Pressure Compartments in Sedimentary Basins: A Review
Pressure compartments are found in sedimentary basins throughout the world. They are defined primarily by hydraulic potentials calculated from pressure measurements but may be indicated by differing brine and hydrocarbon chemistries; by mineralogic differences; by electrical resistivity, sonic velocity, and density of the shales; and by mud weight requirements and drilling rate changes.
Pressure compartments are characterized by an effective seal, in three dimensions, that prevents pressure equilibration to normal hydrostatic pressure. A pressure seal, as opposed to a capillary seal, restricts flow of both hydrocarbon and brine and is formed where the pore throats become effectively closed, i.e., the permeability approaches zero. A leaking pressure seal, called a "rate seal" occurs when the pressure difference caused by subsidence-sedimentation or uplift-erosion or other pressure source is greater than the seal pressure leakage. When the internal fluid pressure in the compartment exceeds the fracture pressure of the seal, the seal will fracture and fluids will escape from the compartment. The fracture and resealing may occur repeatedly.
Multiple pressure seal origins must be invoked to explain their geometric and stratigraphic occurrence. Certainly some pressure seals appear to be stratigraphically controlled with perhaps more or less diagenetic enhancement. Some seals, particularly those that cross stratigraphy, appear to be entirely diagenetic. The lateral seals, which appear to be subvertical to vertical, are possibly due to faulting and fracturing or to lateral facies changes. An extensive investigation of seal origin, recognition, and duration is being undertaken by a consortium of universities under the sponsorship of the Gas Research Institute.
The cause of the abnormal fluid pressure, or flow if there is no seal, is epeirogeny (uplift or subsidence) with accompanying erosion or sedimentation. This changes the temperatures, pressure, and stress in the system leading to either changes in pressure, if sealed, or flow, if not sealed.
The knowledge and understanding of pressure compartments and compartment seals are vital to understanding the basin fluid flow regimes that control diagenesis and hydrocarbon migration and trapping.
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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.