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Rudi Wilhelm1, Craig Limbaugh2
(1) Consultant, Houston, TX
(2) Consultant, Houston

Abstract: Pressure compartments from effective stress profiles - hydrocarbon trap implications for the shelf and deepwater GOM

A successful technique for the prediction of pressure compartments and the evaluation of hydrocarbon traps in the Gulf of Mexico basin is presented. The distinguishing feature of the technique is the integration of geologic information into the interpretation of pressure gradient vs. depth profiles.

The effective stress gradient is computed from seismic velocity data, and is used in conjunction with both the overburden gradient and fracture gradient curves to delineate the hydrocarbon seal failure zone. The effect of water depth is taken into account, and the fluid pressure and seal capacity estimates are calibrated to nearby wells whenever possible.

The effective stress gradients are estimated using Dutta's constitutive model of undercompacted shales. It differs from other published pressure models in that the calculation of pore pressure takes into account its predominant causes: namely, shale mineralogy (cation exchange capacity) and temperature. Since this pressure model does not need the conventionally used normal interval travel time shale compaction trend it is particularly useful in deep water where geo-pressures start shallow below mud line.

The method improves the accuracy of the predicted effective stress. This results in a better estimation of the fluid pressure gradient for trap evaluation and expected maximum possible column height.

Examples from different deltaic and turbidite provinces will be used to describe the technique, the data preparation, and the interpretation of pressure compartments, migration pathways, seal failure and the impact on the hydrocarbon trap.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana