Permeability-porosity relationships of shallow mudstones in the Ursa Basin, northern deepwater Gulf of Mexico
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In the Ursa Basin, Gulf of Mexico, in situ mudstone permeability near the seafloor declines from 1.1 1016 to 5.8 1019 m2over a depth of 578 m. We can reproduce this in situ permeabilityporosity behavior through consolidation experiments in the laboratory. We use uniaxial constantrateofstrain consolidation experiments to measure permeabilityporosity relationships and derive in situ permeabilities of 31 mudstone samples collected at Integrated Ocean Drilling Program (IODP) Sites U1324 and U1322. Although these mudstones have similar grainsize distributions, permeability at a given porosity varies significantly between the samples due to small variations in composition or fabric. We calculate an upscaled permeability relationship based on the observed permeability variation in the samples and determine a resultant largescale permeability anisotropy of around 30. Based on this upscaled relationship and observations of in situ pressure, we calculate upward fluid flow rates of 0.5 mm/yr. We find that given the observed compressibility, permeability, and the geologic forcing at Ursa, overpressures are predicted as observed in the subsurface. The primary mechanism for overpressure generation at Ursa is sediment loading due to rapid burial. Low vertical permeabilities, accompanied by high sedimentation rates, can cause severe overpressure near the seafloor, which controls fluid flow and can reduce slope stability as observed in the Mississippi Canyon region. Such flow systems, especially at intermediate depths on passive margins, are important due to their control over macroscale behavior such as topographic gradient of continental slopes and submarine landslides, but have been largely understudied in the past.
