New Borehole Breakout Derived Stress Constraints and their Implications for Stress Heterogeneity Near High Risk Fault Systems in the Santa Barbara Channel, Southern California

Abstract

The Santa Barbara Channel represents the offshore portion of the Ventura Basin in Southern California. Ongoing transpression related to a regional left step in the San Andreas Fault has led to the formation of E-W trending en-echelon fault systems, with both north and south dips, which accommodate varying rates of localized shortening across the basin. Recent studies have suggested that faults within the northern region of the channel could be capable of a multisegment rupture and producing a Mw 7.7–8.1 tsunamigenic earthquake. However, dynamic rupture models producing these results have not accounted for stress heterogeneity, which is known to limit earthquake size. With limited information available on the stress field in the Santa Barbara Channel, new constraints on crustal stress could prove essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems in this region. We use oriented 4-arm caliper data from 19 deviated wells obtained from industry to identify stress induced borehole breakouts beneath the Holly and Gail oil platforms in the Santa Barbara Channel. A misfit-based forward modeling technique is used to provide quantitative constraints on the orientations and relative magnitudes of the three principal stresses beneath each platform. At platform Gail, we determine a reverse faulting stress regime with SHmax trending N45°E (confidence range N40°E - N55°E). Our results are consistent with local structures, which reflect deeper regional scale trends, and with similar studies onshore nearby. At platform Holly, limited breakout azimuthal coverage in deviated wells restricted our ability to uniquely constrain the stress regime and SHmax orientation. However, our results indicate a reverse, oblique reverse, or strike-slip regime, and breakouts in near-vertical wellbore sections indicate mean SHmax orientations of N56.76°E ± 24.3° and N35.88°W ± 16.1° at different depths. Our results are partially consistent with the expected sense of slip along proximate fault systems, but differ from nearby onshore and offshore results, indicating stress heterogeneity in the north channel region. We conclude that borehole breakouts may record stress heterogeneity near active faults at the short length-scales (depth) important for seismic hazard studies.