Overpressure, Hydrocarbon Entrapment, Seafloor Venting, and Slope Stability: The Dynamic Flow Regime Beneath the Seafloor

Peter B. Flemings, The Pennsylvania State University, University Park, PA

Sedimentation, overpressure, fluid flow, seafloor venting, and submarine landslides are intimately related. Sandstone buried rapidly by low permeability mudstone has a characteristic pressure regime: the sandstone has a hydrostatic pore pressure gradient whereas the bounding low permeability mudstone can have a lithostatic pressure gradient. This simple behavior drives a myriad of exciting geological processes. In the deepwater Gulf of Mexico, reservoir pore pressures at the crest of the Popeye Genesis minibasin equal the least principal stresses and fluids are venting today. Mud volcanoes, gas hydrates, and biological communities overlie this leak point. In the Ursa Basin, Pleistocene sedimentation from the ancestral Mississippi River was so rapid that we find overpressure within a few meters of the seafloor. Permeable sand bodies transmitted this pressure laterally and these pressures contributed to large submarine landslides. The coupled study of stratigraphy and hydrodynamics can be used to predict pressure, estimate trap integrity and migration pathways, predict slope failure, and design safe and economic drilling programs.