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_Journal of Structural Geology 32 (2010) 1806–1826_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg _Structural and petrophysical evolution of extensional fault zones in low-porosity, poorly lithified sandstones of the Barreiras Formation, NE Brazil_ _F. Balsamo a,*, F. Storti a, F. Salvini a, A.T. Silva b, C.C. Lima b Università degli Studi Roma Tre, Dipartimento di Scienze Geologiche, 00146 Roma, Italy; b Cenpes, Petrobras, Rio de Janeiro, Brazil_ _article info_ _Article history: Received 5 February 2009; Received in revised form 2 October 2009; Accepted 5 October 2009; Available online 6 November 2009_ _Keywords: Grain size reduction Fractal distribution Porosity Cataclasis Dilatancy Fault-zone permeability_ _abstract_ _We describe the structural and petrophysical evolution of extensional fault zones developed in low porosity, poorly lithified, quartz-dominated sandstones from the Mio-Pliocene continental Barreiras Formation, NE Brazil. We studied eight fault zones developed as sands were lithified. Fault displacement ranges from a few centimetres to 50 m. A diagnostic feature of the studied fault zones is the lack of deformation bands, which typically develop in high porosity sandstones. Structural and microstructural analyses, grain size and shape analyses, porosity and pore size analyses, and laboratory and in situ permeability measurements show relationships between deformation processes and hydrologic properties. Undeformed rocks are very poorly sorted, medium-to-fine-grained, clay-rich sandstones with an average intergranular porosity of about 3%. Sandstones in damage zones record non-destructive dilatant granular flow and formation of opening-mode intergranular extensional fractures, which increase porosity, pore connectivity and permeability. Deformation in fault cores evolved from particulate flow to compactional cataclastic flow, with progressive grain size reduction increasing the amount of silt-and clay-size fractions. Porosity was dramatically reduced to an average value of 0.2% and permeability is generally lower than the related protoliths. All this evidence highlights a conduit barrier behaviour of the studied fault zones, which significantly differs from the sealing behaviour of deformation band fault zones commonly observed in high-porosity sandstones._ _© 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Porosity and its evolution through time are an important factor controlling what kind of mesoscopic deformation structures develop during rock failure (e.g., Dunn et al., 1973; Vernik et al., 1993; Kwon et al., 2005; Fossen et al., 2007). In the last four decades, considerable attention has been devoted to the understanding of fault zone evolution in high-porosity (>10–15%), lithified to loose granular material both in the field (Aydin, 1978; Aydin and Johnson, 1978; Pittman, 1981; Lucas and Moore, 1986; Antonellini and Aydin, 1994; Fowles and Burley, 1994; Fossen and Hesthammer, 1997; Heynekamp et al., 1999; Cashman and Cashman, 2000; Shipton and Cowie, 2001; Rawling and Goodwin, 2003; Flodin et al., 2003, 2005; Johansen et al., 2005; Minor and Hudson, 2006) and in experimental studies (Borg et al., 1960; Mandl et al., 1977; Mene?ndez et al., 1996; Wong et al., 1997; Zhu and Wong, 1997; Mair et al., 2000). This because of their important influence_ _* Corresponding author. Tel.: +39 0657338049; fax: +39 0657338201. E-mail address: balsamo@uniroma3.it (F. Balsamo)._ _0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.10.010_ _on fluid flow in hydrocarbon reservoirs and groundwater aquifers (e.g., Haneberg, 1995; Walsh et al., 1998; Heynekamp et al., 1999; Aydin, 2000; Fisher and Knipe, 2001; Rawling et al., 2001; Manzocchi et al., 2002; Nelson et al., 2009). Deformation in high-porosity granular materials occurs by development of small displacement deformation structures comprehensively referred to as deformation bands, which evolve into zones of deformation bands and slip surfaces with increasing offset (e.g., Aydin and Johnson, 1978; Fowles and Burley, 1994; Shipton and Cowie, 2001; Fossen et al., 2007). A typical result of deformation band faulting in high-porosity sandstones is that their extensive development in fault damage zones may reduce fault transmissibility, thus providing an effective barrier to fluid flow (e.g., Antonellini et al., 1994, 1999; Sigda et al., 1999; Rotevatn et al., 2007). This hydraulic behaviour differs from the typical conduit behaviour of fault damage zones in low-porosity fully lithified rocks, where deformation is dominated by opening-mode fracturing (e.g., Caine et al., 1996; Billi et al., 2003; Kim et al., 2004)._ _The lower threshold porosity limit for deformation band development is at about 10–15% (e.g., Dunn et al., 1973; Flodin et al., 2003; Wong et al., 1997). Below this threshold limit, shear strength_ _F. Balsamo et al. Journal of Structural Geology 32 (2010) 1806–1826_ _1807_ _becomes a fundamental parameter controlling deformation mechanisms. Joints and slip surfaces are expected to develop in fully lithified sandstones (e.g., Johansen et al., 2005; Fossen et al., 2007). On the other hand, deformation in low-porosity poorly lithified sandstones is still poorly understood. In this paper, we attempt to bridge the gap by describing the structural and petrophysical evolution of extensional fault zones developed in low-porosity, poorly lithified quartz-dominated sandstones of the Barreiras Formation, NE Brazil. The relative compositional maturity and homogeneity of the Barreiras sandstones allow us to discount the effects of clay smearing and tectonic mixing of strongly different sedimentary units within fault zones (e.g., Antonellini and Aydin, 1994; Gibson, 1998; Heynekamp et al., 1999; Caine and Minor, 2009). Results of structural, microstructural, grain size, grain shape, and porosity analyses and permeability measurements are described with the aim of (1) inferring the deformation mechanisms that governed the evolution of these extensional fault zones; (2) proposing an evolutionary model of grain size, grain shape and porosity changes during extensional faulting; and (3) assessing the influence of faulting on fluid flow by establishing a relationship between fault-related permeability variations and fault displacement. The latter provides a useful tool for predicting the expected permeability and transmissibility of sub-seismic and seismic fault zones in sand-dominated clastic reservoirs._ _2. Analytical methods_ _Structural analysis was used to constrain the mesoscale architecture and kinematics of the studied extensional fault zones. Where offset markers were available, stratigraphic separations were measured in the field and then converted into true fault displacement values by using fault kinematics (Butler and Bell, 1989). Fault core thicknesses were measured to determine whether or not there was a predictive statistical relationship that could be used for estimating fault displacement from fault core width when direct measurements were not possible (e.g., Walsh and Watterson, 1988)._ _Undeformed sandstones and rocks in damage zones and fault cores were sampled at each studied field site. About 0.5 kg of material was collected for each sample. After complete disaggregation and chemical removal of Fe-oxides in the laboratory, grain size analysis of about 60 g of granular material was completed by combining standard sieve and laser diffraction analyses in order to account for coarser (gravel-and sand-size) and finer (silt-and clay-size) fractions, respectively (Selley, 2000). A total of 44 samples were analysed including 17 undeformed sandstones, 16 fault core rocks, and 11 damage zone sandstones. Results of grain size data were plotted as frequency distribution curves using the Phi scale arrangement (Krumbein, 1934, 1938). Grain size distributions were quantitatively described in terms of the following statistical parameters: (a) the mean size Phim (a measure of the average size of the curve); (b) the standard deviation So (a measure of the size spread around the mean value, or sorting); (c) the skewness Sk (a measure of the curve symmetry around the mean value, or preferential spread to one side of the mean); and (d) the kurtosis (the degree of concentration of the grain sizes relative to the mean value) (Inman, 1952; Grif?ths, 1952). These parameters were obtained by mathematical methods (Folk and Ward, 1957; Krumbein and Pettijohn, 1938) employing the entire sample grain size populations (McManus, 1988). Grain size distributions were also transformed into equivalent particle numbers (e.g., Storti et al., 2003) assuming spherical particles with a density of 2.65 g cm³ and plotted against the equivalent size classes in bilogarithmic diagrams to obtain their fractal dimensions (D) as the slope of the best-fit lines (e.g., Blenkinsop, 1991)._ _Additional non-destructive sampling was carried out in the three structural domains (undeformed sands, damage zones and fault cores) for blue-dyed epoxying and thin section analysis. Microstructural analyses were carried out with a standard petrographic microscope connected to a digital photo camera. Computer-based image analysis techniques (e.g., Francus, 1998; Heilbronner and Keulen, 2006) were applied to thin section images using the Optimas-6.5? commercial software (e.g., Shipton and Cowie, 2001). Colour TIFF images (2568 × 1938 pixel resolution; 1 pixel = 2.6 mm) were acquired at constant 1.25× magnification under plane polarised light, in order to ensure the appropriate visualization of both microporosity and macroporosity (i.e., pores with equivalent diameter lower and greater than 63 μm, respectively; Choquette and Lloyd, 1970). A total of 45 images were analysed to characterise thin section sectors representative of undeformed, damage zone and fault core rock fabrics, respectively. Microfabric heterogeneities were addressed by acquiring multiple images_ Ключевые слова: gaussian peak, increase, platykurtic curve, grain size, range, material, evolution, structural heterogeneity, johnson, developed, geophysical, surface, standard deviation, fault zone, undeformed sand, ferreira, eds faults, fault core, damage, aydin, damage zone, signi?cantly, brittle faulting, gaussian, size phim, lithied sandstone, scale, internal structure, image, angular particle, society, applied, grain-size distribution, fault journal, core, undeformed, main angularity, lima, studied, geological society, sharp boundary, wa, permeability, undeformed sample, brazil, cuspate boundary, permeability observed, journal structural, maltman, md, structure, sand particle, ner particle, goodwin, sand-size content, sample, fractal dimension, bulk porosity, magnitude ranging, fault, rawling goodwin, deformation band, sediment journal, foliated, rio grande, ?ow, size, increasing displacement, ?ne, architectural element, size class, generally higher, fault displacement, fraction, curve, showing, ne-skewed curve, statistical parameter, sediment, relative abundance, minor, science, frequency, measurement, fault strike, study area, ward, increasing slip, reservoir, undeformed damaged, sand, kindly thanked, permeability reduction, sandstone, deformation, barreiras formation, grain, dramatically reduced, balsamo, tectonophysics, text, distribution, undeformed sandstone, fracture, ?uid, displacement, pore, large pore, deformation mechanism, london, journal sedimentary, intergranular, size fraction, porosity, slip, extremely heterogeneous, journal, plane-polarised-light photomicrograph, average porosity, cataclastic, shear zone, journal structural geology, mm, parameter, sammis, generally, analysed image, data, coarse, analysed, equivalent diameter, core rock, formation, francus, granular, slip surface, mixed zone, icapu village, zone, structural geology, wide range, area, lithi?ed, porous sandstone, clay-size, extensional fracture, fossen, intergranular porosity, uid, peak, cataclastic deformation, shipton, structural, faulting, dimension, abruptly increase, geological, size distribution, sandstone pure, cashman, dilatant structure, main, rock, extensional fault, general decrease, angular, extensional, correlation sample, sedimentary, shape, geology, balsamo journal, high-porosity sandstone, fe-oxide cement, letters, band, undeformed domain, host sandstone, analysed sample, hydraulic behaviour, measured, pore size, barreiras, petrophysical property, average, sand-size fraction, particulate, zone fault, krumbein, fractal, signicantly higher, dilation band, petrophysical evolution, mandl, rawling, silt, particle, sorting, petroleum, salvini, haneberg, permeability measurement, angularity, blue epoxy, frequency histogram, domain, silva, higher, evans, lithied sediment, increasing, analysis, estimated displacement