Changes

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.

[[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.]]

[[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.<ref name=Lawetal_1989 />). Shaded pattern shows overpressured, gas-saturated BCGA. Location of cross section shown on Figure 2.]]

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.

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.