Systematic Variations in Stress State in the Southern San Joaquin Valley: Inferences Based on Well-Bore Data and Contemporary Seismicity

Abstract

Analysis of stress-induced well-bore breakouts in 35 wells from 10 production fields in the southern San Joaquin Valley (SSJV) indicates systematic spatial variations in the direction of the maximum horizontal stresses at three different scales. First, the regional northeast-southwest compressional stress direction seen along the western margin of the San Joaquin Valley in the Elk Hills, Kettleman Hills, and Coalinga areas, gradually changes to approximately north-south compression over a distance of 10-20 km in the SSJV. This major excursion in the stress field seen in the Yowlumne, Yowlumne North, Paloma, and Rio Viejo production fields represents an approximately 40° counterclockwise rotation in the direction of the maximum horizontal stress (SHmax). Th s systematic reorientation is consistent with approximately north-south convergence as seen in the local fold axes and reverse faults of Pliocene age and younger. Second, at the extreme southern reaches of the SSJV in the San Emidio, Los Lobos, Pleito, Wheeler Ridge, and North Tejon fields, another systematic, but localized, reorientation in the stress field indicates an abrupt change to an approximately east-northeast--west-southwest compression over a distance of a few kilometers. This latter reorientation of SHmax stress direction, which is inconsistent with the local east-west--trending fold axes and thrust faults, represents a 40-50° clockwise rotation in the stresses; this reorientation appears to be limited to production fields located within the inferred hanging w ll of the White Wolf fault that ruptured during the 1952 Ms 7.8 Kern County earthquake. Inversion of earthquake focal mechanisms of events located below the perturbed stress field indicates approximately north-south compression. The stress drop associated with the 1952 earthquake may have been responsible for rotating the SHmax stress direction from the regional approximately north-south to approximately east-northeast--west-southwest in the southernmost SSJV, implying that the remote horizontal stresses are comparable in magnitude. Finally, localized stress perturbations observed at the scale of a few meters in several wells may reflect local perturbations in the stress field, resulting from slip along small faults penetrated by the borehole. Knowledge of the nonun formity in the SSJV stress field is important for designing development programs for production fields in terms of borehole stability, propagation of hydraulic fractures, and fluid transport properties.