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_Journal of Structural Geology 32 (2010) 1943-1959_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Quantifying diagenetic and stratigraphic controls on fracture intensity in platform carbonates: An example from the Sierra Madre Oriental, northeast Mexico_ _Oraldo J. Ortega a,*, Julia F.W. Gale b, Randall Marrettc a Shell International E&P, 150 North Dairy Ashford, A-1014 Houston, TX 77079, USA b Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, TX 78713, USA c Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, TX 78712, USA_ _article info_ _Article history: Received 30 September 2009 Received in revised form 29 June 2010 Accepted 2 July 2010 Available online 14 July 2010_ _abstract_ _Normalized fracture intensity measured along 1D scanlines was used to compare fracture intensity for different sedimentary facies, stratigraphic position, bed thickness and degree of dolomitization in carbonate beds of the Cupido and Tamaulipas formations, northeast Mexico. We calculated the fracture intensity for individual beds, for single and combined fracture sets, and the statistical significance of relationships using bivariate weighted regressions and multivariate methods. Results suggest that the degree of dolomitization is positively correlated with fracture intensity and has the strongest correlation, followed by the position of the bed in the stratigraphic cycle and the mud content. Dolomite content, normalized position in the parasequence, environment of deposition, and mud content show a significant degree of correlation among themselves and should be considered dependent variables for statistical purposes. Geological observations suggest that dolomite precipitation and fracturing occurred, at least in part, synchronously in these rocks. A fracture-intensity distribution model integrating sequence-stratigraphic and diagenetic history is proposed for the Cupido and Tamaulipas formations with potential use in similar carbonate successions. Our analysis does not support the classic bed-thickness-fracture-spacing relationship._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Fracture intensity (abundance) is one of the key parameters needed for an understanding of fluid flow in otherwise low-permeability rocks, yet it is difficult to measure directly in the subsurface because of sampling problems. Predictive models (e.g., Mueller, 1991; Rives et al., 1992; Fischer et al., 1995; Bai and Pollard, 2000; Philip et al., 2005) require testing with datasets that face the same problems. Nelson (2001) identified the principal geologic controls on fracture intensity as (1) composition, (2) texture (including grain size and porosity), (3) stratigraphy (bed thickness), and (4) structural position. Structural position has been the subject of many studies of fracture intensity (e.g., Antonellini and Mollema, 2000; Casey and Butler, 2004; papers in Hennings, 2009). Although structural effects are locally important in the folded platform-carbonate rocks of the Sierra Madre Oriental, Mexico, our focus is on diagenetic, bed thickness and stratigraphic controls on fracture intensity. We selected outcrops in the same structural position in the limb region of kilometer-scale folds, away from hinges, and where bedding dip is constant. Variation of fracture intensity between outcrops due to structural position is therefore minimized._ _We used a size-normalized fracture-intensity measurement method described by Ortega et al. (2006). Normalized fracture intensity is determined in the context of varying sedimentary facies, dolomite content, bed thickness, and position of beds in fifth-order stratigraphic cycles. We attempted to answer the following questions: Which stratigraphic parameters exert the strongest control on fracture intensity? Is there a predictable pattern of fracture-intensity distribution?