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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
2004. The American Association of Petroleum Geologists. All rights reserved.
Mineralization, fluid flow, and sealing properties associated with an active thrust fault: San Joaquin basin, California
Renee J. Perez,1 James R. Boles2
1Room 1006, Geological Sciences Department, Webb Hall, University of California at Santa Barbara, Santa Barbara, California 93106-9630; present address: Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, AB T2N-1N4, Canada; [email protected]
2Room 1006, Geological Sciences Department, Webb Hall, University of California at Santa Barbara, Santa Barbara, California 93106-9630; [email protected]
AUTHORS
Renee Perez is a postdoctoral fellow at the University of Calgary, Canada. He obtained his B.S. degree in engineering geology from Universidad Central de Venezuela, and his Ph.D. from the University of California at Santa Barbara. His current research includes clastic diagenesis and the application of thermodynamic, kinetic, and transport concepts to the interpretation of water-gas-rock interactions in sedimentary basins.
James Boles is a professor of geology at the University of California at Santa Barbara. He obtained his B.S. degree from Purdue University, his M.S. degree from the University of Wyoming, and his Ph.D. from the University of Otago (New Zealand). He has conducted research on various aspects of clastic diagenesis for more than 35 years. His current research is on fault diagenesis, pressure solution, and geochemical tools for interpreting 
diagenetic
processes.
ACKNOWLEDGMENTS
We acknowledge editorial work by Emma Perez and Arthur Sylvester, as well as critical reviews by Mark Person, Knut Bj
rlykke, Grant Garven, Marco Antonellini, Peter Hagelakas, and Rick Erickson. Comments by an anonymous reviewer were also helpful. Enrique Novoa interpreted our structural cross section. We also acknowledge Vintage Petroleum, Inc., which, through Tom Seiler, generously provided structural maps, logs, and all subsurface samples. The Department of Energy funded our research under grant no. 444033-22433.
ABSTRACT
Petrographic observations indicate that the distribution of cement and porosity within a Quaternary-age thrust fault in the subsurface of the Wheeler Ridge oil field in California is a function of depth and temperature and varies spatially. At depths shallower than 2.5 km (1.6 mi), porosity increases because of the abundance of open microfractures and plagioclase dissolution. At depths greater than 2.5 km (1.6 mi), the porosity in the fault zone decreases because of calcite cementation in microfractures that ultimately form vein networks. Based on 
18O data, we distinguished veins cemented by intraformational (lateral) flow into the fault from veins cemented by ascending fluids along the fault. Ascending, cementing fluids traveled at least 75–750 m (246–2460 ft) vertically. The petrography suggests that oil migration was the last event following dissolution and calcite cementation in the fault zone. Based on oil chemistry, whole-oil 13C, API gravity, and petrographic data, we propose that hydrocarbons, presently in shallow and deep reservoirs, flowed laterally into the fault zone. Whereas hydrocarbons in shallow reservoirs flowed across the fault into the hanging wall, hydrocarbons in deep reservoirs were trapped against the fault in the footwall. The increase of API gravity with depth and lack of evidence for retrograde condensation indicate a limited vertical migration and reaccumulation of hydrocarbon, suggesting that below 2.5 km (1.6 mi), the thrust behaves as a vertical seal. The sealing properties of the thrust vary spatially and may be controlled by calcite cementation.
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