Present-Day Stress Directions in California Determined Through Borehole Breakout Analysis
Van S. Mount, John Suppe
Borehole elongations or breakouts, observed on unprocessed four-arm dipmeter logs, have been used to map in-situ stress directions throughout onshore California. Approximately 130 wells were analyzed for borehole breakouts in California, with 107 giving useful results. The wells are subvertical (most inclinations > 80°) and range in depth from 920 to 5,760 m, with 90% of the wells being deeper than 1,500 m. Breakouts were observed from depths of 100-5,600 m. The number of broken out intervals per well range from 3 to 38 with an average of 12.
A regionally consistent stress pattern is observed with maximum horizontal compression generally oriented northeast-southwest. Breakout-determined stress directions are consistent with other stress indicators, including stress directions determined from focal mechanisms for earthquakes occurring at depths of 5-15 km. The direction of maximum compression consistently intersects the San Andreas fault at a high angle (80°-90°) and is approximately perpendicular to axes of young thrust-related anticlines.
Heat-flow and seismic observations limit the shear stress on the San Andreas fault to 10-20 MPa, or less. The observed orientation of maximum horizontal compression (nearly perpendicular) to the San Andreas) allows generation of large regional deviatoric stresses of rock-breaking magnitudes (on the order of 100 MPa). Transpressive plate motion is decoupled into a low-stress strike-slip component and a high-stress compressive component. These observations suggest that standard concepts of transpressive wrench tectonics--which envisage drag on a high-friction fault--are wrong. The thrust structures are largely decoupled from the strike-slip fault.
On a more local scale, it is possible to use stress directions determined from breakout data to predict hydraulic fracture directions accurately, as well as young natural fracture orientations, at an early stage of development in tectonically active areas.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.