Can Seismic Character Identify Lithologies Associated with Bluell and Sherwood Shorelines and Shoals?

Abstract

Several new oil fields have been discovered in the Bluell and Sherwood members of the Mission Canyon Formation, reviving the Mississippian shoreline play in the eastern Williston basin. During periodic regressions, carbonates and evaporites were deposited along the shoreline of a shallow sea. Embayments in the shoreline created traps that contain 3 - 8 million bbl of oil. Carbonate shoals developed offshore. Surrounded by impermeable carbonate mud, they trap 1 - 3 million bbl of oil. The ability of seismic character to distinguish lithologies associated with shorelines and shoals was investigated using sonic logs, models, and seismic data. Shorelines: The gross thickness of the Bluell zone can range between 40 and 70 ft, the Sherwood zone between 35 and 80 ft. Changing thicknesses on geologic models had a distinct effect on seismic character. Also, seismic character varied in response to changing stratigraphy above, within, and below the Bluell and Sherwood zones. Carbonate and anhydrite bulk densities and velocities differ by about 10%. Modeling this difference demonstrated a minimal change in seismic character. Seismic character cannot delineate the shoreline transition from carbonate to anhydrite. Other stratigraphic variations alter seismic character more than this lithologic change. Shoals: Velocity and density variations between more » shoal carbonates and intershoal mud can differ by 30%. Sonic log seismograms and seismic data show a distinct character change between these lithologies. Seismic character can be used to delineate carbonate shoals and mud. However, the shoal/mud character change can be modified by the seismic response to other stratigraphic variations. Calibration with nearby wells can reduce interpretational uncertainty. « less