_ _2. Previous fracture-intensity studies_ _Compositional controls on fracture intensity have been studied in laboratory experiments (e.g., Handin et al., 1963) and in outcrops and core (Das Gupta, 1978; Sinclair, 1980; Narr, 1991; Gillespie et al., 1993, 1999, 2001; Mandal et al., 1994; Nelson and Serra, 1995; Nelson, 2001). Handin et al. (1963) found that fracture intensity decreases from quartzites to dolostones, to sandstones, to limestones, and that the increase in ductility with burial is greater for limestones than for other sedimentary rocks. Sinclair (1980) suggested that fine-grained carbonates have higher fracture intensity than coarse-grained carbonates._ _1944_ _O.J. Ortega et al. Journal of Structural Geology 32 (2010) 1943-1959_ _Fig. 1. Landsat image of the Monterrey salient in the Sierra Madre Oriental (SMO) showing kilometer-scale folds affecting Mesozoic rocks. Ridges are dominated by carbonates of the Cupido and Tamaulipas Superior formations. Study locations are in canyons that cut across anticlines. Ch: El Chorro, Es: La Escalera, LP: Las Palmas, M: Monterrey, S: Saltillo. Inset: map of Mexico showing location of landsat image. I: Iturbide._ _Fig. 2. NW-SE schematic stratigraphic section across the Cupido-Tamaulipas platform (modified from Wilson et al., 1984). Black columns show the approximate stratigraphic interval exposed in each area. White columns indicate the approximate section studied at each location. Horizontal distances between localities not at uniform scale._ _O.J. Ortega et al. Journal of Structural Geology 32 (2010) 1943-1959_ _A_ _C_ _B_ _A_ _D1_ _D2_ _A_ _B C_ _Fig. 5. Unfolded fracture orientation data, El Chorro Anticline. (A) Unfolding about a horizontal fold axis. (B) Further rotation of 38 x14 clockwise about a vertical axis as suggested by paleomagnetic data (Kleist et al., 1984). Fracture sets D1 and D2 are youngest and are probably associated with folding. Fracture sets A, B, and C probably formed when bedding was horizontal. Set C lies subparallel to the fold axis and may have been formed during the shortening event that produced the fold._ _than coarse-grained carbonates. Das Gupta (1978) concluded that greater dolomitization correlates with higher fracture intensity; although dolomitized beds were thinner than limestone beds in that study and separation of bed thickness from dolomitization effects on fracture intensity is difficult._ _Carbonate mechanical and fracture stratigraphy was addressed by Corbett et al. (1987), Ferrill and Morris (2008), Laubach et al. (2009), and several papers in a special issue of the Journal of Structural Geology, “Deformation in Carbonates” (Agosta and Tondi, 2009). For example, Zahm et al. (2010) used integration of facies, rock strength, and mud content within a sequence-stratigraphic framework as a means of predicting faulting and jointing in limestone outcrops. Their study found that “fracture deformation intensity” was higher in transgressive system tracts (TST) than in highstand system tracts (HST). The TST in that study also had more argillaceous, thinner cycles and rocks with lower unconstrained strength. There was, however, no dolomitization component in their example and as we will show here, dolomitization overrides facies and mud-content controls on fracture intensity._ _Fig. 3. Variation of facies, bed thickness, and dolomite content among layers studied. The facies definition includes textural groups and depositional environment interpretations. Some histograms exclude multifacies beds or chert beds. Total dolomite content is based on thin-section point counting. From Ortega et al. (2006)._ _3. Study area_ _The study area is in the north-central part of the Sierra Madre Oriental (SMO) fold and thrust belt in northeastern Mexico (Fig. 1). An Upper Hauterivian-Lower Aptian platform-to-basin carbonate system, comprising the Cupido, Tamaulipas Superior, and_ _A_ _T C_ _B_ _D_ _B A_ _C T_ _B B A_ _M C_ _C_ _A B_ _Fig. 4. Unfolded fracture orientation data, San Blas Anticline. (A) La Escalera; (B) Las Palmas. Simple rotations of poles to fractures around fold axis restoring bedding to horizontal. Fold axis is indicated by a crossed large square. Restoration reveals two dominant fracture orientations approximately 60-70 x14 apart. Crosscutting relationships between sets A and B are mutual suggesting that these sets are synchronous. Set