Insight into the Stress Field of the Southern Los Angeles Basin, California Determined from Borehole Breakout and Drilling Induced Fracture Analysis

Justin O. Kain, & Patricia Persaud

In Preparation March 1, 2021, SCEC Contribution #10955

We present a new comprehensive model of the in-situ stress in the upper 3 km of the shallow crust near an inferred blind thrust fault that crosscuts the Wilmington oil field, Southern California. The Giant Wilmington field is located just south of Long Beach, California, and was previously thought to have no major faults capable of large earthquake ruptures. However, a recent study of the Wilmington anticline has identified a blind thrust fault that is capable of producing a multi-segment (Mw 6.3–6.4) earthquake rupture. These estimates are based on the onshore Torrance, the Wilmington blind thrust, and the Huntington Beach fault segments all rupturing together. This rupture model does not take into account stress heterogeneity along strike of these faults (~55 km total length) which may limit the size of earthquakes. To obtain a more realistic model of potential earthquake rupture, information on the stress field along strike of the Wilmington blind thrust fault system is essential.
We analyze digital oriented 4- and 6-arm caliper data and resistivity image logs from 36 (image logs from 16 wells) wells in the Wilmington oil field that are distributed in an ~12 x 3 km2 area and sample a depth up to ~3140 m. Spatially these wells have surface locations within ~500 meters of the Wilmington Blind Thrust. Caliper logs give insight on borehole breakouts, or compressive shear failures, while resistivity images can show both borehole breakouts and drilling induced tensile fractures. This dense data set yields ~450 breakout zones (total length ~670 meters) and 71 drilling induced tensile fracture zones (total length of 170 meters).
Previously a preliminary misfit calculation for eight wells on a single platform was conducted that determined the misfit between the dataset and theoretical stress orientations to obtain a high confidence model of the stress state. We have incorporated the remaining three platforms in our misfit analysis. This resulted in a best fit model of Shmin 2 and Shmax 1.9 and Sv 1.0 with an SHmax orientation of N4°E. Additionally, the near-vertical sections, with wellbore deviations <10⁰, show a dominant of SHmax azimuth of N5°E. These results indicate a reverse stress regime for the Wilmington Anticline, which contrasts with nearby onshore stress regimes and may indicate possible stress heterogeneity.

Citation
Kain, J. O., & Persaud, P. (2021). Insight into the Stress Field of the Southern Los Angeles Basin, California Determined from Borehole Breakout and Drilling Induced Fracture Analysis . Insight into the Stress Field of the Southern Los Angeles Basin, California Determined from Borehole Breakout and Drilling Induced Fracture Analysis , (in preparation).