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_Journal of Structural Geology 32 (2010) 1783–1791_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com locate jsg_ _Evidence of hydraulic connectivity across deformation bands from field pumping tests: Two examples from Tucano Basin, NE Brazil_ _W.E. Medeiros a,c,*, A.F. do Nascimento a,c, F.C. Alves da Silva b, N. Destro d, J.G.A. Demetrioe_ _a Universidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra, Departamento de Geofísica, Programa de Pós-graduação em Geodinâmica e Geofísica, 59072-970, Natal, RN, Brazil_ _b Universidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra, Departamento de Geologia, Programa de Pós-graduação em Geodinâmica e Geofísica, 59072-970, Natal, RN, Brazil_ _c INCT–GP, Instituto Nacional de Geofísica do Petróleo (CNPq), Brazil_ _d Petrobras, Research and Development Center, Av. Horácio Macedo, 950, Cidade Universitária, Ilha do Fundão, 21941-915, Rio de Janeiro, RJ, Brazil_ _e Universidade Federal de Pernambuco, Centro de Tecnologia, Departamento de Geologia, Laboratório de Hidrogeologia, LABHID, 50740-530, Recife, PE, Brazil_ _article info_ _Article history: Received 26 January 2009 Received in revised form 12 August 2009 Accepted 20 August 2009 Available online 27 September 2009_ _Keywords: Pumping tests deformation bands Tucano Basin hydraulic connectivity permeability_ _abstract_ _It is assumed that deformation bands may compartmentalize aquifers or hydrocarbon reservoirs because these low-permeability structures may behave as barriers to fluid flow. To address the question whether there is, at a reservoir scale, hydraulic connectivity across a damage zone dominated by cataclastic deformation bands, we present results of two pumping tests carried out in a fluvial-deltaic phreatic sandstone aquifer from the Ilhas Group at Tucano Basin (NE Brazil) where intense concentration of deformation bands occurs. Both test sites are associated with macroscopic damage zones that are approximately 1 km in length and 15 m thick. In situ permeability measurements show values of 2000 mD for host rock and 0.1 mD for deformation bands. GPR profiles reveal good continuity of the primary sedimentary structures with almost no deformation band vertical offsets (less than 10 cm). The well locations for the pumping tests were chosen so that the damage zone is located between pumping and monitoring wells. Pumping tests in both cases revealed hydraulic connectivity across the damage zone since the observed stationary drawdown at monitoring wells was a considerable fraction of the drawdown observed in the pumped well. In one experiment using eight monitoring wells, the drawdown cone is evolving through the damage zone instead of contouring it. Local deviations in the natural groundwater flow allow to say that the damage zone dimensions are quite large both in horizontal and vertical directions compared to the distances among the wells. The interpretation of all experimental results is that deformation bands do not fully compartmentalize the aquifer. Generalization of this result to hydrocarbon reservoirs has to take into account capillary effects which are not present in the studied case._ _© 2009 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Deformation bands (Aydin, 1978; Antonellini and Aydin, 1994) are one kind of frictional deformation structures found in the uppermost Earth’s crust. They may be defined as tabular structures of finite width resulting from strain localization commonly found in very porous (15–25% of porosity) granular material such as sandstones. Commonly, deformation bands exhibit localized porosity reduction that lack shear offset (Du Bernard et al., 2002). Based on microstructural analysis, Antonellini et al. (1994) classified the deformations bands in three main groups: (i) deformation bands without cataclasis: characterized by the almost complete lack of crushed grains. They can exhibit positive or negative volume variation; (ii) deformation bands with cataclasis: characterized by intense grain size and volume reduction, and (iii) deformation bands with clay smearing or localized shear zones. For a comprehensive review on deformation bands see Fossen et al. (2007)._ _Deformation bands are thought to baff fluid flow during oil production (e.g., Knipe et al., 1997; Gibson, 1994) and to act as seal for hydrocarbon accumulations (e.g., Ogilvie and Glover, 2001) because they tend to have lower permeability than their host rock (e.g., Pittman, 1981; Jamison and Stearns, 1982; Antonellini and Aydin, 1994; Knipe et al., 1997; Gibson, 1994; Fisher and Knipe, 2001; Lothe et al., 2002; Shipton et al., 2002). Tindall (2006) argued that when connectivity across deformation bands (or damage zone) is detected it is interpreted that this is due to open joints. However,_ _1784_ _W.E. Medeiros et al. Journal of Structural Geology 32 (2010) 1783–1791_ _Fig. 1. Deformation bands in outcrops at Site 1: (a) Mesoscopic aspect of the deformation band. Note the positive relief of the structures due to their greater resistance to erosion. (b) Microscopic view of a deformation band exhibiting evidences of cataclasis. The lower contact is sharper than the upper one and is shown by a dashed line. Note the great reduction of grain size in the tectonic matrix. Some survivor grains (S) are still seen along the deformation band._ _Fossen and Bale (2007) suggested that the impact of deformation band on reservoir production is small or negligible in most cases based on observation of paleo-fluid fronts seen in the field and on mathematical considerations on the average permeability of rocks affected by deformation bands._ _In order to address the question whether there is, at a reservoir scale, connectivity across damage zones dominated by deformation bands in porous sandstone, we show here results of two pumping tests of a phreatic sandstone aquifer where intense