Numerical modeling of the origin of calcite mineralization in the Refugio-Carneros fault, Santa Barbara Basin, California: GEOFLUIDS, vol. 7, p. 79-95, 2007.

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Many faults in active and exhumed hydrocarbon-generating basins are characterized by thick deposits of carbonate fault cement of limited vertical and horizontal extent. Based on fluid inclusion and stable isotope characteristics, these deposits have been attributed to upward flow of formation water and hydrocarbons. The present study sought to test this hypothesis by using numerical reactive transport modeling to investigate the origin of calcite cements in the Refugio-Carneros fault located on the northern flank of the Santa Barbara basin of southern California. Previous research has shown this calcite to have low δ13C values of about −40 to −30‰ PDB, suggesting ... continued below

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79-95

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Appold, M. S.; Garven, G.; Boles, J. R. & Eichhubl, P. February 1, 2007.

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Many faults in active and exhumed hydrocarbon-generating basins are characterized by thick deposits of carbonate fault cement of limited vertical and horizontal extent. Based on fluid inclusion and stable isotope characteristics, these deposits have been attributed to upward flow of formation water and hydrocarbons. The present study sought to test this hypothesis by using numerical reactive transport modeling to investigate the origin of calcite cements in the Refugio-Carneros fault located on the northern flank of the Santa Barbara basin of southern California. Previous research has shown this calcite to have low δ13C values of about −40 to −30‰ PDB, suggesting that methane-rich fluids ascended the fault and contributed carbon for the mineralization. Fluid inclusion homogenization temperatures of 80-125° C in the calcite indicate that the fluids also transported significant quantities of heat. Fluid inclusion salinities ranging from fresh water to seawater values and the proximity of the Refugio-Carneros fault to a zone of groundwater recharge in the Santa Ynez Mountains suggests that calcite precipitation in the fault may have been induced by the oxidation of methane-rich basinal fluids by infiltrating meteoric fluids descending steeply dipping sedimentary layers on the northern basin flank. This oxidation could have occurred via at least two different mixing scenarios. In the first, overpressures in the central part of the basin may have driven methane-rich formation waters derived from the Monterey Formation northward toward the basin flanks where they mixed with meteoric water descending from the Santa Ynez Mountains and diverted upward through the Refugio-Carneros fault. In the second scenario, methane-rich fluids sourced from deeper Paleogene sediments would have been driven upward by overpressures generated in the fault zones due to deformation, pressure solution, and flow, and released during fault rupture, ultimately mixing with meteoric water at shallow depth. The models in the present study were designed to test this second scenario, and show that in order for the observed fluid inclusion temperatures to be reached within 200 meters of the surface, moderate overpressures and high permeabilities were required in the fault zone. Sudden release of overpressure may have been triggered by earthquakes and led to transient pulses of accelerated fluid flow and heat transport along faults, most likely on the order of 10’s to 100’s of years in duration. While the models also showed that methane rich fluids ascending the Refugio-Carneros fault could be oxidized by meteoric water traversing the Vaqueros Sandstone to form calcite, they raised doubts about whether the length of time and the number of fault pulses needed for mineralization by the fault overpressuring mechanism were too high given existing geologic constraints.

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Blackwell Science Publishing

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  • Journal Name: Geofluids; Journal Volume: 7; Journal Issue: 1

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  • Report No.: DOE/ER14619-1
  • Grant Number: FG02-96ER14619
  • DOI: 10.1111/j.1468-8123.2006.00165.x | External Link
  • Office of Scientific & Technical Information Report Number: 920144
  • Archival Resource Key: ark:/67531/metadc895679

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  • February 1, 2007

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Appold, M. S.; Garven, G.; Boles, J. R. & Eichhubl, P. Numerical modeling of the origin of calcite mineralization in the Refugio-Carneros fault, Santa Barbara Basin, California: GEOFLUIDS, vol. 7, p. 79-95, 2007., article, February 1, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc895679/: accessed June 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.