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Andrew J. Petty1, Paul J. Post1, William C. Parcell2
(1) Minerals Management Service, New Orleans, LA
(2) University of Alabama, Tuscaloosa, AL

Abstract: Field Distribution and Petroleum Systems of the Andrew Formation (Albian) and James Limestone (Aptian), Mobile, Main Pass, and Viosca Knoll Areas, Northeastern Gulf of Mexico

Hydrocarbon exploration of Lower Cretaceous carbonates on the Outer Continental Shelf began in 1968. Seven fields have been discovered in the Andrew Formation and in the James Limestone. The Andrew Formation's Main Pass Block 253 and Main Pass Block 221 Fields contain oil and are located along the shelf-edge-reef trend, while the James Limestone's Viosca Knoll Block 69, Viosca Knoll Block 114, Viosca Knoll Block 252, Viosca Knoll Block Block 256, and Mobile Block 991 Fields contain gas and are distributed in patch reefs across the shelf behind the shelf-edge-reef trend.

Generation of the Lower Cretaceous-sourced oil initiates at approximately 10,000 feet, which makes the Andrew Formation thermally immature at current depths along the shelf edge. However, fore-reef Lower Cretaceous marine shales became thermally mature after the Miocene deltas covered the Main Pass Area adjacent to the shelf edge reef trend. The Lower Cretaceous-sourced oils migrated from the marine shale source beds into reservoir rocks along the shelf edge.

The James Limestone patch reef reservoirs are regionally sealed by anhydrite beds of the Ferry Lake and Rodessa Formations. The anhydrite beds occur sparsely along the shelf edge interfingering with carbonates of the reef trend. By contrast, on the shelf, the anhydrite beds are more numerous, thicker, and provide a seal for the underlying gas-charged porosity zones of the James Limestone, but at the same time are a migration barrier for the overlying porosity zones of the Mooringsport Formation. Higher sulfur-content gases found in the James Limestone have a Smackover (Upper Jurassic) source that underwent hydrocarbon generation during the Lower Cretaceous.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana