SWARBRICK, RICHARD E.¹, MARK J. OSBORNE², and GARETH S. YARDLEY^3
1University of Durham, Durham, England
²BP Exploration, Sunbury-on- England
³Heriot-Watt University, Edinburgh, Scotland.

Abstract: Magnitude Of Overpressure From The Main Generating Mechanisms

The contribution of all the popular generation mechanisms to reservoir overpressure has been re-assessed. Vertical stress due to sediment loading, leading to disequilibrium compaction during incomplete dewatering, represents a principal mechanism. Continuous burial leads to overpressure magnitude increasing downwards below the depth of fluid retention. Connectivity of highly permeable strata leads to rapid redistribution of fluids and pressures, generally from a downdip location where differential subsidence has created higher overpressure. This lateral transfer results in an underestimate of overpressure using 1-D models. Magnitude of overpressure due to lateral stress is low, except where compressional tectonics are evident.

Overpressure can be created by "fluid expansion" mechanisms, such as aquathermal, clay dehydration, smectite-illite transformation, mineral precipitation/cementation reactions and maturation of source rocks to oil. In each case the amount of volume change is small, the rate of change generally slow, and there is concurrent pressure dissipation through the seals when realistic matrix permeability is considered. The exception is volume change associated with gas generation, but only where rapid gas generation takes place in strata with limited connected pore volume. Gas generation is restricted to deep basin locations.

Overpressure from a hydraulic head and/or buoyancy of the hydrocarbon phase are readily assessed. Osmosis leads to a magnitude of overpressure limited to about 500 psi (4 MPa). We present our basis for concluding that disequilibrium compaction and gas generation are the principal generating mechanisms.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas