Changes

The best example of a BCGA structural sweet spot is the Jonah field in the northern part of the Green River basin, Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 2]], [[:file:BasinCenteredGasFig4.jpg|Figure 3]]). 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 ([[:file:BasinCenteredGasFig4.jpg|Figure 3]]). 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 ([[:file:BasinCenteredGasFig4.jpg|Figure 3]]). The gas chimney has subsequently been identified through the use of sonic velocity data.<ref name=Surdametal_2001 />

The best example of a BCGA structural sweet spot is the Jonah field in the northern part of the Green River basin, Wyoming ([[:file:BasinCenteredGasFig3.jpg|Figure 2]], [[:file:BasinCenteredGasFig4.jpg|Figure 3]]). 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 ([[:file:BasinCenteredGasFig4.jpg|Figure 3]]). 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 ([[:file:BasinCenteredGasFig4.jpg|Figure 3]]). The gas chimney has subsequently been identified through the use of sonic velocity data.<ref name=Surdametal_2001 />

[[File:BasinCenteredGasFig14.jpg|thumb|300px|{{figure number|4}}Thermal maturity map of the Denver basin, Colorado, showing the large thermal maturity anomaly in the Cretaceous Muddy ("J") Sandstone in the Wattenburg field (modified from Higley et al.<ref name=Higleyetal_1992 />). The field is nearly coincident with the 0.9% isoreflectance contour.<ref name=Higleyetal_1992 /> The location of the anomaly is also coincident with the basinward projection of the Colorado Mineral Belt (CMB).]]

[[File:BasinCenteredGasFig14.jpg|thumb|300px|{{figure number|4}}Thermal maturity map of the Denver basin, Colorado, showing the large thermal maturity anomaly in the Cretaceous Muddy ("J") Sandstone in the Wattenburg field (modified from Higley et al.<ref name=Higleyetal_1992 />). The field is nearly coincident with the 0.9% isoreflectance [[contour]].<ref name=Higleyetal_1992 /> The location of the anomaly is also coincident with the basinward projection of the Colorado Mineral Belt (CMB).]]

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>

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>