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1 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305; present address: Department of Physical and Life Sciences, Texas A&M University–Corpus Christi, 6300 Ocean Drive, Corpus Christi, Texas 78412;
peichhubl{at}falcon.tamucc.edu
2 ConocoPhillips, P.O. Box 2197, Houston, Texas 77252-2197;
donfrps{at}conocophillips.com
3 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305;
aydin{at}pangea.stanford.edu
4 Black Diamond Mines Regional Preserve, Antioch, California 94509;
jwaters{at}ebparks.org
5 ChevronTexaco, 3901 Briarpark, Houston, Texas 77042;
DMcCarty{at}chevrontexaco.com
Peter Eichhubl received his M.S. degree at the University of Vienna, Austria, and his Ph.D. at the University of California, Santa Barbara. After research positions at Stanford University and Monterey Bay Aquarium Research Institute, he joined the faculty at Texas A&M University–Corpus Christi. His research interests include the interaction of chemical mass transfer and brittle deformation, diagenesis, and fault and fracture mechanics.Peter D'Onfro earned his B.S. and M.S. degrees from Boston College. After a research position at Los Alamos National Laboratories, he joined Conoco in 1979 and then ConocoPhillips in 2002. He specializes in bed and fault-seal analysis for exploration prospect and reservoir performance evaluations.
Atilla Aydin received his B.S. degree in geology from the Istanbul Technical University in Turkey and both his M.S. degree and his Ph.D. in geology from Stanford University. He is a research professor of structural geology and geomechanics and codirector of the Rock Fracture Project at Stanford University. His research interest includes fracturing and faulting of rocks and fluid flow in fractured and faulted rocks and regions, with application to hydrocarbon migration, entrapment, and recovery.
John Waters directs abandoned mine reclamation and development for the East Bay Regional Park District, a San Francisco Bay area park agency. His responsibilities include the development of an abandoned underground mine for use as a museum and a site for geotechnical research and education. John also serves as a consultant to other agencies and private industry on abandoned-mine–related issues.
Douglas McCarty received his B.S. and M.S. degrees at the University of Montana and his Ph.D. in geology at Dartmouth, with a specialty in clay mineralogy. After a research position at Montana State University, he started work at what is now ChevronTexaco in 1997. His research interests relate to mineralogy and its geological relationships.
The structure, texture, composition, and capillary-pressure resistance were assessed for shale deformed along a normal fault with 9 m (29 ft) of dip separation. Shale is entrained from a 1.6-m (5-ft)-thick source layer into the fault zone and attenuated to about 5 cm (2 in.). A quantitative analysis of shale mineral composition indicates that little material is contributed to the fault rock from the sandstone units that over- and underlie the shale source layer. This finding is in contrast to common predictive models of fault sealing that assume mechanical wear along the fault surfaces. Instead, shale entrainment is inferred to result from incipient distributed shear across a zone of deformation bands in the over- and underlying sandstone, granular flow of the shale, and the increasing localization of deformation in the shale core or along the shale-sandstone interfaces of the evolving fault zone. The composition of deformed shale indicates the effective mixing of clay- and quartz-rich layers of the shaly source unit by granular flow during shale deformation.
Capillary displacement pressures of deformed shale are 30% higher compared to the most clay-rich undeformed shale outside the fault. This increase in sealing capacity, in combination with a 50% anisotropy in capillary displacement pressure, is primarily attributed to the development of a planar fabric in deformed shale. Enhanced clay diagenesis likely contributed to the increase in shale sealing capacity. We conclude that fault seal by shale entrainment involves a variety of structural, textural, and diagenetic processes that require an integrated methodology for improved predictions of fault-sealing capacity.
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