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Structure, Petrophysics, and Diagenesis of Shale Entrained Along a Normal Fault Shale and Sand from the Host Rock to the Fault

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Structure, Petrophysics, and Diagenesis of Shale Entrained Along a Normal Fault Shale and Sand from the Host Rock to the Fault Structure, petrophysics, AUTHORS Peter Eichhubl Department of Geologi- and diagenesis of shale cal and Environmental Sciences, Stanford University, Stanford, California 94305; present entrained along a normal address: Department of Physical and Life Sci- ences, Texas A&M University–Corpus Christi, fault at Black Diamond Mines, 6300 Ocean Drive, Corpus Christi, Texas 78412; [email protected] California— Implications Peter Eichhubl received his M.S. degree at the University of Vienna, Austria, and his Ph.D. at the University of California, Santa Barbara. for fault seal After research positions at Stanford University and Monterey Bay Aquarium Research In- Peter Eichhubl, Peter S. D’Onfro, Atilla Aydin, stitute, he joined the faculty at Texas A&M John Waters, and Douglas K. McCarty University–Corpus Christi. His research inter- ests include the interaction of chemical mass transfer and brittle deformation, diagenesis, and fault and fracture mechanics. ABSTRACT Peter S. D’Onfro ConocoPhillips, P.O. The structure, texture, composition, and capillary-pressure resis- Box 2197, Houston, Texas 77252-2197; tancewereassessedforshaledeformedalonganormalfaultwith [email protected] 9 m (29 ft) of dip separation. Shale is entrained from a 1.6-m (5-ft)- Peter D’Onfro earned his B.S. and M.S. degrees thick source layer into the fault zone and attenuated to about 5 cm from Boston College. After a research posi- (2 in.). A quantitative analysis of shale mineral composition indi- tion at Los Alamos National Laboratories, he cates that little material is contributed to the fault rock from the joined Conoco in 1979 and then ConocoPhillips sandstone units that over- and underlie the shale source layer. This in 2002. He specializes in bed and fault-seal finding is in contrast to common predictive models of fault seal- analysis for exploration prospect and reservoir ing that assume mechanical wear along the fault surfaces. Instead, performance evaluations. shale entrainment is inferred to result from incipient distributed Atilla Aydin Department of Geological shear across a zone of deformation bands in the over- and underlying and Environmental Sciences, Stanford Univer- sandstone, granular flow of the shale, and the increasing localization sity, Stanford, California 94305; of deformation in the shale core or along the shale-sandstone in- [email protected] terfaces of the evolving fault zone. The composition of deformed Atilla Aydin received his B.S. degree in geology shale indicates the effective mixing of clay- and quartz-rich layers of from the Istanbul Technical University in Turkey the shaly source unit by granular flow during shale deformation. and both his M.S. degree and his Ph.D. in ge- Capillary displacement pressures of deformed shale are 30% ology from Stanford University. He is a research higher compared to the most clay-rich undeformed shale outside professor of structural geology and geome- the fault. This increase in sealing capacity, in combination with a chanics and codirector of the Rock Fracture 50% anisotropy in capillary displacement pressure, is primarily at- Project at Stanford University. His research in- tributed to the development of a planar fabric in deformed shale. terest includes fracturing and faulting of rocks Enhanced clay diagenesis likely contributed to the increase in shale and fluid flow in fractured and faulted rocks sealing capacity. We conclude that fault seal by shale entrainment and regions, with application to hydrocarbon involves a variety of structural, textural, and diagenetic processes migration, entrapment, and recovery. that require an integrated methodology for improved predictions of John Waters Black Diamond Mines Re- fault-sealing capacity. gional Preserve, Antioch, California 94509; [email protected] John Waters directs abandoned mine recla- Copyright #2005. The American Association of Petroleum Geologists. All rights reserved. mation and development for the East Bay Re- Manuscript received September 18, 2004; provisional acceptance December 7, 2004; revised manuscript gional Park District, a San Francisco Bay area received April 21, 2005; final acceptance April 22, 2005. DOI:10.1306/04220504099 AAPG Bulletin, v. 89, no. 9 (September 2005), pp. 1113–1137 1113 park agency. His responsibilities include the INTRODUCTION development of an abandoned underground mine for use as a museum and a site for The entrainment of shale along faults in sandstone-shale sequences, geotechnical research and education. John commonly referred to as shale smear, potentially impedes the flow also serves as a consultant to other agencies