Summary of Published Literature on Anomalous Pressures: Implications for the Study of Pressure Compartments
The phenomenon of anomalous geopressure has been a focus of study for almost 40 years. Until recently, most studies were concerned with either the detection of overpressure or with the processes by which anomalous pressure is generated. Two processes, disequilibrium compaction and hydrocarbon generation/maturation, probably account for the majority of overpressured rocks observed worldwide. Because anomalous pressure is often associated with undercompacted rocks, various techniques based on detecting under-compaction (in addition to formation pressure tests) are commonly used to detect anomalous pressure indirectly. Whereas formation pressure tests can only be obtained in porous and permeable reservoir-type rocks, indirect techniques can only be applied in thick shale sequences. Therefore, data sets obtained by these two different approaches are mutually exclusive.
There has been a great deal of uncertainty with regard to the existence and nature of pressure seals. Overall, researchers studying anomalous pressure fall into one of two categories: those who accept the existence of effective seals and those who do not. Researchers who do not believe seals exist support classic hydrologic interpretations that suggest anomalous pressures are bounded by low-hydraulic-conductivity rocks and are, therefore, geologically ephemeral. Those who do believe seals exist, for the most part, do not present specific evidence of what comprises a seal. Although the sealing capacity of various rocks for hydrocarbon accumulations has been investigated, very little research has been conducted specifically concerning the seals that contain anomalous pressures. The influence of capillary forces on hydrocarbon entrapment is widely recognized, but capillary phenomena have largely been ignored in studies dealing with geopressures, even though hydrocarbon generation is commonly cited as the mechanism by which overpressure is generated. The delineation of pressure compartments, especially those of known geologic duration, is an important tool by which potential seals can be mapped and investigated. Without specific knowledge of seal characteristics, well-founded interpretations of seal effectiveness and pressure compartment longevity cannot be made.
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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.