| ABSTRACT
Overpressure can be produced
by the following processes: (1) increase of compressive stress, (2) changes
in the volume of the pore fluid or rock matrix, and (3) fluid movement
or buoyancy. Loading during burial can generate considerable overpressure
due to disequilibrium compaction, particularly during the rapid subsidence
of low-
permeability sediments. Horizontal stress
changes can rapidly generate and dissipate large amounts
of overpressure in tectonically active areas.
Overpressure mechanisms involving change in volume must be well sealed
to be effective. Fluid volume increases associated with aquathermal expansion
and clay dehydration are too small to generate significant overpressure
unless perfect sealing occurs. Hydrocarbon generation and cracking to gas
could possibly produce overpressure, depending upon the kerogen type, abundance
of organic matter, temperature history, and rock permeability; however,
these processes may be self-limiting in a sealed system because buildup
of pressure could inhibit further organic metamorphism. The potential for
generating overpressure by hydrocarbon generation and cracking must be
regarded as unproven at present. Fluid movement due to a hydraulic head
can generate significant overpressure in shallowly buried, "well-plumbed"
basins. Calculations indicate that hydrocarbon buoyancy and osmosis can
generate only small amounts of localized overpressure. The upward movement
of gas in an incompressible fluid also could generate
©Copyright
1997. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript
received October 17, 1995; revised manuscript received September 4, 1996;
final acceptance January 20, 1997.
2Department
of Geological Sciences, Durham University, South Road, Durham DH1 3LE,
United Kingdom. Osborne e-mail: M.J.Osborne@
durham.ac.uk;
GeoPOP web site http://www.dur.ac.uk/~dgl0zz7/
We
wish to thank the companies that support the Geosciences Project on Overpressure
(GeoPOP) at the universities of Durham, Newcastle, and Heriot-Watt: Agip,
Amerada Hess, Amoco, ARCO, Chevron, Conoco, Elf Exploration, Mobil, Norsk
Hydro, Phillips Petroleum UK Company Limited, Statoil, and Total. We also
thank Neil Goulty (Durham) for commenting on an earlier draft of this paper.
Osborne thanks Gordon Macleod (Newcastle) for help with geochemical modeling. |
