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ABSTRACT: Stratigraphy and Hydrocarbon Accumulations in the Mississippi Canyon Block 486 Field

W. C. Riese, J. E. Geitgey, S. McAllister, D. Van Nostrand

Oil and gas accumulations in the Mississippi Canyon 486 field occur in sediments of Pleistocene age which were deposited by sediment mass transport and turbid flow in upper to middle slope settings. Sediment transport was from north to south and was influenced by the presence of relief on the sea floor above a shale ridge which was being emplaced during deposition of the sediments. This relief was expressed as a north-plunging ridge that split the southward-flowing sediment stream and allowed small [sic] peripheral fans to accumulate along its western flank. Movement of this shale mass after sedimentation caused down-to-the-west faults to develop along its western flank.

Our analysis of the reservoirs facies present in the area suggests that lateral facies changes in the turbidite fans were sufficient to trap hydrocarbons. Although the field sits downthrown to faulting developed along the western flank of the shale ridge, well control and seismic amplitude distributions suggest that the hydrocarbons are not principally trapped by these faults.

Detailed electrofacies analysis, coupled with seismic stratigraphic analysis at the parasequence scale, suggests that these fan systems may have accumulated through the development of leveed-channel systems. Electrofacies distributions are analogous to those found in deltaic systems, and subtle phase changes in seismic reflection character--similar to those observed in fans in shelf settings--are interpreted to be channels. Facies changes related to these channels and their levees may be restricting hydrocarbon accumulations to the equivalent of crevasse splays.

Alternatively, these fan systems may have accumulated as geographically restricted lobes on the flank of the shale ridge, analogous to the fans which are reservoirs in the Joliett field in Green Canyon 184. Facies changes related to the lateral decrease in reservoir-quality sands onto the shale ridge appear in some respects to be responsible for the hydrocarbon accumulations present.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990