Bands of localised deformation in weakly cemented sands: the example of Bédoin (southern France)

Abstract

Weakly cemented sands from southern France exposing deformation bands were studied as a sandstone reservoir analogue. The objective was to analyse the observed localised deformation in the presence of imperfections (diagenetic or from deformation processes). 2D nano- (Scanning Electron Microscopy) and 3D micro(X-ray Computed Tomography) image analysis techniques were employed to inspect samples of deformation bands from the outcrops and laboratory-deformed samples of undisturbed sand. Artificially cemented samples without imperfections, prepared with a novel procedure, were also tested for comparison. Syntaxial quartz overgrowths were identified not only in compactional shear bands but also in undisturbed sand and in a dilation band. Kaolin clay was present at the time of localised deformation, since it interacted with free silica to form micro-quartz, but more clay infiltrated afterwards forming menisci bonds. Kaolinite illitization, observed in multi-strand bands and cluster, and silica cementation required temperatures > 70°C to occur. Therefore, it is supported the hypothesis that the B´edoin sand reached a burial depth of 2 - 3 km. The mechanical tests showed that all the samples developed localised deformation before or at the peak stress; however, peak stress was reached later in artificially prepared samples than in the natural ones. The weakly cemented sands exhibited a behaviour close to that of sands. Weaker imperfections were observed, by performing Digital Volume Correlation, to trigger localised deformation, whereas stronger imperfections shifted the bands far from them. Artificially cemented sand samples, with higher cement content than the natural ones, seemed to develop less stress concentration at grain contact, thus generating networks of shear bands, rather than fewer bands as observed in the natural samples. Porosity and permeability analyses were performed through image processing on the samples 3D X-ray tomographies. Porosity values enabled the distinction of zones in each band type; in particular, a dilation band core reported a porosity value (30%) similar to that of the undisturbed sand. Permeability was zero in the strands of compactional shear bands, whereas the dilation band core had values closer to that of the undisturbed sand (10+4 mD).