Grain Deformation Processes in Porous Quartz Sandstones – Insight from the Clusters of Cataclastic Deformation Bands

Presented at: Powders and Grains 2017 – 8th International Conference on Micromechanics on Granular MediaPorous sandstones represent classical reservoirs for water or hydrocarbons. Deformation in such granular material is due to tectonic events and occurs through the process of cataclasis, implying the crushing of the grains to a diversity of smaller clasts. Cataclasis is generally accompanied by porosity and permeability decrease. Although it is known that cataclastic deformation localizes to form individual bands and clusters of bands, the parameters controlling the distribution of this deformation were not well understood until recently. We used scanline measurements to show a favoured localization and clustering of the deformation on the case of normal-fault stress regime and potentially in strike-slip fault regime. The reverse regime favours the formation of distributed networks of conjugate deformation bands. At the scale of a cluster, field data reveals that the minimum modal grain size value of the host sandstone(s) controls the band density. Finally, microscopic cathodoluminescence analysis reveals enhanced quartz cementation for high degree of cataclasis. Hence, because band clustering, high degree of cataclasis and band cementation are favoured in normal-fault stress regime, tectonic extension appears to be favourable conditions for the formation of efficient barriers to fluid-flow in porous sandstone reservoirs.publishedVersio

Development of a structural, petrophysical and mechanical model of faults in porous sandstone environment : implications for the migration and trapping of fluids

La cataclase est un processus de déformation efficace en termes de réduction de porosité et de perméabilité dans les grès poreux, constituant des aquifères et réservoirs d’hydrocarbures classiques. Un enjeu majeur concernant la déformation dans les grès consiste à identifier les processus contrôlant l’évolution des structures cataclastiques et reconnaître les paramètres influençant l’expression de la déformation à l’échelle microscopique et à l’échelle du bassin. Dans cette étude, nous nous concentrons sur l’analyse structurale des amas (« clusters ») de bande de déformation cataclastiques afin de considérer une déformation suffisamment localisée représentant un potentiel rôle de barrière sur les fluides. Nous choisissons sept sites d’étude présentant des clusters formés en tectonique extensive et contractive, dans différent régimes Andersoniens, à différentes profondeurs d’enfouissement, et dans des grès aux lithologies variées. Nous utilisons une approche analytique afin d’estimer l’évolution de l’état de contrainte des grès jusqu’à la déformation. L’utilisation de modèles numériques permet d’analyser l’influence de certains paramètres physiques sur la structuration de la déformation. Nous montrons que la position de l’enveloppe de rupture du grès (dépendant de sa lithologie) semble déterminer la morphologie de la déformation. D’autre part, les clusters formés en régimes Andersoniens normal, décrochant et inverse semblent respectivement couramment se former sur la même partie de l’enveloppe.Les clusters formés en régime normal montrent des épaisseurs fines à moyennes, des densités de bande importantes et forment, avec d’autres clusters, des réseaux d’échelle kilométrique souvent localisés à proximité d’une faille majeure. Ils représentent une barrière potentielle pour les fluides. Les clusters formés en régime décrochant ont des épaisseurs et des densités de band moyennes. Parce qu’ils semblent éparses, ces clusters ne forment probablement aucun frein pour les fluides. Les clusters formés en régime inverse ont des épaisseurs et des densités de bande moyennes si la rupture est atteinte sur la partie fragile de l’enveloppe. Ils semblent potentiellement plus épais avec des densités de bands faibles voire deviennent de simples réseaux de bandes distribuées si l’enveloppe de rupture est atteinte sur sa partie ductile. Parce qu’ils sont courts et éparses, ces clusters ne représentent pas de frein pour les fluides.Nous relions le développement des clusters et leur morphologie à l’agencement microscopique des clasts dans le matériel déformé. La faible compaction du matériel déformé des clusters créés en régimes normal et décrochant semble être à l’origine de