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Abstract
Chapter from: M
61: Basin Compartments and Seals
Edited by
Peter J. OrtolevaAuthors:
E. Sonnenthal and Peter J. Ortoleva Methodology and Concepts
Published 1994 as
part of Memoir 61
Copyright © 1994 The American Association of Petroleum
Geologists. All Rights Reserved. |
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Chapter 26
*
Numerical Simulations
of Overpressured Compartments in Sedimentary BasinsE. Sonnenthal
Peter J. Ortoleva
Indiana University
Bloomington, Indiana,
U.S.A.
*
ABSTRACT
In many sedimentary basins there are regions
where the fluid pressure exceeds the hydrostatic pressure and may approach
lithostatic values. These overpressured compartments occur in actively
forming basins as well as Paleozoic intracratonic basins. The development
and maintenance of fluid overpressures in compartments, over tens to hundreds
of millions of years, must require pressure-generating mechanisms and rock-sealing
processes to retard the loss of fluid. Several pressurizing processes have
been invoked for different basins and include thermal expansion, organic
reactions, disequilibrium mechanical compaction, poroelastic deformation,
and pressure-solution. Whether the seal is discordant or concordant with
bedding, it must be diagenetically altered or mechanically compacted for
it to have sufficiently low permeability to retard appreciable fluid loss.
Mechanisms of seal formation are therefore dependent on pressure and temperature
and thus may be tightly coupled to pressure-generating mechanisms. The
thermal, tectonic, and depositional history of the basin directly affects
these processes.
This paper describes a model and gives
computer simulations in two dimensions of coupled fluid flow, compaction,
hydrofracturing, deposition, and subsidence in sedimentary basins. The
mechanism of compaction used is a water-film diffusion model of pressure
solution for quartz in a periodic array of truncated spheres. Although
this process is not the only possible compaction mechanism, it is important
in quartz-rich sandstones and illustrates the coupling of overpressuring
and diagenesis. Results of simulations show that regions of overpressure
can encompass several lithologies with the upper transition to normal pressure
cutting across dipping beds, which were deformed by differential tectonic
subsidence. Some compartments are enclosed within individual beds, sealed
by greater compaction on the margins of the beds. Interiors of these overpressured
compartments become fractured and much more permeable as the fluid pressure
exceeds the sum of the horizontal stress and rock strength. Compaction
and thermal expansion of the fluid combine to cause overpressuring that
slows down the rate of compaction by reducing stresses on grain contacts.
Rocks surrounding the
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