most likely occur near the upper boundary of the BCGA.

In Figure 6, the top of overpressure and BCGA in the Washakie basin is shown as a fairly smooth, uniform line cutting across structural and stratigraphic boundaries. In this case, if very closely spaced pressure data were available along the line of section, the pressure boundary would most likely not be as smooth as shown but would probably be highly irregular, with significant areas of high relief. The areas of high, positive relief, or bumps, may be indicative of structural and/or stratigraphic sweet spots that occur at or near the upper boundary of the BCGA. In the absence of closely spaced pressure data, it is difficult to identify a sweet spot. However, some techniques can be used to identify and focus more expensive techniques such as three-dimensional (3-D) seismic surveys. Those techniques may include lineament, thermal maturity, and present-day temperature mapping. Aeromagnetic, gravity, and surface geochemical surveys also may be useful in the identification of potential sweet spots. Surdam<ref name=Surdam_1997 /> and Surdam et al.<ref name=Surdametal_1997 /> described methods employing sonic logs to identify sweet spots in several basins in Wyoming.

[[File:BasinCenteredGasFig6.jpg|thumb|300px|{{figure number|1}}Cross section BB' showing spatial distribution of BCGA superimposed on structure through the Washakie basin (modified from Law et al., 1989). Shaded pattern shows overpressured, gas-saturated BCGA. Location of cross section shown on Figure 2.]]

The best example of a BCGA structural sweet spot is the Jonah field in the northern part of the Green River basin, Wyoming (Figures 3, 4). As previously discussed, the Jonah field is a gas chimney, rooted in a regionally pervasive BCGA described by Law (1984) and producing from multiple sandstone reservoirs in the Upper Cretaceous Lance Formation. Alternatively, Cluff and Cluff<ref name=Cluffandcluff_2001>Cluff, R. M., and S. G. Cluff, 2001, Overpressure determination from sonic and resistivity log anomalies, Jonah field, northern Green River basin, Wyoming, ''in'' J. W. Robinson and K. W. Shanley, eds., Tight gas fluvial reservoirs: A case study from the Lance Formation, Green River basin, Wyoming: RMAG Short Course Notes 2, unpaginated.</ref> have interpreted the Jonah field to be a remnant of a larger, much more shallow BCGA than presently identified. The Jonah field is a wedge-shaped area with the north, south, and west boundaries of the field defined by westward converging faults (Figure 4). The eastern boundary is undefined. The geologic characteristics of the Jonah field are given by Montgomery and Robinson<ref name=Montgomeryandrobinson_1997>Montgomery, S. L., and J. W. Robinson, 1997, [http://archives.datapages.com/data/bulletns/1997/07jul/1049/1049.htm Jonah field, Sublette County, Wyoming: Gas production from overpressured Upper Cretaceous Lance sandstones of the Green River basin]: AAPG Bulletin, v. 81, p. 1049-1062.</ref> and Warner.<ref name=Warner_1998>Warner, E. M., 1998, Structural geology and pressure compartmentalization of Jonah field, Sublette County, Wyoming, ''in'' R. M. Slatt, ed., Compartmentalized reservoirs in Rocky Mountain basins: Rocky Mountain Association of Geologists, p. 29-46.</ref> <ref name=Warner_2000>Warner, E. M., 2000, Structural geology and pressure compartmentalization of Jonah field based on 3-D seismic data and subsurface geology, Sublette County, Wyoming: The Mountain Geologist, v. 37, no. 1, p. 15-30.</ref> According to Warner<ref name=Warner_2000 /> the top of overpressure (top of gas-saturated reservoirs) within the field occurs at depths of 7700 ft (2347 m) at the west end of the field (updip end of field) and 9500 ft (2896 m) at the east end of the field (downdip end of the field). Outside the field, the top of overpressure and gas-saturated reservoirs occur at depths ranging from 11,200 to 11,600 ft (3414-3536 m).<ref name=Warner_2000 /> Thus, there is 2500-3000 ft (726-914 m) of relief on the top of overpressuring from outside the field to inside the field (Figure 4). The gas chimney has subsequently been identified through the use of sonic velocity data.<ref name=Surdametal_2001 />

In [[:file:BasinCenteredGasFig6.jpg|Figure 1,]] the top of overpressure and BCGA in the Washakie basin is shown as a fairly smooth, uniform line cutting across structural and stratigraphic boundaries. In this case, if very closely spaced pressure data were available along the line of section, the pressure boundary would most likely not be as smooth as shown but would probably be highly irregular, with significant areas of high relief. The areas of high, positive relief, or bumps, may be indicative of structural and/or stratigraphic sweet spots that occur at or near the upper boundary of the BCGA. In the absence of closely spaced pressure data, it is difficult to identify a sweet spot. However, some techniques can be used to identify and focus more expensive techniques such as three-dimensional (3-D) seismic surveys. Those techniques may include lineament, thermal maturity, and present-day temperature mapping. Aeromagnetic, gravity, and surface geochemical surveys also may be useful in the identification of potential sweet spots. Surdam<ref name=Surdam_1997 /> and Surdam et al.<ref name=Surdametal_1997 /> described methods employing sonic logs to identify sweet spots in several basins in Wyoming.

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<ref name=Higleyetal_1992>Higley, D. K., D. L. Gautier, and M. J. Pawlewicz, 1992, Influence of regional heat flow variation on thermal maturity of the Lower Cretaceous Muddy ("J") Sandstone, Denver basin, Colorado, ''in'' The petroleum system-status of research and methods, 1992: U.S. Geological Survey Bulletin 2007, p. 66-69.</ref> shows the presence of an anomaly associated with a BCGA (Figure 14). The anomaly, defined by reflectance values greater than 0.9% R_o, 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.<ref name=Meyerandmcgee_1985>Meyer, H. J., and H. W. McGee, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0006/0900/0933.htm Oil and gas fields accompanied by geothermal anomalies in the Rocky Mountain region]: AAPG Bulletin, v. 69, p. 933-945.</ref>

 

 

 

 

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<ref name=Higleyetal_1992>Higley, D. K., D. L. Gautier, and M. J. Pawlewicz, 1992, Influence of regional heat flow variation on thermal maturity of the Lower Cretaceous Muddy ("J") Sandstone, Denver basin, Colorado, ''in'' The petroleum system-status of research and methods, 1992: U.S. Geological Survey Bulletin 2007, p. 66-69.</ref> shows the presence of an anomaly associated with a BCGA ([[:file:BasinCenteredGasFig14.jpg|Figure 4]]). The anomaly, defined by reflectance values greater than 0.9% R_o, 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.<ref name=Meyerandmcgee_1985>Meyer, H. J., and H. W. McGee, 1985, [http://archives.datapages.com/data/bulletns/1984-85/data/pg/0069/0006/0900/0933.htm Oil and gas fields accompanied by geothermal anomalies in the Rocky Mountain region]: AAPG Bulletin, v. 69, p. 933-945.</ref>

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,<ref name=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.

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,<ref name=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.