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_Journal of Structural Geology 32 (2010) 1850–1863 An experimental investigation of the development and permeability of clay smears along faults in uncemented sediments F. Cuisiat*, E. Skurtveit Norwegian Geotechnical Institute, P.O. Box 3930, Ullevaal Stadion, N-0806 Oslo, Norway Abstract: The generation of clay smears along faults in uncemented sediments has been studied through laboratory experiments in a newly developed high stress ring shear apparatus. The main objective is to investigate basic mechanisms involved in the deformation process of sediments during faulting and formation of clay smears. The experimental test program comprises ring shear tests on sand with embedded clay segments (sand–clay sequence) under constant effective normal stress. Visual inspection of the samples after testing, analyses of thin sections and permeability measurements across the shear zone are used to characterise geometrical continuity, thickness and sealing potential of the smear. Deformation processes such as grain reorientation, clay smear and cataclasis are identified from the tests. The complexity of the shear zone is observed to increase with the effective normal stress applied to the specimen and the number of clay segments used in the ring (multilayered sand–clay sequences). At low effective normal stress, in clay-rich sediments, clay smear is the most efficient mechanism for permeability reduction. The permeability across the smear decreases with ring rotation (or shear displacement) and effective normal stress. A maximum decrease of two orders of magnitude compared to the permeability of the surrounding sand is observed after 90 x14 rotation under 10.5 MPa effective normal stress. Sand–sand juxtaposition shear is dominated by grain rolling causing only minor permeability reduction. At high effective normal stress, permeability measurements across clay smear and sand–sand juxtaposition yield similar values indicating that the permeability reduction is dominated by grain size reduction in the sand. Keywords: Laboratory Clay smear Fault Permeability Ring shear 1. Introduction Faults play a major role in the distribution and movement of fluids, hydrocarbons or groundwater in the Earth’s crust. Faults may act as barriers, baffles, or mixed conduit-barrier systems to fluid flow depending on the composition of the faulted rock, the stress conditions at the time of faulting, and post-faulting burial and temperature history (Antonellini et al., 1994; Caine et al., 1996; Sibson, 2000). An understanding of fault zone internal structure and associated hydraulic properties is a requisite to many fields such as petroleum exploration and production, groundwater modelling, earthquake rupture, and sequestration of CO2 (e.g. Hickman et al., 1995; Sibson, 2000; Bense and Van Balen, 2004; Rutqvist et al., 2007; Annunziatellis et al., 2008; Wibberley et al., 2008). * Corresponding author. Tel.: +47 22 02 31 55; fax: +47 22 23 04 48. E-mail address: fabrice.cuisiat@ngi.no (F. Cuisiat). Faults in sediments can be classified upon the phyllosilicate content of their host lithology and burial depth at time of faulting (Fisher and Knipe, 1998, 2001). Faults in clean sandstones (<15% clay) at shallow depth produce a disaggregation zone with only local grain rearrangement, no grain-fracturing and similar or even higher permeability than the host rock (Fisher and Knipe, 2001; Bense and Van Balen, 2004). Faulting of the same sediments at greater depth results in grain-fracturing (cataclasis) with clogging of the pore space by smaller grain fragments. Experimental data show that the onset of grain-fracturing may start at depths as low as 500 m, or 5 MPa effective vertical stress (Chuhan et al., 2002). Post-faulting burial may lead to quartz cementation for temperatures greater than circa 90°C or circa 3 km in basins with normal temperature gradient. Quartz cementation occurs most intensively at burial depths between 3.5 and 5 km (120–170°C) (Bjørlykke and Høeg, 1997). In impure sands with clay content between 15 and 40%, or other contexts of similar mineralogy, faulting leads to a fault gouge or phyllosilicate framework fault rock (Fisher and Knipe, 2001) with mixing of sand and clay often structured parallel to the shear plane (Rutter et al., 1986; Wibberley and Shimamoto, 2003; van der Zee and Urai, 2005). In clay-rich sequences (i.e. clay content higher than 40%), faulting is commonly associated with clay smears, that is shearing of an offset clay layer into the fault zone (Fig. 1). Smearing of low permeability clay has been presented as one of the most efficient mechanisms for fault sealing. From outcrop observations, laboratory experiments, and numerical modelling (e.g. Lindsay et al., 1993; Lehner and Pilaar, 1997; van der Zee and Urai, 2005; Sperrevik et al., 2000; Clausen and Gabrielsen, 2002; Egholm et al., 2008) several processes have been suggested to explain the occurrence of clay smear such as: clay abrasion, lateral clay injection from source layer and shearing within fault, and material instabilities. Nevertheless, no real mechanics-based predictive model for fault seals is yet available in the literature. In this paper, we present the results of ring shear experiments to study the development and permeability of clay smear along faults. The use of a ring shear apparatus in geosciences is fairly widespread, for instance to characterise the residual shear strength and behaviour of soils and granular materials as well as the interaction between soil and structure (e.g. Bishop et al., 1971; Tika et al., 1996; Lupini et al., 1981; Hungr and Morgenstern, 1984; Sassa et al., 2003). However, the use of a ring shear device as an analogue for fault formation and clay smear is more limited, although some early work was conducted by Mandl et al. (1977). The principle is illustrated in Fig. 2. The ring shear sample consists of an annular sand specimen confined