Evidence of sub kilometer-scale variability in in-situ stress directions near active faults: an example from the Newport-Inglewood Fault, southern California, Abstract T23C-2972
Patricia Persaud, Joann M. Stock, & Deborah E. SmithPublished 2015, SCEC Contribution #6218
As part of the Southern California Stress Map update, we determined the orientation of stress-induced compressive failures or borehole breakouts, which are reliable indicators of the orientation of the maximum horizontal stress direction (SH), in over 40 wellbores in the Los Angeles basin. The oriented caliper logs provided by various oil companies, and oil field operators allowed us to investigate the variation of SH in the three oil fields in the Newport-Inglewood Fault zone. We considered ellipticity of the borehole to be a breakout indicator if the maximum difference in caliper lengths was greater than 5-15% of the smallest caliper, the tool rotation was less than 10° over a depth range of 3 m, and the minimum difference in caliper lengths was less than 0.65 cm. More than 50% of our dataset comes from near-vertical wells. For wells deviated by more than 5°, we used a caliper arm that was not close to the high side of the tool to determine rotation as this introduced less variability in the calculated azimuth. In the Inglewood oil field, a dense dataset of 24 wells in ~2 km2, SH varies from N9°E to N32°E over a depth range of 1-3 km and within 400 m of the fault in the western fault block, with more variability occurring in wells father away. At depths below 2 km, SH takes on a more northerly orientation. In contrast, SH is oriented E-W in the eastern fault block, based on constraints from two wells. In the Wilmington oil field, data from 11 wells yields elongation directions (breakout heights ≥3 m) suggesting a dominantly thrust faulting stress regime. The pattern for Huntington Beach breakouts, however, indicates a strike-slip stress regime. The short-length-scale variations in SH direction are attributed to the proximity to faults or fault segmentation. These observational constraints on in-situ stress are useful for validating the Community Stress Model and indicate the likely complexity that may be found in stress fields near other active faults.
Citation
Persaud, P., Stock, J. M., & Smith, D. E. (2015). Evidence of sub kilometer-scale variability in in-situ stress directions near active faults: an example from the Newport-Inglewood Fault, southern California, Abstract T23C-2972. Poster Presentation at American Geophysical Union Fall Meeting.