of hydrocarbons and water across fault contacts that juxtapose and private industry on abandoned-mine– sandstone against sandstone (Smith, 1980; Lindsay et al., 1993; related issues. Knipe et al., 1997). Such faults may form economic hydrocarbon Douglas K. McCarty ChevronTexaco, traps depending on the petrophysical properties of the entrained 3901 Briarpark, Houston, Texas 77042; and deformed shale and its extent and continuity along the fault [email protected] surface. The effect of shale on impeding fluid flow is attributed Douglas McCarty received his B.S. and M.S. to two processes commonly referred to as membrane or static seal- degrees at the University of Montana and his ing and hydraulic-resistance or dynamic sealing (Watts, 1987; Heum, Ph.D. in geology at Dartmouth, with a spe- 1996; Brown, 2003). Membrane-sealing behavior results from the cialty in clay mineralogy. After a research po- capillary pressure that resists flow of the nonwetting fluid phase sition at Montana State University, he started in a two- or multiphase system across necks in the continuous pore work at what is now ChevronTexaco in 1997. system of the shale (Watts, 1987). The size of the trap is, in this case, His research interests relate to mineralogy and controlled by the hydrocarbon-column height that the seal can its geological relationships. support because of its capillary-pressure resistance for flow of the nonwetting hydrocarbon phase. Hydraulic-resistance sealing is con- ACKNOWLEDGEMENTS trolled by the matrix or fracture permeability of the shale, which is applicable to single- and multiphase fluid systems. The size of a We thank Ray Sullivan for insightful discussions trap formed by a hydraulic-resistance seal is controlled by the rate on the Domengine depositional environment of reservoir charge relative to the leakage rate. Although the seal- and for providing an expanded stratigraphic section. Park ranger Pat Dedmon provided ing behavior of any given faults may vary with depth, lithologic assistance during sample collection and fault juxtaposition, and reservoir and fault fluid composition (Brown, excavation. The personnel of the Black Dia- 2003; Bretan et al., 2003), trap size in many hydrocarbon systems mond Mines Regional Preserve are thanked is believed to be controlled by membrane-sealing behavior (Scho- for general logistic support. The manuscript walter, 1979). The prediction of fault properties that control the benefited from reviews by AAPG Bulletin re- membrane-sealing behavior is thus of fundamental importance viewers Stephen Dee, Ted Doughty, and Ron for predicting oil and gas in place in hydrocarbon exploration. Nelson. This study was supported by the in- Seal prediction of faults containing entrained shale has thus dustrial affiliates of the Stanford Shale Smear focused on two controlling factors: (1) the geometric properties of Project, with laboratory analyses and printing the shale layer and (2) the petrophysical properties that determine costs paid for by ConocoPhillips. the sealing capacity of the shale. Studies on the geometric prop- erties have largely addressed the shale continuity as a function of shale composition, slip distance, and structural style (Weber et al., 1978; Smith, 1980; Bouvier et al., 1989; Lindsay et al., 1993; Gibson, 1994; Childs et al., 1997; Lehner and Pilaar, 1997; Yielding et al., 1997; Koledoye et al., 2000, 2003; Younes and Aydin, 2001; Aydin and Eyal, 2002; Doughty, 2003). Current models predicting the sealing capacity of shale in faults are based on reservoir calibra- tions of measured pressure differentials across faults, with a single parameter that relates to fault rock composition (Yielding et al., 1997; Skerlec, 1999; Harris et al., 2002; Yielding, 2002). This pa- rameter, referred to as the shale gouge ratio by Yielding et al. (1997, p. 900–901), relates to the volume fraction of clay minerals or phyllosilicates integrated over the thickness of shale and sand units that have slipped past a point on the fault over the finite slip distance. The shale gouge ratio is considered to equal the volume fraction of clay minerals of the entrained shale assuming perfect mixing of clay and nonclay minerals and an equal contribution of 1114 Structure, Petrophysics, and Diagenesis of Shale Entrained along a Normal Fault shale and sand from the host rock to the fault. This mond Mines Regional Preserve, Antioch, California parameter is then empirically calibrated to pressure (Sullivan and Waters, 1980; Sullivan et al., 1994, 2003). differentials across faults in the same field. This underground setting was chosen to minimize the The ability of the shale gouge ratio and similar effects of surface alteration on shale composition and single-parameter criteria to sufficiently characterize the petrophysical properties while permitting access for shale-sealing capacity appears, based on our review of controlled sampling
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