This paper was prepared by the Houston Geological Society Abnormal Pressure Study Group Subcommittee on Origins, to serve petroleum geologists who may be responsible for planning and executing drilling programs.

The normal compactional process produces a stress system in sedimentary rock. A stress system is in equilibrium when the overburden pressure on a given rock equals the sum of the fluid pressure and the grain pressure within the rock. Processes which impose changes in the stress system may generate abnormal pressures, and there are several different modes of origin. Abnormal pressures may be generated if changes in overburden pressure result from vertical compression, horizontal compression, or uplift. Abnormal pressures also may result if changes in fluid pressure result from fluid density contrast or recharge, or if mechanical or physical processes (such as faulting, adsorption, osmosis, or diagenesis) inhibit the expulsion of fluid from compacting rocks.

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