Abstract: Reservoir Potential of Lowstand Shoreface Sandstones: An Example from the Green River Basin, Wyoming
John C. Horne, Alan J. Scott, John R. Forster
Recent research by sequence stratigraphers in foreland basins suggests that lowstand shoreface deposits are important sites of significant reservoir-quality sandstone accumulation during relative sea level lowstands. A detailed study of the Frontier Formation in the Green River basin illustrates the characteristics and reservoir potential of lowstand shore face deposits associated with the middle Turonian lowstand. The recent realization of significant gas reserves in this facies in the Table Rock Field establishes a new Frontier sandstone play in the eastern Green River basin. Predicting the trend, reservoir quality, and trapping geometries of these lowstand shoreface sandstones will be critical to the future success of the play.
During the middle Turonian eustatic sea level drop, exposure and erosion of most of the western Green River basin occurred. This eustatic fall of sea level was enhanced by the development of a foreland bulge (paleo-Moxa Arch) in the western part of the basin. Only the southeastern part of the Green River basin remained submerged.
The sea level drop reduced accommodation space and forced the shoreline to shift basinward. With less accommodation space, the waves eroded into finer grained lower shoreface and offshore deposits. Wave-reworked, coarse-grained, upper shoreface deposits accumulated over this scoured surface forming a sharp-based reservoir-potential sandstone. These lowstand shoreline sandstones elongate along depositional strike, are capped by a transgressive surface and marine shales, and rest sharply on finer grained offshore deposits. Because the rate and magnitude of the middle Turonian sea level fall was significant, most of these lowstand shoreface deposits are detached, creating opportunities for updip pinchouts of porosity.
AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994