Flow focusing in overpressured sandstones: Theory, observations, and applications

Abstract

Dipping permeable sandstone bodies encased in overpressured low permeability mudstone have a characteristic pressure field: sandstone pressures follow the hydrostatic gradient while mudstone pressures have a steeper (often lithostatic) gradient. This pressure distribution drives fluid into the base of the sandstone and expels it at the crest. We use mudstone pressures predicted from porosity and measured sandstone pressures to describe the spatial variation in pressure in two Eugene Island 330 reservoirs (Gulf of Mexico). In one severely overpressured reservoir, bounding mudstones are less compacted at the reservoir crest than at the reservoir base, and we interpret that flow is focused along the reservoir and expelled at the crest. In the second reservoir, mudstone is compacted around the base of the sandstone, and we interpret pore fluids were drawn into the sandstone. Dipping sandstone bodies encased in overpressured mudstone regulate hydrocarbon migration, affect borehole stability, and impact slope stability.