Numerical Modeling Analysis of the Role of Episodic Fault Overpressuring and Rupture in Methane Transport and Massive Calcite Cementation in the Santa Barbara Basin, California

Appold, Martin¹, Grant Garven², James R. Boles³, Peter Eichhubl⁴ (1) University of Missouri, Columbia, MO (2) Johns Hopkins University, Baltimore, MD (3) University of California, Santa Barbara, Santa Barbara, CA (4) University of Texas at Austin, Austin, TX

Massive calcite fault cements occur along portions of the Refugio-Carneros fault intersecting the Oligocene-Miocene Vaqueros Sandstone on the subaerial northern flank of the Santa Barbara basin. These calcite cements record the ascent of methane-rich hydrothermal basinal fluids that mixed with and became oxidized by meteoric water entering the basin at outcrops of the Vaqueros and other steeply dipping aquifers. Meteoric water appears to have flowed toward the fault in response to a southward topographic gradient. Methane-rich fluids could have reached the mineralized portions of the fault via at least two scenarios. In the first, overpressures in the submarine, central part of the basin, perhaps generated by disequilibrium compaction or hydrocarbon generation, may have driven methane-rich fluids northward. In the second, methane-rich fluids sourced from deeper Paleogene sediments would have been driven upward episodically by overpressures generated in the fault zone and released during fault rupture. The present study evaluated this second scenario using numerical reactive transport modeling and showed that transient pulses of fluid flow could transport enough heat to raise temperatures in the upper levels of the fault to 80-120° C, provided that fault overpressures were around at least 80% of lithostatic and the hydraulic conductivity of the fault during rupture was around at least 100 m/yr. The models also showed calcite precipitation to be concentrated at the intersection of the fault with the Vaqueros Sandstone, but raised doubts about whether the length of time and the number of fault pulses needed were too high given existing geologic constraints.