Abstract: Use of Shear-Wave Anisotropy in Reservoir Characterization in Cymric Oil Field, CA

DE, GOPA S. and DONALD F. WINTERSTEIN, and SCOTT J. JOHNSON

Shear-wave anisotropy data from crossed-dipole wireline logs and nine-component VSPs from several wells in Cymric oil field show that there is a well-defined polarization azimuth of the fast shear-wave which is also the most rigid direction of the rock fabric of siliceous shales. Under induced stresses, these rocks would likely fracture with the fracture plane oriented along the rigid direction.

The rigid direction of the siliceous shale was seen to change by large amounts both with depth and from well to well at the same equivalent depths. In the relatively undeformed part of the reservoir, the fast shearwave polarization azimuth was aligned with the known regional maximum horizontal stress direction. However, in the highly deformed part of the reservoir, the fast shearwave polarization azimuth changed by approximately 55 degrees. Differences in the shear-wave birefringence magnitudes where lithologies are similar probably indicate differences in stress magnitudes. These results show that regional stress maps are insufficient for inferring local stresses at Cymric. Such stress azimuth and magnitude changes in turn have major consequences for intensive developments of Cymric oil field.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California