Many field, experimental, and theoretical studies have contributed greatly to our understanding of the occurrence and formation of deformation bands in porous granular materials, but questions remain regarding the mechanics of strain localization, and how the orientation, thickness and internal strain (shear relative to volume change) of deformation bands is influenced by loading history and evolving rock properties. Here we report on triaxial rock deformation experiments using a non-traditional sample geometry to investigate band formation across the brittle-ductile transition. Five-cm diameter cylinders of Berea sandstone were machined with a circular (8.77 cm radius) notch to form a dog-bone sample geometry. In triaxial compression, the sample geometry obviates end-effects without creating heterogeneous stress gradients that can influence localization. Samples were instrumented to measure local strains in the neck region and acoustic emissions (AE), and then shortened to failure at confining pressures of 50 to 250 MPa. Deformation bands formed at all conditions, and photo mosaics of the outer sample surface were used to determine the thickness and orientation of the bands. Band thickness increases from several to tens of mm thickness and the angle between the bands with the shortening axis changes from 35 to 80 degrees, as confining pressure increases from 50 to 250 MPa, respectively. Mechanical data, including local strain measurements through yield, were used to test theoretical models for the onset of localization and formation of deformation bands as an instability in the constitutive description of homogeneous deformation. Generally, theoretical predictions compare favorably with the observed onset of localization determined by marked changes in the AE rate, and are consistent with the formation of compacting shear bands at higher mean stress. Predictions of changes in band orientation with mean stress are largely consistent with observed trends, but deviate from the observed orientation by as much as twenty degrees.

Many field, experimental, and theoretical studies have contributed greatly to our understanding of the occurrence and formation of deformation bands in porous granular materials, but questions remain regarding the mechanics of strain localization, and how the orientation, thickness and internal strain (shear relative to volume change) of deformation bands is influenced by loading history and evolving rock properties. Here we report on triaxial rock deformation experiments using a non-traditional sample geometry to investigate band formation across the brittle-ductile transition. Five-cm diameter cylinders of Berea sandstone were machined with a circular (8.77 cm radius) notch to form a dog-bone sample geometry. In triaxial compression, the sample geometry obviates end-effects without creating heterogeneous stress gradients that can influence localization. Samples were instrumented to measure local strains in the neck region and acoustic emissions (AE), and then shortened to failure at confining pressures of 50 to 250 MPa. Deformation bands formed at all conditions, and photo mosaics of the outer sample surface were used to determine the thickness and orientation of the bands. Band thickness increases from several to tens of mm thickness and the angle between the bands with the shortening axis changes from 35 to 80 degrees, as confining pressure increases from 50 to 250 MPa, respectively. Mechanical data, including local strain measurements through yield, were used to test theoretical models for the onset of localization and formation of deformation bands as an instability in the constitutive description of homogeneous deformation. Generally, theoretical predictions compare favorably with the observed onset of localization determined by marked changes in the AE rate, and are consistent with the formation of compacting shear bands at higher mean stress. Predictions of changes in band orientation with mean stress are largely consistent with observed trends, but deviate from the observed orientation by as much as twenty degrees.

