Integrated Approach to Decipher the Petroleum Charge History in the Horn Mountain Field, Gulf of Mexico

Alexei V. Milkov, Leon I. P. Dzou, Evelyn M. Goebel, David A. Fisher, Neal F. McCaslin, Joshua C. Turner, and David F. Bergman
BP America, Houston, TX

Oil is produced at the Horn Mountain field (Mississippi Canyon 126/127) from two middle Miocene reservoirs (M and J) deposited in sand-filled channels and associated levees environments. The accumulations are trapped by a major west-east fault dipping to the north and stratigraphic pinch-outs at the crest of the structure. Three major faults separate reservoirs into structural compartments. A petroleum charge model is proposed based on the integration of sub-regional burial and structural history, sand distribution, PVT properties, and geochemistry of entrapped petroleum fluids. Microbial gas accumulated in both reservoirs prior to the oil charge. Thermogenic oil and associated gas were derived primarily from Jurassic source rocks (marine shales), with a minor contribution from Cretaceous marine shales. Petroleum migrated vertically through the thick section of mudstones and occasional marls till it encountered the sub-regional carrier bed (M sand), and then migrated laterally up-dip to the trap. The early expelled (least mature) petroleum charged the central fault block, dissolved the pre-existing microbial gases and spilled into the western and eastern fault blocks. Continued arrival of more mature oil filled the trap to the common oil-water contact in the M sand. The pressure at the top of the oil column eventually exceeded capillary entry pressure of the cap rock, and the oil leaked from the M sand into the J sand. This model implies that faults serve as fluid flow barriers (not migration pathways) in the smaller J reservoir. This conclusion is consistent with pressure, geochemistry, and production data available to date.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005