between upper and lower rings. One or several segments of clay are embedded within the sand. After applying the vertical (normal) stress sn onto the specimen, the lower ring is rotated, thus dragging the clay material onto the shear plane located at the separation of the upper and lower confining rings. The linear displacement along the shear plane corresponds to a throw on a fault plane, while the width of the embedded clay segment corresponds to the thickness of a clay sequence through which the fault develops. By performing several tests with different conditions (effective normal stress, maximum rotation, sand density, clay type, etc.) parameters may be varied to assess their effect on the development of the shear band and clay smear properties. Early experimental studies dedicated to clay smear and shear band formation carried out at the Norwegian Geotechnical Institute (NGI) were performed in a classical geotechnical ring shear apparatus which allowed for large deformations, but was limited to low stress levels equivalent to ca. 50 m burial under hydrostatic conditions (Sperrevik et al., 2000; Clausen and Gabrielsen, 2002; Kvaale, 2002). More recently, a new ring shear apparatus was designed and constructed at NGI to investigate shear band formation and clay smear in unconsolidated sediments at greater burial depths (Torabi et al., 2007; Cuisiat et al., 2007). The new equipment allows for normal effective stresses up to 20 MPa, which correspond to a depth of ca. 2600 m under hydrostatic conditions or even higher in over-pressured reservoirs. These depth ranges cover the depth at which deformation (faulting) may have occurred for most North Sea reservoirs. In the absence of carbonate cementation, North Sea sediments remain loosely cemented until depths of 2.5–3 km until diagenesis takes place (Bjørlykke and Høeg, 1997). In the new ring shear apparatus, the flow resistance and permeability can be measured across the specimen through 48 drainage points evenly distributed around the upper and lower rings (Fig. 2 left) during testing. The possibility of running permeability tests across the sheared specimen greatly improves the quantification of the sealing potential of clay smear and other deformation processes, and how this potential varies with normal stresses and shear displacement (i.e. fault offset) for different sediment types. The main motivation of the work described in this paper is to pursue the experimental work initiated earlier (Cuisiat et al., 2007; Torabi et al., 2007) in order to increase our database of fault properties. The work focuses on the mechanisms associated with clay smear along faults in uncemented sediments as well as the associated permeability changes. The final aim is to develop an experimental database for quantification of fault texture, petrophysical properties, deformation and strength, and fluid flow properties of faults in unconsolidated sediments. 2. Clay smears: observation and prediction Experimental studies (Weber et al., 1978; Sperrevik et al., 2000; Clausen and Gabrielsen, 2002; Karakouzian and Hudyma, 2002) as well as outcrop studies of clay smears in loose sandstone-shale sequences (Weber et al., 1978; Lehner and Pilaar, 1997; Doughty, 2003), and lithified sandstone–shale sequences (Lindsay et al., 1993) suggest that clay smearing is strongly dependent on the original thickness of th_ Ключевые слова: estimated, shear stress, conning ring, shear strength, thickness, burial, sample, loading, experiment, linear, structural geology, uncemented sediment, depth, sand grain, specimen, sandsand juxtaposition, test, permeability reduction, burial depth, oedometer cell, time, manzocchi, unit weight, laboratory experiment, pore space, friction angle, clay smear, property, shear apparatus, equal, formation, apparatus, grain crushing, soil, oslo, geotechnical, permeability, experimental, ratio, slipped interval, shear direction, average, empirical relationship, takizawa, mpa, change, opening, shear displacement, knipe, normal fault, shear band, stress, rotation, van, sand clay, takahashi, smear, fisher, volume, segment, fault formation, time faulting, society, skurtveit journal, bishop, mechanism, shale smear, composite smear, increase, wibberley, dominant mechanism, mpa mpa, fault plane, tika, constant, high, faulting, grain, elsevier, condition, ssf, mechanics, model, fault, total volume, porosity reduction, fault sealing, lindsay, permeability measurement, sassa, journal structural, engineering, fault zone, troll clay, marine, porosity, unconsolidated sediment, permeability clay, proceedings, lunne, shear, journal, area, smearing, band, sand shear, clay content, normal, sediment, reduction, sand–sand, van balen, sand permeability, ?ow, effective, consolidated, sand wedge, fault formed, volumetric deformation, previous parameter, efcient mechanism, illustrated, ring, structural, shale, normal stress, depth time, cuisiat, study, experimental study, sibson, estimated burial, effective normal stress, sand, north sea, higher, fault throw, shearing, unit, layer, sealing potential, porosity change, clay segment, geology, displacement, permeability increase, effective normal, ring shear, maximum, mandl, zone, shear zone, deformation, measurement, observed, crushing, reaction force, mpa effective, sequence, juxtaposition, sand–sand juxtaposition, rutter, journal structural geology, sealing, normal loading, geotechnique, clay, permeability data, higher permeability, strength, linear displacement, rock, sgr, internal friction, granular material, cuisiat skurtveit, deformation process, mm, fault rock, lupini, result, clay smearing, outcrop study, ring rotation, yielding, pressure, post-faulting burial, testing analysis, petroleum, testing, evenly distributed, note, skurtveit, table, upper ring, torabi, hydraulic property, deformation band, bulletin, shale bed, smear permeability, average permeability, permeability change, continuous smear, fracture observed