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The AAPG/Datapages Combined Publications Database

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Abstract


 
Chapter from: M 61: Basin Compartments and Seals 
Edited by 
Peter J. Ortoleva

Authors:
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.
 
Chapter 26

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Numerical Simulations of Overpressured Compartments in Sedimentary Basins

E. Sonnenthal
Peter J. Ortoleva
Indiana University
Bloomington, Indiana, U.S.A.


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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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