Les bandes de déformation sont des structures géologiques se mettant en place dans les matériels granulaires présentant une forte porosité (>15%). Ces structures peuvent être compactantes ou dilatantes et peuvent présenter une composante cisaillante. À l’échelle microscopique, il est possible d’observer une réorientation des grains, une compaction ou un cisaillement intense peut entrainer la fracturation de ces derniers (cataclase) pour former une fine zone déformée modifiant ainsi la porosité et la perméabilité de la roche. Ces bandes ont un impact sur la circulation des fluides en formant des barrières ou des drains dans le réservoir. La formation de ces structures est étroitement liée à la tectonique et aux paramètres sédimentologiques du matériel hôte. Comprendre et pouvoir prédire le mode de mise en place, les orientations et la distribution de ces bandes est l’objectif principal de cette thèse.Dans cette étude nous avons analysé un site de bandes de déformation observé dans le bassin de Tremp, au sein de la formation d’Aren, dans la zone Central Sud-Pyrénéenne. Nous avons pu définir la nature de ces structures grâce à des analyses macro- et micro-structurales couplées à une étude d’anisotropie magnétique permettant de déduire la direction de raccourcissement à l’origine de la mise en place de ces bandes. Deux principaux types de bandes cataclastiques sont alors mis en évidence: (1) des bandes de compaction pures, perpendiculaires au raccourcissement et (2) des bandes de compaction à composante cisaillante, obliques à cette même direction de raccourcissement.En comparaison avec le calendrier tectonique de la région et des données d’enfouissement relatif dans le temps de la formation étudiée. Nous avons fait l’hypothèse que ces deux familles de bandes sont apparues à faible enfouissement (< 1 km de profondeur), soit peu de temps après le dépôt et associées à la croissance du pli du Sant Corneli-Boixols. De telles structures ne sont pas communes pour un enfouissement superficiel et le faciès calcarénitique est mis en cause pour expliquer leurs apparitions.La mise en place de simulations analytiques basées sur des résultats d’expérimentations géomécaniques ont ensuite permis de contraindre le régime tectonique, l’orientation et les valeurs de ces contraintes nécessaires à la formation de ces deux types de bandes par rapport à l’enfouissement et la croissance du chevauchement. Les valeurs de contraintes attendues sont ici très faibles dans le cas d’une déformation très précoce.Enfin, nous avons testé ces observations et nos hypothèses à des modélisations numériques dans lesquelles nous avons analysé l’impact de la croissance d’un chevauchement et d’un pli de propagation de rampe. La distribution des contraintes et les potentielles bandes de déformation mises en place au sein d’un réservoir poreux situé en avant de ce pli ont été étudiées. Nous avons alors montré que nos hypothèses d’apparition superficielles de bandes de déformation étaient dépendantes de la position des enveloppes de ruptures (elles-mêmes dépendantes de la lithologie de la roche). Pour expliquer la mise en place des bandes étudiée dans ce mémoire, une résistance mécanique très faible de la roche est nécessaire pour permettre de former des bandes à moins d’un kilomètre de profondeur. / Deformation bands are geological structures that occur in porous and granular material presenting a high porosity (>15%). These structures can be identified as compactive or dilatant, a shear component is also often observed. At the microscopic scale, it is possible to observe a grain rearrangement and an intense compaction and or shearing can lead to grain crushing (known as cataclasis), to form a thin deform zone that will modify the porosity and permeability of the rock. Deformation bands have a non-negligible impact on fluid flow, creating a barrier or a drain in the potential reservoir. The formation of such structures is mainly linked to the tectonic activity but also to the facies and other sedimentological parameters of the host rock. The understanding and the prediction of the occurrence and distribution of the bands is the main objective of this thesis.In this study we analyzed a deformation band site found in the Tremp basin, in the Aren formation localized in the South Central Pyrenean Zone. We defined the nature of these structures with macro and microstructural analysis and by adding a study of the magnetic anisotropy to constrain the shortening direction responsible to the band formation. We evidence two major types of bands showing different orientations and behavior: (1) Pure compaction bands (PCB), perpendicular to the shortening and (2) Shear enhanced compaction bands (SECB), oblique to the same shortening.In comparison with tectonic schedule in the studied area and time vs. burial data of the formation, we deducted that both types of bands took place at a shallow burial (<1km depth), which means short times after deposition. This localized deformation, showing mainly cataclasis, is associated to the growth of the Sant Corneli-Boixols fold and thrust belt. Such structures are not common at a shallow depth and we propose that the calcarenite facies of the host rock is the key factor to explain the band occurrence.Thereafter, we made analytical simulations based on geomechanical experimentations results that allowed us to constrain the stress state and orientations needed to create these structure and to determine the timing of formation compared to the burial of the layers during the growth of the Boixols thrust. The stresses magnitudes are expected to be really low in the case of an early deformation.Finally, we tested and compared our observations and hypothesis to numerical modeling where we analyzed the impact of the growth of a fold and thrust belt on the stress state and orientations and the analysis of potential deformation bands occurrence. The stress distribution and the potential occurrence of deformation bands in a porous reservoir presenting different characteristics and located in front of this fold were studied.With the modelizations results, we exposed that our hypothesis of shallow deformation bands are dependent from the position of failure envelopes (that are dependent on the rock lithology). To explain the band formation we studied in this thesis, a weak mechanical strength of the host rock is needed to form deformation bands at less than a depth of one kilometer. The pure compaction bands are associated to a potentially early layer-parallel shortening (LPS).

Cette étude expérimentale traite la localisation des déformations sur un grès poreux: le grès de Vosges. Un nombre des essais triaxiaux sont effectués sous des pressions de confinement (i.e., 20 MPa - 190 MPa) et des déformations axiales différentes pour mieux comprendre la réponse mécanique de ce grès. La localisation des déformations a été étudiée dans des différentes échelles en appliquant une variation de mesures de champs (full-field methods) comme la Tomographie Ultrasonore (en 2D), les Emissions Acoustiques (en 3D), les Rayons X (en 3D), et la Corrélation des Images (en 3D). Les méthodes expérimentales ont été appliquées avant, pendant et après les essais triaxiaux. Des coupes fine ont été observées sous le microscope optique et électronique (SEM). La combinaison des multiples techniques expérimentales, qui ont des différentes sensitivités et résolutions, a décrit la procédure de la formation et l’évolution des bands de déformation observées sur le grès de Vosges. Des bandes de cisaillement ont été formées sous des pressions intermédiaires et des bandes de compaction sous des pressions élevées. Des bandes de compactions pure n’ont pas été observées.Les bandes de déformations observées se sont caractérisées comme des zones de déformation de cisaillement localisée et/ou de compaction. En plus, elles se sont caractérisées comme des structures de fable vitesse ultrasonore, des places d’origine des fissures inter- ou intra- granulaires, et des places des densités de matériel élevées.Deux mécanismes principales ont été observées au niveau de grain dans les bandes de cisaillement et de bandes de compaction (shear-enhanced compaction bands): d’un cote c’est la fissuration des grains (endommagement) et de l’autre cote c’est la réduction de porosité (sur la forme de compaction). Les deux mécanismes i présent différences sur leurs proportions et leur ordre d’occurrence dans le temps. / This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time.