Controls on Induced Fractures in Gas Shales of Appalachian Basin

William J. Gregg

A large-scale nine-year geotectonic field effort by the United States Department of Energy has been conducted in the Devonian gas shales of the Appalachian basin. This project was initiated to develop new insitu fracture stimulation methods in this low-permeability gas reservoir. Extensive mechanical testing in over 30 wells in the basin has shown that a well-developed preferred direction of fracture exists in the gas shales throughout an area of over 100,000 mi². The preferred direction manifests itself as a fracture direction in point-load testing, as a minimum tensile strength plane in Brazilian testing, and as a directional ultrasonic velocity minimum in wave-velocity testing. Previous studies have suggested a relationship between either in-situ stresses or paleostresses and the preferred direction of fracture; however, no particular microstructure has been conclusively demonstrated to be associated with the preferred direction. The present study documents the existence of such a microstructure in the Devonian gas shales. This structure is defined by a crystallographic and spatial arrangement of mica grains in crenulated structures that strike parallel to the preferred fracture direction. This mica fabric represents an incipient cleavage due to very low strains imposed on the eastern portion of the basin during late Paleozoic deformation. In-situ stress trajectories, therefore, cannot be assumed to be parallel to the preferred fracture directions shown by mechanical testing, as suggested by previous workers.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.