concentration of deformation bands occur. In both studied sites, because deformation bands are more resistant to erosion than the host rock they show a typical positive relief at the mesoscopic scale (Fig. 1a). Microscopically they are characterized by cataclastic processes with grain size reduction (Fig. 1b). The degree of cataclasis shown by the deformation bands of the Ilhas Group can vary from moderate to high (Alves da Silva et al., 2005). Sometimes they exhibit strong comminution with production of gouge-like material in discontinuous stripes; in other occasions survivor grains can be visualized within a finer tectonic matrix as show in Fig. 1b. The deformations bands limits can be sharp or gradational in respect to the normal sandstone and sometimes one can find both types of limits in the same structure, as also shown in Fig. 1b._ _Both test sites are associated to faults with a length of 1 km. The associated damage zones have a thickness of approximately 15 m. As a general rule, the well locations were chosen so that the deformation bands are located between pumping and monitoring wells. We stress that the results of our pumping tests must be interpreted in terms of piezometric head connectivity between the wells and not in terms of mass transfer between them._ _We are going to present strong evidence that there is hydraulic connectivity across the deformation bands damage zones on a reservoir scale. In the following sections we present the regional geographical and geological contexts of the test sites, petrophysical and geophysical characterization of the aquifer sandstone where the tests were performed, description of the pumping tests methodology and their results and finally discussions and conclusions._ _Fig. 2. Geological sketch of Sites 1 and 2 (S1 and S2 respectively) in Tucano Basin, Bahia State, NE Brazil._ _W.E. Medeiros et al. Journal of Structural Geology 32 (2010) 1783–1791_ _1785_ _association to the Gondwana break-up and formation of the South Atlantic Ocean to the east of the rift system (Magnavita and Cupertino, 1987; Magnavita, 1992)._ _The Tucano Basin is characterized by sets of NS trending normal faults that tilted the sedimentary strata eastward displaying a domino-like structural style (Magnavita and Cupertino, 1987; Magnavita, 1992). The basin sedimentary infillings can be related to distinct tectonic stages: deposition of the Brotas Group (Aliança and Sergi Formations) and the basal portion of the Santo Amaro Group (Itaparica Formation) represent the pre-rift stage whilst deposition of Candeias Formation (Santo Amaro Group), Ilhas and Massacara Groups, and Salvador Formation are associated to the rift phase (Caixeta et al., 1994)._ _The pumping tests described hereafter were done in sites dominated by the sandstones of the Ilhas Group (Fig. 2). A fluvial-deltaic sedimentation is proposed to this group (Cupertino, 1990). The two chosen sites are located near the south margin of the VazaBarris River (Fig. 2). Although the Vaza-Barris River is the main river of the region it is intermittent because the climate in the area is dry. The main rain season usually occurs from May to August._ _In outcrops at the studied sites (Fig. 3a–c) the normal faults trend NNE (Fig. 3c) and dip steeply to the ESE (Figs. 3b and 4). In the damage zone of such faults three main sets of deformations bands are intensely developed (Fig. 4)._ _3. Petrophysical and geophysical characterization of the Ilhas Group sandstone_ _Fig. 3. (a) Outcrop showing deformation bands in different orientations in Site 1. (b) View of the damage zone in the Site 1 showing a swarm of deformation bands. (c) Fault plane with part of the associated damage zone (upper part of the picture) in the Site 2. The hammer handle points towards the North. ‘‘U’’ and ‘‘D’’ identify the foot wall and hanging wall respectively._ _2._ Ключевые слова: thanked, subvertical, previous test, cased, hydrocarbon reservoir, petroleum geologists, fault, fully compartmentalize, jardim, silva, considerable fraction, impact, damage, ilhas group, magnavita, rio grande, region, aydin, lunn, fully, ciencias exatas, compared, bulletin, marsily, geologists bulletin, cataclasis, black ellipsis, aspect, order, result, host rock, reservoir, scale, location, undamaged portion, site showing, volume, journal structural, permeability measurement, evidence, cupertino, journal, test site, sandstone, natural, larger, offset, pumping test, based, alves, open fracture, portuguese, universidade federal, wall, goggin, geophysical characterization, additional criterion, association, monitoring, ?eld, chosen, hydraulic connectivity, bale, data, thesis, water, grain, association petroleum, scan, piezometric, possibility, local, damage zone, reservoir scale, measured, waste water, drawdown cone, brazil, measurement, real test, da, cataclasis characterized, characterized, aquifer, ?uid ?ow, large, including, lothe, miranda, american association, band, vertical dimension, electrical conductivity, geological context, rate, proc, case, quantitative, drilled, fossen, tindall, md, hydraulic, tucano, ?ow, structural, main, foot wall, deformation band, observation, compartmentalize, rio, horizontal, deformation, point, waste, gibson, vertical, geology, piezometric head, medeiros, jatoba boletim, drawdown, issue, head, fact, nascimento, zone, pittman, wa, main set, fault zone, conductivity, petroleum, hanging wall, ?uid, pumping, intense concentration, antonellini, tucano basin, distance, cupertino magnavita, cone, ne, structure, situ, gpr, time, american, medeiros journal, site, contouring, direction, presence, group, outcrop, account, basin, liu, permeability, test, dimension, stress, set, porosity, norte centro, ilhas, dry season, knipe, fracture, connectivity, percolation core, pumped, seismic data, observed, petrobras, shipton, studied site, identify, rock, structural geology, uid, electric conductivity, geologists