University of California-Santa Cruz, United States

Integrated Ocean Drilling Program (IODP) Expedition 308 is the first part of a two-component program dedicated to the study of overpressure and fluid flow on the Gulf of Mexico continental slope. We are examining how sedimentation, overpressure, fluid flow, and deformation are coupled in passive margin settings. Expedition 308 tested a multidimensional flow model by examining how physical properties, pressure, temperature, and pore fluid composition vary within low-permeability mudstones that overlie a permeable and overpressured aquifer. We drilled, logged, and made in situ measurements in a reference location where little overpressure was deemed to be present: the Brazos-Trinity Basin. We contrasted these measurements with experiments performed in a region of very rapid Pleistocene sedimentation where overpressure is known to be present: the Ursa region of the northern Gulf of Mexico. Drilling documented severe overpressure in the Ursa region. Postcruise studies will illuminate controls on slope stability, seafloor seeps, and large-scale crustal fluid flow. Two key components of the experimental plan were to take substantial whole-core geotechnical samples for later shore-based analysis and to deploy the temperature and dual pressure probe (developed jointly between the Massachusetts Institute of Technology, Pennsylvania State University, and IODP at Texas A&M University) to measure in situ pressure. Expedition 308 science met many of the objectives proposed in the original IODP Proposal 589-Full3 and provided the foundation to implement long-term in situ monitoring experiments in the aquifer and bounding mudstones in a future expedition designed to meet the full objectives of IODP Proposal 589-Full3. An important achievement of Expedition 308 is to have successfully recorded in situ formation pressure and temperature in an overpressured basin. This is the first time that we know of that such measurements have been obtained.