Offshore sediment overpressures: Overview of mechanisms, measurement and modeling

Offshore sediment pore fluid overpressures (fluid pressure in excess of hydrostatic) contribute to potential mechanical instability of these sediments, and during offshore drilling explorations, present potential hazards to equipment and operational crews. Overpressure is likely to be found where low permeability layers have inhibited pore fluid escape. Rapid sedimentation (or disequilibrium compaction), tectonic loading and lateral transfer increase the overpressure. Aquathermal expansion, hydrocarbon generation, mineral diagenesis, organic maturation, and fluid charging increase the pore fluid volume. Dissolution and cementation can create low permeability boundaries and alter porosity. Particularly acting in combination, these mechanisms have the potential to cause near-lithostatic overpressures and potentially unstable sediments. By understanding the causal processes and the rates of overpressure genesis, potentially unstable zones can be detected and modeled more efficiently. Better measurements and models will lead to more accurate analysis of potentially unstable offshore slopes, and enable safer and more cost effective drilling practices.