Description:

_Journal of Structural Geology 32 (2010) 1590-1608_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Evolution of cataclastic faulting in high-porosity sandstone, Bassin du Sud-Est, Provence, France Elodie Saillet *, Christopher A.J. Wibberley a Geosciences Azur, UMR 6526, 250 Av. Albert Einstein, 06560 Valbonne, France b TOTAL EP, CSTJF, Av. Larribau, 64018 Pau, France Article info Article history: Received 14 April 2009 Received in revised form 9 February 2010 Accepted 17 February 2010 Available online 6 March 2010 Keywords: Reservoirs Porous sandstones Cataclastic Deformation Bands (CDBs) “Bassin du Sud-Est” Abstract Cataclastic deformation structures in Cretaceous high-porosity sands in the Bassin du Sud-Est, SE France were surveyed by scan-lines to examine: (i) the role of tectonic loading path on cataclastic deformation band (CDB) network development, (ii) the development of larger ultracataclastic faults as strain increases. Deformation during Pyrenean-Provençal shortening resulted in a persistent high density (>10 m-2) of conjugate reverse-sense CDB zones (displacements up to ~30 cm), with no generation of larger faults. High-low density undulations occur for each pair of the conjugate set in an alternating manner, suggestive of network hardening, with a wavelength of several tens of metres being in the order of mechanical bed thickness. For two study areas which experienced significant Oligocene-Miocene extension, a moderate, undulating background density (~4 m-2) of normal-offset CDBs was recorded, which became focussed in places into clusters (~50 m-2) a few metres wide. Thus tectonic loading path may strongly influence strain distribution. CDB zones develop by the addition of successive bands at the edges until, at a thickness of around 5 cm, new bands tend to stray further away from the zone edges. Coarser sands have thicker CDB zones, suggesting that host grain size, along with mechanical bed thickness, could be an important contributor to the scale limit in CDB zone growth. Larger ultracataclastic faults and discrete slip zones localised within or at the edges of some clusters of CDB zones, post-date cluster development rather than inducing it. This stage of deformation evolution is only reached in extension, not in shortening, suggesting the infeasibility of achieving the critical state line during horizontal compression. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Fluid circulation in the crust, and in particular hydrocarbon migration in reservoirs, is highly dependent on fault geometrical and hydromechanical properties (e.g., Manzocchi et al., 1998; Matthi et al., 1998; Wibberley et al., 2008). Faulting in porous sandstone often produces zones of deformation bands rather than planar fracture surfaces (Aydin, 1978; Aydin and Johnson, 1978, 1983; Underhill and Woodcock, 1987; Antonellini et al., 1994; Davis, 1999; Fossen et al., 2007). Cataclastic deformation bands (CDBs) are brittle shear zones that form through a combination of compaction and cataclasis. Porosity and grain size reduction associated with CDB formation are thought to cause strain hardening, further deformation then being accommodated by deformation of the wall rock, adjacent to the initial band (Aydin, 1978; Aydin and Johnson, 1978, 1983; Underhill and Woodcock, 1987). Continued * Corresponding author. Tel.: +33 492942682. E-mail address: saillet@geoazur.unice.fr (E. Saillet). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.02.007 deformation may possibly result in the development of localised slip surfaces at the edge of deformation band zones (Aydin and Johnson, 1983; Antonelini and Aydin, 1995; Shipton and Cowie, 2001). Some, but not all of these different field observations have been understood through laboratory experiments (Wong et al., 1997; Mair et al., 2000; Torabi et al., 2007). Descriptions of such deformation distributions in high-porosity sandstones are quite varied, ranging from examples of deformation band localisation as “damage zones” around larger faults, in relay zones (often expressed as “ladder zones” cf. Schultz and Balasko, 2003) or in fault-tip folds, to zones of deformation distributed over distances much greater than typical damage zone widths (e.g., ~100 m) with no obvious relationship to larger structures (Aydin, 1978; Underhill and Woodcock, 1987; Jamison and Stearns, 1982; Shipton and Cowie, 2001; Du Bernard et al., 2002a,b; Wibberley et al., 2007). Yet although recent advances have been made in understanding the mechanics of yielding to generate a single deformation band (e.g., Schultz and Siddharthan, 2005; Aydin et al., 2006; Wibberley et al., 2007), no unifying mechanical model exists for explaining and predicting distributions of deformation bands in terms of regional controls such as tectonic loading paths. Furthermore, the evolution of a network, and mechanical controls on this evolution, are still unclear, particularly with respect to localisation processes (a) at the scale of a single deformation band – what controls deformation localisation into a band, and deformation band growth? (b) at the scale of a cluster of deformation bands – do they become clustered around previously formed larger faults as fault “damage zones”, or do the clusters of deformation bands form first, by early deformation localisation, with continued localisation of deformation generating the larger faults within or at the edges of these clusters? Some example of these behaviours exist (Fossen et al., 2000, 2007; Shipton and Cowie, 2001), yet understanding the mechanical controls on fault distribution is fundamental in order to provide better fault distribution prediction in reservoir flow simulations from limited structural input data (Saillet, 2009). This paper presents a