Numerical Simulations of Overpressured Compartments in Sedimentary Basins
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 tohundreds 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 illustratesthe 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. Compactionand 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 overpressured compartment continue to compact and further tend to seal it from its surroundings. Application of such coupled models to specific basins would require detailed information on the dominant overpressuring mechanisms and geologic history, yet could yield much insight into the formation of compartments and deep-basin fluid flow patterns.
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Basin Compartments and Seals
Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.