INTEGRATED GEOLOGICAL ASSESSMENTS OF SHELF AND SLOPE RESERVOIRS FOR CARBON STORAGE IN THE CENTRAL GULF OF MEXICO

Offshore reservoirs in the Central Gulf of Mexico provide numerous opportunities for enhanced oil recovery (EOR) and permanent storage of CO₂. Preliminary assessment indicates a P₅₀ storage resource of about 473 Gt. Sedimentology, structure, pressure, geothermic, and subsurface stress determine which reservoirs offer potential for storage and EOR. The objective of this research, which is sponsored by the SECARB Offshore Partnership, is to characterize the regional geology and quantify the storage resource in oil reservoirs and saline formations of the Central Gulf, and this study focuses on the outer shelf in the South Marsh through Grand Island protraction areas and the upper slope in the Ewing Bank and Green Canyon area. This research is assessing the variation in reservoir thickness, depositional environment, geologic structure, porosity, permeability, temperature, and geomechanical properties of the Miocene-Pleistocene reservoirs in the region. Porosity and permeability decrease with depth and distance from the shoreline, and effective permeability is strongly sensitive to hydrocarbon saturation. Reservoir temperature is highly variable, particularly in the Miocene section, and geothermal gradient is influenced by advection related to active hydrocarbon generation and migration at depth, as well as the thermal mass of the water column above the seabed. Minimum original reservoir pressure is brine-compensated hydrostatic, and hydrocarbon overpressure is widespread, particularly in the Upper Miocene section, where original pressure in places reaches fracture pressure. Overpressure and associated fluid migration appear to drive fault slip and is an important mechanism facilitating gravity gliding, especially in the upper slope. Oil accumulations tend to be near the margins of minibasins and are commonly developed in or near fault systems, whereas saline formations are widespread in the interiors of the minibasins and are broken by few faults. Enhanced oil recovery operations can be designed to minimize operational risk along faults, whereas saline formations in the interiors of minibasins pose minimal risk for large-scale permanent storage.