The Mechanical Properties of Rock through an Ancient Transition Zone in the Appalachian Basin
Hydrostatic compression tests were used to measure the variation in mechanical properties of Devonian core cut through an ancient pressure transition zone in the Appalachian Plateau of western New York. The properties of this core are strongly influenced by low aspect ratio microcracks developed during stress relief. In the siltstones, linear compressibility decreases with increasing confining pressure to a minimum value between 14 and 16 x 106 MPa1 in all directions. In the shales, linear compressibility parallel to bedding is approximately 10 x 106 MPa1 for all confining pressures, but normal to bedding it decreases with increasing confining pressure throughout the range of test pressures (140 MPa), reaching minimum values between 25 and 40 x 106 MPa1 at high pressure. The high linear compressibility normal to bedding reflects compression and closure of a dense population of bedding-parallel microcracks. Microcrack porosity is lowest in the core 50 m above the ancient transition zone and is higher further above and below the zone. In conclusion, rocks within the ancient transition zone have lower intrinsic compressibilities than overlying rocks (8-12 x 106 MPa1 versus 9-18 x 106 MPa1). Such a marked change in elastic properties near the transition zone may serve as a seismic reflector.
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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.