statistical and quantitative field study aimed at providing a model of deformation patterns and fault growth mechanisms in high-porosity sandstones. This study presents three different cases of Cretaceous high-porosity sands and sandstones in the Bassin du Sud-Est, Provence, France (Fig. 1). The field data were recorded along scan-lines from excellent quarry exposures from the Orange, Massif d'Uchaux and Bédoin areas (Fig. 1). These field data allow us to provide detailed descriptions of the distribution of deformation and its evolution, leading to interpretations in terms of fault growth mechanisms and fault network development in high-porosity sandstones. This part of the study concerns only the geometrical evolution of the deformation. The impact of deformed structures in sandstones on fluid flow, evaluated from permeability and geometric properties of the structures, will be addressed in a separate publication. 2. Geological setting and data acquisition 2.1. Regional context of the Bassin du Sud-Est The Bassin du Sud-Est is a triangular region between the Massif Central to the North West, the Alps to the East, and the Mediterranean Sea to the South. It is a Mesozoic cratonic basin on the edge of the Alpine orogen, ~200 km long and 100-150 km wide. The total thickness of the sedimentary units is up to 10,000 m in the central area, but this thickness decreases to 2,000-3,000 m towards the edge of the basin (Delfaud and Dubois, 1984). From the Triassic to the Cretaceous, sedimentary deposits are essentially marine, corresponding to basin rifting related to Tethys opening. In the middle Cretaceous, the sedimentary units correspond to detrital sands deposited during the beginning of basin inversion. From late to end Cretaceous, the sands were deposited only in a continental environment, with locally high sedimentary rates (Debrand-Passard et al., 1984). In the West of the Bassin du Sud-Est, much of the resulting Cenomanian-Turonian deposits are high-porosity sands and poorly to moderately consolidated sandstones. This paper presents a combined study of three areas in the Bassin du Sud-Est, the Bédoin, Massif d'Uchaux and Orange areas (Fig. 1). These studies were carried out on high-porosity sand and sandstone outcrops in active and abandoned quarries (Figs. 2-4), which provide excellent 2-D and 3-D exposures of deformation band networks and larger faults. These sands and sandstones are composed of a large range of quartz grain sizes, varying between the study areas. They also contain a few clay lamellae and, in places, limestone beds containing a few shallow marine fossils. These sands generally have a marine origin from deltaic to beach sands. Fig. 2 (a) Location of the study area at Orange. Modified from the 1:50,000 geological map of Orange, BRGM. (b) Location of the scan-line within the sandstones from the “Quartier de l'Étang” quarry. Ключевые слова: cros, mechanical property, deformation band, ladder zone, wibberley journal, observation, text, slip zone, journal structural geology, schultz, cdb density, geometric property, thicknessedisplacement relationship, grain crushing, sedimentary, thicknessedisplacement ratio, society, strikeslip fault, thickness, transition, plastic deformation, scan-lines, development, relative timing, host sand, clustering tendency, density distribution, geological map, surface, uchaux, high-porosity, method, additional band, larger fault, average density, density, stress, data, wibberley journal structural geology, b?doin, cenomanian age, displacement, outcrop, saillet wibberley, generated, geology, bed, reverse cdbs, high porosity, proportional increase, structural, high-porosity sand, main, relay zone, cohesion crumble, woodcock antonelini, orange area, porosity, clay lamella, deformation continues, tectonic, france, evans, previous study, cluster, density proles, journal structural, cdb zone, tectonophysics, differential stress, structural geology, sandstone, measurement method, case, conjugate structure, generation, study, soil mechanic, total, cretaceous, geophysical, thickness growth, band, higher density, localisation, density prole, massif uchaux, aydin, single, mechanical, marine environment, study area, scale, slip, incohesive sand, ?eld, wibberley journal structural, number, geological, host rock, massif, fault, cdbs, sandstone pure, slip surface, adjacent bed, bdoin area, field appearance, field view, relationship, bin size, fault zone, ii, saillet, area, high density, stress perturbation, orange, work-hardening process, square metre, cataclastic, unit, remaining structure, journal, growth, rectilinear scan-lines, porous, quarry, du, feature, ultracataclastic, set, process, rst set, mandl, single band, deformation occurs, wa, yield envelope, discrete slip, host, scan-line, high-porosity sandstone, deformation, sand, scan, cdb, larger, cataclastic deformation, individual, discrete, geological society, damage zones, shear zone, recorded, successive band, increase, strand, individual band, critical, fossen, recorded based, result, elsevier, shipton, pyreneaneprovenal shortening, material softening, naked eye, ultracataclastic fault, sand unit, damage zone, mechanic, statistical analysis, massif duchaux, zone, spatial distribution, event, eds, spatial correlation, high, porous sandstone, orange brgm, larger structure, uid, conjugate, path, internal structure, hobbs, distribution, loading path, size, heterogeneous distribution, edge, generally synchronous, ladder zones, conjugate set, microscopic scale, deformed structure, bassin, mechanics, wibberley, tectonic event, rock, applied, stress path, structure, ?rst, separate structure, faulting, sedimentary unit, scale limit, individual cdbs, sud-est, deformed material, grain, normal, fault growth, underhill, total length