Estimation of Underground Permeability and Porosity of Slope Sediments in the Northern Gulf of Mexico, IODP Expedition 308

The Brazos-Trinity Basin #4 and Ursa Basin on the northern continental slope off Texas and Louisiana in the Gulf of Mexico are ideal locations for studying the sedimentation and fluid flow processes leading to the development of overpressure because of their contrasting sedimentation histories. The former is a small basin with a slow sedimentation rate and with no overpressure development. Whereas the sedimentation in the latter is high, resulting in the development of overpressure which is likely to have triggered a large-scale slope failure, slumping and faulting. The purposes of Expedition 308 are to document how pressure, stress and geology are coupled to control fluid migration on passive margins, through drilling at sites U1319 and U1320 in the Brazos-Trinity Basin #4 and at sites U1322 and U1324 in Ursa Basin. Quantitative basin analysis, i.e., analysis of sedimentation, fluid flow and compaction processes at least in two dimensions, is possible based on the measured hydraulic properties of those core samples at elevated effective pressures corresponding to various depths. We have just started systematic measurement of permeability, porosity and specific storage capacity of core samples, using an intravessel deformation and fluid-flow apparatus at Kyoto University. Samples were first dried at temperatures of 90-100°C for more than 3 days to evaporate pore water. Permeability and porosity have been measured with N2 gas flow method and with a picnometer, respectively, at confining pressures up to 200 MPa and pore pressures to 2 MPa. The age of drill cores range from the Pleistocene to the Quarternary, and we plan to evaluate the long-term cementation effect by measuring porosity for samples from all stratigraphic horizons. The complete data set will allow one to estimate permeability and porosity structures at depths even greater than 10 km. Our measurements show that permeability of silt in the shallow part of Brazos-Trinity Basin #4 ranges from 10-15 m2 at an effective pressure, Pe, of 3 MPa, but it decreases down to 10-20 m2 as Pe increases to 80 MPa. This dramatic pressure effect on permeability must be incorporated in the basin analysis. More comprehensive data will be reported at the meeting.