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ABSTRACT: Gulf of Mexico Subsalt Deformation Zones: Weak Formation Integrity Rather than Anomalous High Pressure Zones


A study of pressure gradient profiles, mudweights and drilling report data from subsalt wells in the Gulf of Mexico indicates that reported shale deformation zones underneath salt are not anomalously high pressure zones, but rather, low formation integrity zones. Pore pressure gradients were calculated from shale transit time and resistivity logs, and supplemented by detailed drilling reports and available wireline pressure test data for 21 subsalt wells located in the shelf and deepwater areas of the central Gulf of Mexico.

The study indicates that pore pressure gradients beneath salt sheets remain constant from deformation zones into the deeper subsalt section and are high (16.5-20 KPa/m, 14-17 PPG) but not anomalously high for the depth and age of the subsalt section. Unusually low shale resistivities found in sections beneath salt indicate higher pore pressure gradients, but sonic values from the same intervals do not corroborate the higher pressures, suggesting that the low shale resistivities may be caused instead by highly saline formation brines. Lost circulation and well flow problems encountered when drilling out of salt indicate that some subsalt deformation zones have low formation integrity.

Results of this study do not support current subsalt exploration models that invoke hydrocarbon bearing fluids migrating upward to subsalt reservoirs through an undercompacted, high pressure shale deformation zone. Instead, some subsalt sections appear to be unstable zones of lower fracture strength with implications for both sealing capacity and subsalt well design. New models are needed to predict pressure and fracture gradients in subsalt deformation zones to evaluate migration and seal risk and ensure better design of subsalt wells.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana