Abstract: Intraslope Basin Reservoirs Deposited by Gravity-Driven Processes: Ship Shoal and Ewing Banks Areas, Offshore Louisiana

John M. Armentrout, Steve J. Malecek, Vinod R. Mathur, Gary L. Neuder, Gerry M. Ragan

Seismic facies and high-resolution biostratigraphic analysis provide a sequence stratigraphic framework for interpreting lateral distribution of sand-prone facies and reservoir connectivity in the Ship Shoal 351-358 to Ewing Bank 988 area, offshore Louisiana. The interval of interest is an isochron thick interpreted as a lowstand systems tract deposited at bathyal water depths within an intraslope-basin. This basin is approximately 50 kilometers from the age equivalent shelf/slope break. The isochron thick was deposited between the late Pliocene Discoaster pentaradiatus and early Pleistocene Discoaster brouweri Condensed Sections.

The D. pentaradiatus-D. brouweri sequence consists of the synclinal fill of a salt withdrawal basin forming an isochron thick that thins onto adjacent salt-cored structural highs. This isochron interval was subdivided into four seismic facies and each is calibrated with local well data. The seismic facies are: Facies (1) hummocky-mounded facies with an internal reflection character of discontinuous variable amplitude, calibrated to thick, flat-based blocky to fining-upward sandstones interpreted as channel deposits; Facies (2) local-sheet facies consist of tabular sets of high-amplitude, relatively continuous reflections correlated with thin blocky sandstones. These sandstones are interpreted as local sheets deposited as channel-fed depositional lobes; Facies (3) clinoform-wedge facie are characterized by continuous, uniform amplitude reflections and are calibrated with mudstones interpreted as part of a downslope prograding complex that downlaps and onlaps the hummocky-mounded facies; and Facies (4) parallel-continuous facies of relatively uniform amplitude correlated with mudstones that drape the other three facies throughout the study area and are interpreted as hemipelagic mudstone. Mapped patterns of these seismic facies suggest a network of channel systems within a slope valley supplying sand by gravity-driven processes into a local salt withdrawal intraslope-basin.

Following the above analysis, three wells and two side-tracks were drilled to further test the prospectivity of the area. Rock type and hydrocarbon predictions based on seismic facies analysis were confirmed by the wells.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas