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_Journal of Structural Geology 32 (2010) 1363–1374_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Integrated fracture prediction using sequence stratigraphy within a carbonate fault damage zone, Texas, USA Christopher K. Zahm*, Laura C. Zahm, Jerome A. Bellian Bureau of Economic Geology, The University of Texas at Austin, Jackson School of Geosciences, University Station, Box X, Austin, TX 78712, United States Article info Article history: Received 21 February 2008 Received in revised form 18 May 2009 Accepted 20 May 2009 Available online 6 June 2009 Keywords: Sequence stratigraphy Fractures Carbonate TST HST Fault damage zone Abstract Fracture deformation intensity is heterogeneous as a result of the interplay between the stratigraphic architecture and the degree of faulting. Prediction of the distribution of fractures requires careful consideration of the sequence stratigraphic framework in concert with the structural deformation process. Deformation intensity was found to vary by facies that were divided into distinct mechanical units and characterized within a sequence stratigraphic framework. Deformation was found to be more intensely developed within the transgressive systems tract (TST) versus the highstand systems tract (HST). In the HST, facies observed in the outcrop from this study had less mud, thicker cycles, and higher uncon?ned strength (average of 57 MPa for non-argillaceous facies). In contrast, the TST cycle sets have a higher argillaceous component and cycles are thinner. The facies have a higher mud content and lower uncon?ned strength (average of 49 MPa for non-argillaceous facies). The reduction in rock strength is compounded by thinner beds and increased frequency of argillaceous wackestone beds. The TST facies also affect the geometry of secondary faults, creating asperities along the fault plane that cause further deformation. Overall, the integration of facies, rock strength and mud content within a sequence stratigraphic framework provides an improved methodology for prediction of deformation within a carbonate fault damage zone. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fractures in the subsurface must be correlated to mappable surfaces such as stratigraphic horizons or faults if three-dimensional characterization is to be accomplished in the subsurface. The stratigraphic architecture is a primary controlling factor on fracture development as it dictates the distribution and variability of sedimentary facies which influence the style, intensity, and location of fractures. Furthermore, understanding the impact that fractures may exert on ?uid flow in the subsurface requires the incorporation of fractures within an integrated model that includes petrophysical properties of the matrix and fractures—specifically porosity, permeability, and water saturation. The stratigraphic framework represents the foundation for construction of models that incorporate rock properties. A logical progression for characterizing fracture development is to relate fractures to the existing stratigraphic derived rock property model. Structural deformation such as faulting also has a significant impact on fracture development, and ultimately on ?ow behavior in the subsurface (Caine et al., 1996). It is important to consider the influence of both stratigraphy and structural deformation in concert, rather than separately, if predictability of fracture distribution is to be maximized. The relation between fractures and stratigraphy—namely mechanical stratigraphy—has been demonstrated by correlating bed thickness with fracture intensity (McQuillan, 1973; Ladeira and Price, 1981; Huang and Angelier, 1989; Narr and Suppe, 1991; Gross, 1993; Wu and Pollard, 1995; Bai and Pollard, 2000; Lorenz et al., 2002; Renshaw et al., 2003). Alternatively, fracture intensity has been correlated to petrologic rock properties such as strength or ductility (Cook and Erdogan, 1972; Huang and Zhang, 1995; Biot et al., 1983; Corbett et al., 1987; Thiercelin et al., 1987; Friedman et al., 1994; Rijken and Cooke, 2001; Underwood et al., 2003). Other researchers have focused on the relations between fractures within strati?ed units and fault zones (Cox and Scholz, 1988; Treagus, 1988; Childs et al., 1996; Walsh et al., 1999, 2003; Wilkins and Gross, 2001; Ferrill and Morris, 2003; Schopfer et al., 2007). Many of the aforementioned studies have been performed in siliclastic rocks and have had relatively good success correlating fracture intensity to bedding thickness, while correlation to rock strength has been less straightforward. Examples of fracture characterization in carbonates are less common, especially with respect to correlating fracture development to sedimentary facies. Exceptions include Friedman et al. (1994) and Corbett et al. (1987), who found a correlation between increased fracture intensity and bed thickness, especially if mineralogy of the chalk is considered (specifically smectite presence) in the Upper Cretaceous Austin Chalk Formation. Gross (1993), and Gross et al. (1995), looked at fracture terminations and opening-mode fracture development in the diatomaceous, chert-rich Miocene Monterrey Formation. Gross and Eyal (2007) studied through-going fractures in chalk carbonates of Israel. In both the Monterrey and chalks of Israel, detailed analysis of facies or rock properties were not performed. Underwood et al. (2003) studied Silurian dolomites without achieving relevant correlation between facies, rock strength, and fracture intensity largely due to complication of pervasive dolomitization that homogenized rock strength. Ferrill and Morris (2008) documented the controls of mechanical stratigraphy on the variable response of the strata to deformation, which are controlled by the relative amounts of competent versus incompetent strata. Other researchers have focused on the relations between fractures within strati?ed units and fault zones (Cox and Scholz, 1988; Treagus, 1988; Childs et al., 1996; Walsh et