Changes in shear‐wave birefringence and polarization azimuth from well to well in the Cymric oil field

Abstract

In 1989 we observed a major change with depth in shear-wave (S-wave) polarization azimuth near the Plio-Miocene unconformity in the Cymeric oil field of California in nine-component VSP data. Since then we recorded VSPs in two additional wells that showed similar large changes in S-wave polarization at the unconformity, but the polarization azimuths were shifted several tens of degrees relative to those of the first VSP. S-wave birefringence magnitude also changed by large amounts from well to well at equivalent depths. For the first VSP the polarization azimuth of the fast S-wave was N 60{degrees}E above the unconformity and N 10{degrees}E below. For the later VSPs, recorded less than a mile NW of the first VSP, the polarization azimuth of the fast S-wave ranged from N 10{degrees}E to N 35{degrees}E above and N 30{degrees}W to N 60{degrees}W below the unconformity. Azimuths from one of the VSPs were corroborated by analyses of crossed-dipole data. Because of the low rigidity of the rock at Cymric and the high magnitudes of S-wave birefringence, such changes in S-wave polarization azimuth likely indicate changes in direction of horizontal principal stresses. If so, maps of regional stress are not sufficient for inferring local stresses at Cymric.more » Such stress changes in turn have major consequences for intensive development of oil fields, especially where techniques such as induced fracturing or horizontal drilling are employed. The differences in birefringence magnitudes where lithologies are similar probably indicate differences in stress magnitudes.« less