Using Borehole Breakouts in Deviated Offshore Wells to Constrain Stress Regimes Beneath The Santa Barbara Channel, Offshore Southern California

Edward H. Pritchard, Patricia Persaud, & Joann M. Stock

Submitted August 15, 2017, SCEC Contribution #7752, 2017 SCEC Annual Meeting Poster #209

The Santa Barbara Channel is an E-W trending marine basin that represents the western continuation of the onshore Ventura basin. Beneath this channel lie several active, E-W trending en echelon fault systems, with both N and S dips, that control a series of tight, asymmetric, anticlinal folds along the North Channel and Mid-Channel regions. Although controversial, recent results have suggested that some faults in the North Channel region could be capable of large, multisegment ruptures, which could potentially generate earthquakes of up to Mw 7.8 (Hubbard et al., 2014). Not controversial, however, is the fact that further knowledge of the stress regime related to these fault systems, and other analogous systems within the channel, would greatly contribute to our understanding of a potential rupture along them.

In this study, oriented 4-arm caliper well log data obtained from industry are used to determine the orientations of stress induced shear failures along well bore walls, called borehole breakouts, beneath the Holly and Gail drilling platforms in the Santa Barbara Channel. Analysis of breakout orientations along 18 differently oriented, highly deviated wells allows for constraint of the current in situ stress regime beneath each of the platforms. The best-fit stress regime can then be used to inform the nature of slip along nearby faults, many of which are blind and display no surface indications of slip.

At the Holly platform, located roughly 19 km west of Santa Barbara, and proximate to the Pitas Point, North Channel, and Red Mountain fault systems, lower hemisphere polar projections of breakout orientations in deviated well sections within 1.7 km depth indicate a mainly thrust faulting stress regime, although a strike-slip component is not currently excluded. At the Gail platform, located midway between Ventura and Santa Cruz Island, and proximate to the Western Deep Fault, polar projections of breakouts within 2.8 km depth indicate that a predominantly thrust faulting stress regime also exists beneath this platform. However, a few inconsistencies in the breakout orientations at each platform suggest variability in the stress regime, leading to the hypothesis that the stress field beneath these regions may change with depth, from a shallow degenerate-like thrust faulting stress state, with the horizontal principal stresses roughly equal in magnitude yet greater than the vertical principal stress (SH = Sh > Sv), to a deeper, less degenerate regime.

Key Words
Borehole,breakouts,stress,offshore,santa barbara,channel,fault

Citation
Pritchard, E. H., Persaud, P., & Stock, J. M. (2017, 08). Using Borehole Breakouts in Deviated Offshore Wells to Constrain Stress Regimes Beneath The Santa Barbara Channel, Offshore Southern California. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)