Published 1994 as part of Memoir 61
Copyright © 1994 The American Association of Petroleum Geologists.  All Rights Reserved.

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 10^-6 MPa^-1 in all directions. In the shales, linear compressibility parallel to bedding is approximately 10 X 10^-6 MPa^-1 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 10^-6 MPa^-1 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 10^-6 MPa^-1 versus 9-18 X 10^-6 MPa^-1). Such a marked change in elastic properties near the transition zone may serve as a seismic reflector.