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.