Sesimic Amplitute Versus Offset (AVO) Character of Geopressured Transition Zones
The presence of diagenetically cemented seals distinguishes between possible mechanisms for the maintenance of abnormal fluid pressures in sedimentary basins. The velocity gradients and Poisson’s ratio variations surrounding a cemented seal affect the pre-stack amplitude versus offset behavior of seismic reflections from the seal. Acoustic synthetic seismograms based on well logs from sealed transitions demonstrate their unusual AVO character. A long-offset COCORP reflection line near Port Lavaca, Texas, shows similar effects from a seal at 6000 ft (1800 m) depth. Local conductivity logs and regional drilling mud weight compilations establish the presence of this pressure transition zone. It is associated with a strong, low-frequency reflector near its base and displays linear AVO trends markedly stronger than from deeper reflections within the overpressured compartment. The fact that a seal exhibits such prominent physical property characteristics suggests that AVO analysis techniques may locate other seals in basins worldwide, where proper conditions for diagenetic sealing rather than shaliness and subsidence may be needed to maintain geopressures.
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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.