How Does Lithology Impact Borehole Breakouts in central California?

Hannah Hackenmueller, Carleton College
Amelia A. Carson, Carleton College
Max B. Posner, Carleton College
Joshua R. Davis, Carleton College
Sarah J. Titus, Carleton College
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Abstract

Borehole breakouts are used to characterize stress directions in the shallow crust. Workers infer the maximum horizontal compressive stress direction from breakouts under the assumption that the host rocks are homogeneous and isotropic. In central California, previous studies determined the maximum horizontal stress directions to be at high angles to the San Andreas fault, especially in the San Joaquin valley, providing evidence for the weak fault hypothesis.

We revisit wells from the earlier studies in the San Joaquin valley, northeast of the San Andreas fault, where well logs are available. We also add data from new sites both northeast and southwest of the San Andreas fault. Using four-arm caliper logs, we identify breakouts with updated criteria from the World Stress Map project. We use dip logs to constrain the direction of bedding. Last, we use a variety of electrical logs, as well as mud logs and drilling reports to obtain lithologic data for each well.

From caliper logs, we determine that it is not straightforward to replicate the breakout results of previous studies. The issue may be related, in part, to the updated criteria used to identify breakouts from four-arm caliper logs. From dip logs, we observe that dip directions are often correlated with breakouts. In rare cases, breakout directions and bedding both change with depth in a single well, also supporting a correlation between the datasets. Last, from various electrical logs, we determine that the local rocks are typically composed of marine and terrestrial sandstones, siltstones, shales, cherts, and occasionally basement rocks. While breakouts occur in all sedimentary rock types, they seem to be more common in shales and finer-grained rocks and sometimes initiate and terminate when a significant change in grain size occurs. We are continuing to expand these datasets to further characterize the relationships between stress directions, bedding, and lithology in order to address whether rock anisotropy may play a role in breakout formation.

Session

Deformation in the upper crust