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1 Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Texas 78758; peter.eichhubl{at}beg.utexas.edu
2 Department of Geology, Temple University, Philadelphia, Pennsylvania 19122
3 Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Texas 78758
Peter Eichhubl received his M.S. degree from the University of Vienna, Austria, and his Ph.D. from the University of California, Santa Barbara. After research positions at Stanford University and the Monterey Bay Aquarium Research Institute, he joined the Bureau of Economic Geology at the University of Texas in Austin in 2006. His research interests include the interaction of brittle deformation and diagenesis, and fault and fracture mechanics.
Nicholas C. Davatzes has a Ph.D. in geology from Stanford University. He conducted postdoctoral research at Stanford and as a Mendenhall Fellow at the U.S. Geological Survey. His research explores the interaction of faulting, stress, and fluid flow. He is currently an assistant professor at Temple University and a visiting professor at the School for Renewable Energy Science.
Stephen P. Becker received his B.S. and M.S. degrees in geology from the University of Missouri-Rolla (now Missouri University of Science and Technology) in 2001 and 2005 and his Ph.D. in geochemistry from Virginia Tech in 2007. He currently holds a postdoctoral position at the Bureau of Economic Geology at the University of Texas at Austin, working on the application of fluid inclusions to understanding fracturing and fluid-flow histories.
ABSTRACT
The Moab fault, a basin-scale normal fault that juxtaposes Jurassic eolian sandstone units against Upper Jurassic and Cretaceous shale and sandstone, is locally associated with extensive calcite and lesser quartz cement. We mapped the distribution of fault-related diagenetic alteration products relative to the fault structure to identify sealing and conductive fault segments for fluid flow and to relate fault–fluid-flow behavior to the internal architecture of the fault zone. Calcite cement occurs as vein and breccia cement along slip surfaces and as discontinuous vein cement and concretions in fault damage zones. The cement predominates along fault segments that are composed of joints, sheared joints, and breccias that overprint earlier deformation bands. Using the distribution of fault-related calcite cement as an indicator of paleofluid migration, we infer that fault-parallel fluid flow was focused along fault segments that were overprinted by joints and sheared joints. Joint density, and thus fault-parallel permeability, is highest at locations of structural complexity such as fault intersections, extensional steps, and fault-segment terminations. The association of calcite with remnant hydrocarbons suggests that calcite precipitation was mediated by the degradation and microbial oxidation of hydrocarbons. We propose that the discontinuous occurrence of microbially mediated calcite cement may impede, but not completely seal, fault-parallel fluid flow. Fault-perpendicular flow, however, is mostly impeded by the juxtaposition of the sandstone units against shale and by shale entrainment. The Moab fault thus exemplifies the complex interaction of fault architecture and diagenetic sealing processes in controlling the hydraulic properties of faults in clastic sequences.
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