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Crossing conjugatel

Crossing conjugate

normal faults

David A. Ferrill, Alan P. Morris, John A. Stamatakos, and Darrell W. Sims

A B S T R A C T

Normal faults commonly develop in two oppositely dipping sets having dihedral angles of around 60?, collectively referred to as con-jugate normal faults. Conjugate normal faults form at a range of scales from cm to km. Where conjugate normal faults cross each other, the faults are commonly interpreted to accommodate ex-tension by simultaneous slip on the crossing faults. Using two-dimensional geometric modeling we show that simultaneous slip on crossing conjugate normal faults requires loss, gain, or localized re-distribution of cross-sectional area. In contrast, alternating sequen-tial slip on the crossing faults can produce crossing fault patterns without area modi?cationin cross section. Natural examples of crossing conjugate normal faults from the Volcanic Tableland (OwensValley, California), Bare Mountain (Nevada),and the Bal-cones fault zone (Texas)all indicate formation by sequential rather than simultaneous slip. We conclude that truly simultaneous activ-ity of crossing normal faults is likely to be limited to extremely small displacements due to rate-limiting area change processes. If their associated movement is truly simultaneous, crossing normal faults are virtually unrestorable and should show evidence of signi?cantcross-sectional area change (e.g.,area increase may be indicated by salt intrusion along fault, area decrease by localized dissolution or mechanical compaction may be indicated by extreme displacement gradients at fault tips). In the absence of such evidence, even the most complicated crossing fault pattern should be restorable by se-quentially working backward through the faulting sequence. In common with other structures that affect permeability and that cross at high angles, conjugate normal fault systems are likely to produce bulk permeability anisotropy in reservoir rocks that can be approximated by a prolate (elongate)permeability ellipsoid, with greatest permeability parallel with the line of intersection. Char-acterization of the fault pattern in a faulted reservoir provides the basis for interpreting the bulk permeability anisotropy in the res-ervoir, an important step in optimizing well placement. Copyright ?2000. The American Association of Petroleum Geologists. All rights reserved. Manuscript received January 21, 1999; revised manuscript received January 19, 2000; ?nalacceptance February 25, 2000.

AAPG Bulletin, v. 84, no. 10(October2000), pp. 1543–15591543A U T H O R S David A. Ferrill ?Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220Culebra Rd., San Antonio, Texas 78238–5166;dferrill@swri.eduDavid A. Ferrill received a B.S. degree from Georgia State University (1984),an M.S. degree from West Virginia University (1987),and a Ph.D. from the University of Alabama (1991).Before joining the Center for Nuclear Waste Regulatory Analyses (CNWRA)at Southwest Research Institute in 1993, he was an exploration geologist at Shell Offshore Incorporated. David is now a principal scientist and conducts analyses of faulting, fracturing, and seismicity related to radioactive waste disposal, and structural geological training and contract consulting for the oil and gas industry. Alan P. Morris ?Division of Earth and Physical Sciences, University of Texas at San Antonio, San Antonio, Texas, 78249Alan P. Morris received a B.S. degree from Imperial College (1973)and a Ph.D. from the University of Cambridge (1980).He has studied deformed rocks from Svalbard to the Basin and Range. Alan taught structural geology at Wayne State University (Detroit)and was a petroleum industry consultant for Geo-Logic Systems. He now teaches at the University of Texas at San Antonio, is a consultant to the CNWRA in San Antonio, and writes educational software for middle schools. John A. Stamatakos ?Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220Culebra Rd., San Antonio, Texas 78238–5166John A. Stamatakos received his B.A. degree from Franklin and Marshall College (geology,1981) and his M.S. and Ph.D. from Lehigh University (paleomagnetism,structural geology, and tectonics; 1988and 1990). Prior to joining the Southwest Research Institute in 1995, he spent two years as a postdoctoral research scientist at the Eidgeno ¨ssische Technische Hochschule in Zurich (geophysics)and three years as a visiting professor and research scientist at the University of Michigan. John is currently a senior research

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