We compare shale porosity-derived pore pressure predictions with measured sand pressures in multiple horizons of the Eugene Island 330 field (offshore Louisiana) and propose mechanisms to predict the observed differences. The predicted shale pressures from our wireline-derived undercompaction model systematically underpredict pressures at depth, where sands are in hard geopressure. The deviation between predicted and measured pressures at depth is interpreted to result from structural relief effects or late-stage increases in fluid pressure. Pressures within a dipping sand layer will equilibrate to maintain a fluid pressure gradient (i.e. 0.465 psi/ft for water-filled sand), while pressures within an impermeable shale will maintain a lithostatic pressure gradient (~0.94 psi/ft in this area). This results in shale pressures which differ from sand pressures as a function of position on structure. Sand fluid pressure can exceed shale pressure by as much as 0.42 psi per foot of relief. Alternatively, measured pressure may be greater than porosity-predicted pressure due to late-stage pressure generation mechanisms such as clay dewatering, aquathermal pressuring, or hydrocarbon migration. As shale compaction is largely irreversible, an increase in pore pressure following compaction would not result in a corresponding increase in porosity. We demonstrate in the OI sand (~8000 ft TVD) the portion of the pressure difference which can be attributed to structural relief and the portion which we attribute to late-stage pressure generation.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah