Stress orientations from well bore breakouts in the Coalinga Region

Borehole breakouts are caused by unequal stress concentrations around a borehole resulting in shear failure of the borehole wall and creating hole elongation perpendicular to the maximum horizontal in situ stress. Breakouts in Tertiary sedimentary rocks were identified in 10 holes in the West Coalinga, Cantua Creek, and West Haven oil fields in the western San Joaquin Valley. The deeper and more reliable (1.5 to 3.7 km) breakouts were oriented northwest, indicating northeast compressive stress, perpendicular to the major fold axes in the San Joaquin Valley and at a 70°–90° angle to the major strike-slip faults in the Coast Ranges. This stress direction is consistent with a reverse slip component on northwest-trending faults in the region and with the focal mechanisms reported for the May 1983 Coalinga earthquake. The existing literature on stress measurements in central and southern California indicates that on a regional scale, the stress regime is transitional between thrust and strike-slip faulting. A hydraulic fracture test in the Kettleman Hills oil field at 3 km depth indicates that this stress regime exists in the upper crust near Coalinga as well.