C crosscuts sets A and B at both localities. Set D restores to a horizontal attitude suggesting it might be related to tectonic stylolites and folding. T are poles to tectonic stylolites and M are poles to late, mud-filled fractures._ _D_ _A_ _A C_ _B_ _Fig. 6. Unfolded fracture orientation data, Iturbide Anticline. Unfolding is about a fold axis plunging 20 x14 southeast. Fracture set D is the youngest and is probably associated with tectonic stylolites (T) and folding. Fracture sets A, B, and C probably formed when bedding was horizontal - they are close in orientation to similarly labeled sets at other locations._ _1946_ _O.J. Ortega et al. Journal of Structural Geology 32 (2010) 1943-1959_ _Table 1 Summary of number of fractures measured and scanline lengths._ _Locality_ _Bed number_ _Apertures measured_ _Escalera_ _1_ _2_ _3_ _4_ _5B_ _6A_ _6B_ _7_ _8_ _9_ _10_ _11_ _12_ _Set A 202 233 229_ _8 200 210 218 204_ _206_ _Set B_ _148 219 241 206_ _240 205 213 224 84 202 197_ _Set C_ _62 9_ _Set D_ _Scanline length (mm)_ _Set A_ _705 1024 1689_ _Set B_ _1511 1568 959 1537_ _2007 1422 1918 927 1664_ _876_ _2311 1372 851 1588 13183 12370 921_ _Iturbide_ _1_ _215_ _2_ _189_ _3_ _4_ _5_ _182_ _6_ _7_ _8_ _9_ _10_ _233 61 3721 86 2400 166 204 163 194 2261 210 389 411 220 200 220_ _210_ _3683_ _Palmas_ _1_ _2_ _3_ _4_ _5_ _6_ _7_ _8_ _9_ _10_ _21 217 223 30 59 248 121 19 93 72 219_ _Dolomite content, D (%)_ _0.040 0.035 0.030 0.025_ _Set AB weighted regression I_ _0.00043 D - 0.0074 r2_ _0.5495_ _0.020 0.015_ _0.010 0.005_ _._ Ключевые слова: sle, increase, northeastern mexico, outcrop, diagenetic history, mineralogists gulfcoast, aapg, grainstone shoal, linear regression, mechanical property, ortega journal, marrett, las palmas, scale, positive correlation, determination, petroleum geologists, dolomitized layer, mud content, society, suggesting, sorted number, bed, studied, fracture intensity, ab, wa, mud, layer, dominant control, weighted regression, journal structural, fracture-intensity, carbonate, el, environment, university, axis, vein, calcite, interpreted environment, nelson, fracture set, fold axis, collapse breccia, elsevier, limestone, sierra, parasequence environment, ferrill, stratigraphic cycle, coef?cient determination, spacing, laubach, content, fold, austin, timing, aapg bulletin, mudstone bed, aperture, cupido, ortega, thickness, study area, reservoir, individual set, escalera, position, fold geometry, horizontal, weighted, austin mexico, crosscut set, oriental, deformation, zahm, abundant, weak correlation, relationship, iturbide, distribution, fischer, fracture, la, dip, individual, olson, intensity number, coef?cient, abundant set, lehmann, bed studied, journal structural geology, dolomitization, journal, intensity, mexico, palmas canyon, location, fracture size, combined, fracturing occurred, iturbide mudstones, mm, kinematically compatible, study, scale-independent approach, canyon, madre, wide, data, chorro, number, formation, variable, palmas, highly dolomitized, set set, partly, carbonate succession, sinclair, las, structural geology, corbett, limestone bed, facies, control fracture, fracture measured, dolomitized, dolomite content, tectonic, kleist, set weighted, structural, degree, correlation, diagenetic, control, dolomite precipitation, uncorrelated variable, parasequence, geological, eds, bedding, rock, stratigraphy, issue, stratigraphic position, fracture spacing, regression, sedimentary, subtidal, mueller, analyzed, rives, probable timing, texas, monterrey salient, number fracture, bed thickness, camerlo, narr, geology, american association, fracture-intensity analysis, sets, set, event, coef?cients, fracturing, independent variable, normalized fracture, subtidal cycle, stylolites, locality, landsat image, economic paleontologists, cupido formation, normalized, bulletin, escalera canyon, crosscut, dolomite, stratigraphic, subtidal parasequences, petroleum, cycle, stratigraphic control, monterrey, structural position, special, fractures, sequence stratigraphy, analysis, tectonic stylolites, joint spacing, diagenesis, regression bed, schematic distribution