l’étroite localisation des bandes à cause de la présence de plans de faiblesse dans le matériel déformé. Pour le même degré de déformation, la compaction plus élevée du matériel en régime inverse favoriserait la distribution des bandes.Le passage à la faille tel qu’observé dans les clusters en régime normal est permis par la présence entre les grès de niveaux incluant des minéraux fragiles. Ces niveaux permettent l’initiation et la propagation d’une surface de glissement majeure dans les grès adjacents. L’initiation d’une faille est aussi favorisée lorsque des grès poreux sont juxtaposés contre une lithologie indurée. Notre étude montre que la cimentation de quartz des parties les plus déformées des clusters est fréquente, même dans le cas de clusters ayant été enfouis à des profondeurs inférieures à 800 m. Cette cimentation est catalysée par l’intense degré de cataclase, semble être précipitée par « self-healing » et altère les propriétés pétrophysiques des clusters.Deformation through cataclasis, which corresponds to grain crushing, is an effective process of porosity and permeability reduction in porous sandstones, classical aquifers and hydrocarbon reservoirs at depth. A major stake concerning the deformation in sandstone is to understand what processes govern the growth of the cataclastic structures and to recognize what parameters influence the expression of the deformation at microscopic scale and at basin scale.In this study, we focus on the analysis of cataclastic deformation band clusters in order to consider a significantly concentrated deformation regarding the potential of fluid flow baffling. We select seven study sites presenting clusters formed in extensional and contractional tectonics, under different Andersonian regimes, at various burial depths and in sandstones of varying lithologies. To complement the structural analysis, we use an analytical approach to estimate the stress-state evolution of the sandstones leading to deformation. Numerical modeling allows the analysis of the influence of physical parameters on the structuring of the deformation.We show that the position of failure along the failure envelope of the sandstone (which depends on its lithology) seems to determine the morphology of deformation. On the other hand, normal, strike-slip and thrust Andersonian regime clusters respectively seem to form frequently on the same part of the envelope.Normal regime clusters (favorably formed in extensional tectonics) have thin to medium thickness, with high band density and form, with other clusters, networks of km-scale length - often localized near a major fault. They are likely to baffle fluid flow. Strike-slip regime clusters (favorably formed in contractional tectonics) have medium thickness with medium band densities. Due to their sparseness, they seem unlikely to form a baffle for fluids. Thrust regime clusters (favorably formed in contractional tectonics) have medium thickness and medium band density if failure is attained on the brittle part of the envelope. They seem potentially thicker, with low band density and tend to form arrays of deformation bands if failure is attained on the cap of the envelope. Because they are short and sparse, they do not represent an effective baffle for fluid flow.We relate the process of cluster growth and their resulting morphology to the microscopic arrangement of the clasts in the deformed material. The minor compaction in the deformed material of normal and strike-slip regime clusters seems to be at the origin of the dense localization of the bands through the presence of weaker planes in the deformed material. For the same degree of deformation, the more compacted material in thrust regime clusters would favor the distribution of the bands.Faulting of normal regime clusters is enhanced by the presence of layers including weak minerals between the sandstones. These weak layers are responsible for the initiation and propagation of major slip-surfaces in the adjacent sandstone from small displacements. The initiation of major slip-surfaces is also favored when porous sandstone is juxtaposed with a hard lithology.We find that the quartz cementation of the most deformed parts of the clusters is common, even in clusters that were never buried below 800 m. This cementation is promoted by an intense degree of cataclasis, seems to form by “self-healing”, and may reduce the petrophysical properties of clusters

