Abstract: Systematic Variations in Stress State in the Southern San Joaquin Valley: Inferences Based on Wellbore Data and Contemporary Seismicity

David A. Castillo

Detailed knowledge of the stress field is an important element to consider when designing production fields for purposes of ensuring borehole stability, containment of hydraulic fractures and, understanding fluid transport properties. Analysis of wellbore breakouts from production fields in the southern San Joaquin Valley (SSJV) indicates three systematic spatial variations in the direction of the maximum horizontal stresses (S_Hmax). The regional NE-SW S_Hmax stress direction seen in production fields located along the western margin of the San Joaquin Valley (e.g., Coalinga and Elk Hills), changes to ~N-S compression over a distance of 10-20 km in the SSJV. This regional excursion in the SHmax stress direction in SSJV production fields (e.g., Yowlumn ), reflects a counterclockwise rotation of ~40°. This reorientation in stresses is consistent with ~N-S convergence inferred from trends in the local fold axes and reverse faults of Pliocene age and younger. In production fields located within the southern extreme boundaries of the SSJV (e.g., Wheeler Ridge), a second reorientation in the S_Hmax stress direction indicates a change to ~ENE-WSW compression. This latter S_Hmax reorientation is inconsistent with the local E-W trending fold axes and thrust faults, and interestingly, is restricted to production fields located within the inferred hanging wall of the White Wolf fault that ruptured during the 1952 M_s 7.8 Kern County earthquake. It appears that the stress drop and/or perturbati ns in the local pore pressures, associated with the 1952 earthquake may have been responsible for rotating the stresses away from the regional ~N-S to ~ENE-WSW direction, implying that the remote horizontal stresses may be comparable in magnitude. However, inversion of earthquake focal mechanisms of events located below the perturbed stress field indicates ~N-S compression and an increase in the horizontal stress differential. Finally, small-scale stress changes observed in individual wells may reflect local perturbations in the stress field resulting from slip along faults penetrated by the borehole.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California