A Sonic Log Study of Abnormally Pressured Zones in the Powder River Basin of Wyoming
Debra Maucione, Vladimir Serebryakov, Paul Valasek, Yue Wang, Scott Smithson, 1994. "A Sonic Log Study of Abnormally Pressured Zones in the Powder River Basin of Wyoming", Basin Compartments and Seals, Peter J. Ortoleva
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Most hydrocarbon production from the Powder River basin in northeastern Wyoming is from an abnormally pressured Cretaceous section. The preliminary identification and delineation of abnormally pressured zones by surface seismic methods would greatly enhance hydrocarbon recovery. The present velocity study shows the usefulness of a detailed sonic log analysis to find abnormally pressured zones. Because the conclusion of the log response studypositively indicated the presence of abnormal pressure, surface seismic data will also show these anomalies. Velocity profiles, pressure profiles, and abnormal pressure-gradient calculations all indicate that the Cretaceousshales are overpressured throughout the basin, whereas the Cretaceous sandstones are present in overpressured, underpressured, and normally pressured regimes. Pressure compartment boundaries appear to be stratigraphically controlled. Three profiles in different parts of the basin— T46N, R69-76W (Hilight field area), T51N, R69-77W (Amos Draw-Kitty field area), and T41N, R69-76W (north of the Powell field area)—delineate the top of overpressure near the Parkman Sandstone-Steele Shale contact and the bottom seal along the Fuson Shale.
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Basin Compartments and Seals
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.