SWARBRICK, RICHARD E.1, MARK J. OSBORNE2, and GARETH S. YARDLEY3
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