SMITH, MICHAEL A., Minerals Management Service, New Orleans, Louisiana

Abstract: Geological Controls and Variability in Pore Pressure in the Deepwater Gulf of Mexico

MMS geological reviews of exploration and development plans and applications for permit to drill on Gulf of Mexico OCS leases contain a discussion of possible abnormal pressure zones. Geopressure is the depth at which fluid pressure exceeds normal hydrostatic pressure, typically requiring mud weights of 12.5 ppg. Burial rates, geothermal gradients, compaction, and diagenetic reactions are the primary factors affecting the onset of geopressure. In deep-water wells, the large seawater column causes greater depths to abnormal pressure. However, geological factors that control the deposition of turbidite systems, sequence stratigraphy, major faults, unconformities, and salt tectonics also affect pore pressure. In complexly faulted structures, formation pressures may be compartmentalized or vary between different sands. We have analyzed predicted and actual pore pressures, sedimentation rates, and formation temperatures in the deepwater Gulf of Mexico and prepared trend maps of the occurrence of geopressures for this province.

Overpressured sands have caused various drilling problems in deepwater wells. Shallow water flow can occur in undercompacted slope fan/channel sands a few thousand feet below the seafloor, particularly in tilted and rotated slump blocks. In the centroid concept, pore pressure in a reservoir sand at the crest of a high-relief overpressured structure exceeds pore pressure in the bounding shale. The top of a large structure near the seafloor may contain fluid pressures that are similar to the fracture gradient in adjacent shales. If unplanned additional casing strings are required in shallow formations, the target sand may be unreachable or the limited wellbore size uneconomic.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England