SYSTEMATIC VARIATIONS IN STRESS STATE IN THE SOUTHERN SAN-JOAQUIN VALLEY - INFERENCES BASED ON WELL-BORE DATA AND CONTEMPORARY SEISMICITY

Analysis of stress-induced well-bore breakouts in 35 wells from 10 pro duction fields in the southern San Joaquin Valley (SSJV) indicates sys tematic spatial variations in the direction of the maximum horizontal stresses at three different scales. First, the regional northeast-sout hwest 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 ove r a distance of 10-20 km in the SSJV. This major excursion in the stre ss field seen in the Yowlumne, Yowlumne North, Paloma, and Rio Viejo p roduction fields represents an approximately 40-degrees counterclockwi se rotation in the direction of the maximum horizontal stress (S(Hmax) ). This systematic reorientation is consistent with approximately nort h-south convergence as seen in the local fold axes and reverse faults of Pliocene age and younger. Second, at the extreme southern reaches o f the SSJV in the San Emidio, Los Lobos, Pleito, Wheeler Ridge, and No rth Tejon fields, another systematic, but localized, reorientation in the stress field indicates an abrupt change to an approximately east-n ortheast-west-southwest compression over a distance of a few kilometer s. This latter reorientation of S(Hmax) stress direction, which is inc onsistent with the local east-west-trending fold axes and thrust fault s, represents a 40-500 clockwise rotation in the stresses; this reorie ntation appears to be limited 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. Inversion of earthquake focal m echanisms of events located below the perturbed stress field indicates approximately north-south compression. The stress drop associated wit h the 1952 earthquake may have been responsible for rotating the S(Hma x) stress direction from the regional approximately north-south to app roximately east-northeast-west-southwest in the southernmost SSJV, imp lying that the remote horizontal stresses are comparable in magnitude. Finally, localized stress perturbations observed at the scale of a fe w meters in several wells may reflect local perturbations in the stres s field, resulting from slip along small faults penetrated by the bore hole. Knowledge of the nonuniformity in the SSJV stress field is impor tant for designing development programs for production fields in terms of borehole stability, propagation of hydraulic fractures, and fluid transport properties.