 |
Chapter 30
*
The Mechanical
Properties of Rock through an Ancient Transition Zone in the Appalachian
BasinIrene Meglis*
Terry Engelder
The Pennsylvania State
University
University Park, Pennsylvania,
U.S.A.
*
ABSTRACT
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. |
---|