Weimer, Paul, Barry C. McBride, and Mark G. Rowan
University of Colorado, Boulder, Colorado

Abstract: Evaluating the Petroleum Systems of the Northern Gulf of Mexico Through Integrated Basin Analysis

The northern deep Gulf of Mexico is the most active exploration area in the United States today. To understand the complexity of these petroleum systems, a case study was conducted in the Green Canyon and Ewing Bank areas by integrating sequence stratigraphy, structural geology (including structural restorations), geothermal modeling, and migration pathway analyses.

The continental slope consists of a multilevel salt system, with the autochthonous Middle Jurassic Louann salt having been deformed into a series of shallow allochthonous layers during the Cenozoic. Intraslope basins were created by the Neogene sedimentary loading of allochthonous salt systems. End-member allochthonous salt systems include salt sheets with a nearly horizontal base salt, and bulb-shaped salt stock systems. Source rocks occur below the shallow salt, and include Tithonian and Turonian strata.

Reservoirs consist of intraslope turbidite systems, and include sheets and amalgamated sheets, channel-fill, and thin beds in levee deposits. Production rates and development plans vary for the different reservoirs. Amalgamated sheet sands can partially or entirely fill one minibasin, and generally have the highest sustainable production rates. Thin beds in levees have given surprisingly high rates of production, although production typically declines significantly after a few years. Channel-fill deposits have fairly high rates of production, especially where they are amalgamated.

Allochthonous salt affects the evolution of the petroleum systems in three ways. First, salt formed the traps during the Pliocene and Pleistocene. Second, the high thermal conductivity of salt retarded the maturation of subsalt source rocks and caused late generation and migration from the early Miocene through the Present. The critical moment of peak oil generation of source rocks varies spatially and temporally as a function of allochthonous salt and sediment overload. Third, the initial shape of the base of salt—sheet vs. bulb—affects the petroleum migration pathways. Migration of oil is primarily vertical from source rocks to the base of salt, where it deflects laterally up the dip of base of slope. Petroleum pools in the shallowest places below the salt. When a salt weld forms, petroleum migration is unimpeded and continues vertically into the reservoirs.

All fields/discoveries in the area are associated with zones of paleosubsalt petroleum concentration. Present-day salt geometries do not delineate many of these zones because of the weld formation during the Pleistocene. This integrated approach was developed to help evaluate the overall petroleum systems for this basin, and, ultimately, help reduce risk for this and similar basins globally.

AAPG Search and Discovery Article #90929©1998-1999 AAPG Distinguished Lecturers