Natural Fractures, Mechanical Anisotropy, Stress, and Fabric in North-Central Oklahoma

William D. Rizer

The regional fracture pattern exposed on surface outcrops of Permian and Pennsylvanian limestones and sandstones in north-central Oklahoma consists of a systematic set striking east-northeast and an orthogonal set striking north-northwest. Both sets are normal to bedding and show no evidence of shear offset. Detailed mapping of fractures exposed in Permian limestone in a quarry at Vap's Pass, Kay County, Oklahoma, reveals that individual fractures of both sets are comprised of joined straight-line segments of slightly different strike. The systematic set strikes between N70E and N85E and the non-systematic set between N10W and N45W.

Both the systematic and non-systematic fracture orientations are parallel to a (microcrack) fabric in the plane of bedding inferred from point load testing in the laboratory. Neither fracture set appears related to the depositional fabric as determined by thin-section analysis of the preferred orientations of grain boundaries and long axes.

Results of point load tests on oriented (Pennsylvanian) shale core from Conoco's Borehole Test Facility, 3 mi southwest of the Vap's Pass exposure, indicate a different orientation for mechanical anisotropy, N50E. That direction is roughly parallel to maximum horizontal in-situ stress determined from bore hole breakouts and to large-scale extensional fractures observed in the core.

The relations among surface and subsurface fracture orientation, depositional fabric, mechanical anisotropy, and the in-situ stress field are discussed.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.