Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists.  All Rights Reserved.

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 study positively 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 Cretaceous shales 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.