Seismic Anisotropy of Overpressured Shale in the Gulf of Mexico Basin

Massive shale deposits from the North American continent constitute one of the major sedimentary rock types in the Gulf of Mexico (GOM) basin. These shale deposits are subject to overpressure primarily by two mechanisms: smectite-to-illite transition and disequilibrium compaction. Obviously, an accurate pore-pressure prediction is important for risk assessment in the GOM. However, estimating pore pressure with empirical equations based solely on seismic velocity is questionable because of shale anisotropy. For the continental shelf of offshore Louisiana, shale anisotropy has been parametrized to the rock properties of shale volume and areal distribution, pore pressure, and overpressure mechanisms. Using seismic and well-log data, these anisotropic parameters are estimated in the northern and southern blocks in the study area. A quantitative anisotropic correlation between the northern and southern blocks was expressed with Thomsen’s coefficients. We found that both the disequilibrium compaction and the smectite-to-illite transition mechanisms produce an increase in seismic anisotropy. Geomechanical models are suggested to account for these observations.