al., 1999, 2003; Wilkins and Gross, 2001; Ferrill and Morris, 2003; Schopfer et al., 2007). It is clear from these studies that the faulting process is the primary deformation mechanism, but the heterogeneity observed in fracture intensity within each zone is heavily controlled by the strata, specifically the vertical stacking of facies. The present contribution builds upon work done by Ferrill and Morris (2008), which demonstrated the relationship between fracture development and the relative proportion of incompetent to competent strata within the vertical section. This study further focuses the discussion within a sequence stratigraphic framework which is more predictive than the purely lithofacies derived context they utilize. For instance, an examination of a well log or core will give a precise measurement of the incompetent to competent rock strength ratio for an individual sample location. However, describing the facies within the context of sequence stratigraphic framework allows prediction and distribution of the relative facies proportions in 3D, enabling facies and fracture prediction within the interwell region of subsurface reservoirs. This study documents brittle deformation intensity, primarily opening-mode and shear fractures, using sequence stratigraphy and extensional faulting as an integrated methodology for a distributed zone of significant fracture development. We propose that understanding of the sequence stratigraphic framework provides an improved predictability for fracture development within fault zones. Furthermore, we contend that characterization of carbonate fault damage zones in absence of a stratigraphic framework will always give a misleading prediction of fracture distribution and, ultimately, permeability within the fractured subsurface. 2. Regional setting Extensional faults associated with the Balcones Fault Zone of central Texas occur within primarily Aptian–Albian age carbonates, often the youngest rock exposed, developed during the Late Oligocene to Miocene (Weeks, 1945). These large scale extensional faults are parallel to the paleo-hinge line, set up by the transition between the Comanche Shelf and the Gulf of Mexico Basin. Regional faults exhibit high fracture intensity, which become areas where significant groundwater recharge occurs to the Edwards aquifer (Collins and Hovorka, 1997). Faults have variable amounts of offset throughout the Balcones Fault Zone, creating a hierarchy of fault size based on offset (Fig. 1). Regional scale faults have offsets greater than 30 m, with a maximum up to 259 m (Hill, 1890; Sellards, 1919; Hovorka et al., 1998; Collins, 2000). In standard seismic acquisition and processing, faults of this scale are resolvable. Local faults have offsets between 5 and 30 m, which have been mapped throughout most of the San Antonio area by Collins (2000). Faults of this scale are sub-seismic resolution and represent a significant characterization challenge in subsurface reservoirs. At the outcrop exposure scale, numerous secondary faults (offsets between 10 cm and 5 m) exist which are only detectable by advanced geophysical characterization techniques and direct sampling such as rock coring or downhole logging tools. This study characterizes a local fault with numerous secondary faults in an area north of Bulverde, Texas (Fig. 1, Latitude: 29.775982 x14, Longitude: -98.424807 x14). The exposed section is within the Albian age, Lower Glen Rose Formation. Depositionally, the facies are comprised of predominantly shallow subtidal carbonates of the Albian 6 Co' Ключевые слова: dip change, architecture, sellards, subsurface reservoir, study area, tst, carbonate, zahm, mapped, secondary fault, natural fracture, morris schopfer, mechanical unit, geological, mechanical stratigraphy, american, journal, rock strength, fracture development, bureau, hst, thiercelin, classication, content, le, tract, facies type, opening-mode joint, austin, collins, grid, outcrop locality, lorenz, weeks, strength, geologists, study, fracture orientation, mcquillan, report, dreuzy, argillaceous, facies, scale, framework, road cut, high-frequency sequence, outcrop, journal structural, hanging wall, stratied unit, deformation intensity, bed, damage, geology, layered rock, mapped fault, classi?cation, integrated, exposure, formation, distribution, argillaceous wackestone, stratigraphic architecture, facies fault, stratigraphic framework, aapg bulletin, aapg, fault damage, faulting, vertical succession, composite measured, goldhammer, development, thinner bed, structural, rock mechanics, ultimately permeability, compared, fracture, non-argillaceous facies, weighting, measured, lucia, sequence, represents, fracture distribution, joint, long, bed thickness, orientation, argillaceous unit, association, vertical, main fault, joint spacing, american association, dip, opening-mode, journal structural geology, survey, area, excavation exposure, damage zone, reservoir, structural geology, fault plane, normal fault, san, cut, competent facies, mud, fault, texas, faulting process, cyclicity, misleading prediction, rock, renshaw, read, road, signi?cant, excavation wall, university, regional fault, gross ferrill, kerans, secondary, length, gross, bulletin, permeability, fracture intensity, stratigraphic, stratigraphy, ferrill, fracture termination, loucks, stratigraphic cyclicity, higher, opening-mode fracture, pollard, shear fracture, hill, ha, treagus, lidar, deformation, geophysical, unit, study characterizes, grid represent, additional, improved predictability, morris, intensity, normal, tract hst, dunham, sequence stratigraphy, biot, association petroleum, interwell region, layered, model, characterization, zone, fault zone, cycle, prediction, gardner, set, offset, thickness, tract tst, excavation, fracture aperture, fracture spacing, outcrop exposure, friedman, odling, wa, change, lidar survey, petroleum geologists, subsurface, specically address, signicant fracture, economic geology, outcrop face, fracture mechanics, petroleum, mechanical, manzocchi