Elaboration d'un modèle structural, pétrophysique et mécanique des failles en milieu gréseux poreux : implication pour la migration et le piégeage des fluides

Deformation through cataclasis, which corresponds to grain crushing, is an effective process of porosity and permeability reduction in porous sandstones, classical aquifers and hydrocarbon reservoirs at depth. A major stake concerning the deformation in sandstone is to understand what processes govern the growth of the cataclastic structures and to recognize what parameters influence the expression of the deformation at microscopic scale and at basin scale.In this study, we focus on the analysis of cataclastic deformation band clusters in order to consider a significantly concentrated deformation regarding the potential of fluid flow baffling. We select seven study sites presenting clusters formed in extensional and contractional tectonics, under different Andersonian regimes, at various burial depths and in sandstones of varying lithologies. To complement the structural analysis, we use an analytical approach to estimate the stress-state evolution of the sandstones leading to deformation. Numerical modeling allows the analysis of the influence of physical parameters on the structuring of the deformation.We show that the position of failure along the failure envelope of the sandstone (which depends on its lithology) seems to determine the morphology of deformation. On the other hand, normal, strike-slip and thrust Andersonian regime clusters respectively seem to form frequently on the same part of the envelope.Normal regime clusters (favorably formed in extensional tectonics) have thin to medium thickness, with high band density and form, with other clusters, networks of km-scale length - often localized near a major fault. They are likely to baffle fluid flow. Strike-slip regime clusters (favorably formed in contractional tectonics) have medium thickness with medium band densities. Due to their sparseness, they seem unlikely to form a baffle for fluids. Thrust regime clusters (favorably formed in contractional tectonics) have medium thickness and medium band density if failure is attained on the brittle part of the envelope. They seem potentially thicker, with low band density and tend to form arrays of deformation bands if failure is attained on the cap of the envelope. Because they are short and sparse, they do not represent an effective baffle for fluid flow.We relate the process of cluster growth and their resulting morphology to the microscopic arrangement of the clasts in the deformed material. The minor compaction in the deformed material of normal and strike-slip regime clusters seems to be at the origin of the dense localization of the bands through the presence of weaker planes in the deformed material. For the same degree of deformation, the more compacted material in thrust regime clusters would favor the distribution of the bands.Faulting of normal regime clusters is enhanced by the presence of layers including weak minerals between the sandstones. These weak layers are responsible for the initiation and propagation of major slip-surfaces in the adjacent sandstone from small displacements. The initiation of major slip-surfaces is also favored when porous sandstone is juxtaposed with a hard lithology.We find that the quartz cementation of the most deformed parts of the clusters is common, even in clusters that were never buried below 800 m. This cementation is promoted by an intense degree of cataclasis, seems to form by “self-healing”, and may reduce the petrophysical properties of clusters.La cataclase est un processus de déformation efficace en termes de réduction de porosité et de perméabilité dans les grès poreux, constituant des aquifères et réservoirs d’hydrocarbures classiques. Un enjeu majeur concernant la déformation dans les grès consiste à identifier les processus contrôlant l’évolution des structures cataclastiques et reconnaître les paramètres influençant l’expression de la déformation à l’échelle microscopique et à l’échelle du bassin. Dans cette étude, nous nous concentrons sur l’analyse structurale des amas (« clusters ») de bande de déformation cataclastiques afin de considérer une déformation suffisamment localisée représentant un potentiel rôle de barrière sur les fluides. Nous choisissons sept sites d’étude présentant des clusters formés en tectonique extensive et contractive, dans différent régimes Andersoniens, à différentes profondeurs d’enfouissement, et dans des grès aux lithologies variées. Nous utilisons une approche analytique afin d’estimer l’évolution de l’état de contrainte des grès jusqu’à la déformation. L’utilisation de modèles numériques permet d’analyser l’influence de certains paramètres physiques sur la structuration de la déformation. Nous montrons que la position de l’enveloppe de rupture du grès (dépendant de sa lithologie) semble déterminer la morphologie de la déformation. D’autre part, les clusters formés en régimes Andersoniens normal, décrochant et inverse semblent respectivement couramment se former sur la même partie de l’enveloppe.Les clusters formés en régime normal montrent des épaisseurs fines à moyennes, des densités de bande importantes et forment, avec d’autres clusters, des réseaux d’échelle kilométrique souvent localisés à proximité d’une faille majeure. Ils représentent une barrière potentielle pour les fluides. Les clusters formés en régime décrochant ont des épaisseurs et des densités de band moyennes. Parce qu’ils semblent éparses, ces clusters ne forment probablement aucun frein pour les fluides. Les clusters formés en régime inverse ont des épaisseurs et des densités de bande moyennes si la rupture est atteinte sur la partie fragile de l’enveloppe. Ils semblent potentiellement plus épais avec des densités de bands faibles voire deviennent de simples réseaux de bandes distribuées si l’enveloppe de rupture est atteinte sur sa partie ductile. Parce qu’ils sont courts et éparses, ces clusters ne représentent pas de frein pour les fluides.Nous relions le développement des clusters et leur morphologie à l’agencement microscopique des clasts dans le matériel déformé. La faible compaction du matériel déformé des clusters créés en régimes normal et décrochant semble être à l’origine de l’étroite localisation des bandes à cause de la présence de plans de faiblesse dans le matériel déformé. Pour le même degré de déformation, la compaction plus élevée du matériel en régime inverse favoriserait la distribution des bandes.Le passage à la faille tel qu’observé dans les clusters en régime normal est permis par la présence entre les grès de niveaux incluant des minéraux fragiles. Ces niveaux permettent l’initiation et la propagation d’une surface de glissement majeure dans les grès adjacents. L’initiation d’une faille est aussi favorisée lorsque des grès poreux sont juxtaposés contre une lithologie indurée. Notre étude montre que la cimentation de quartz des parties les plus déformées des clusters est fréquente, même dans le cas de clusters ayant été enfouis à des profondeurs inférieures à 800 m. Cette cimentation est catalysée par l’intense degré de cataclase, semble être précipitée par « self-healing » et altère les propriétés pétrophysiques des clusters

Tectonic regime controls clustering of deformation bands in porous sandstone

International audiencePorous sandstones tend to deform by the formation of low-permeability deformation bands that influence fluid flow in reservoir settings. The bands may be distributed or localized into clusters, and limited recent data suggest that tectonic regime may exert control on their distribution and clustering. In order to explore this suggestion, we performed a synthetic analysis based of 73 sets of bands, including 22 new sets measured for a reverse Andersonian regime that fill the important gap in data for this context. We find a surprisingly strong correlation between clustering and tectonic regime, where bands clearly are more distributed in the reverse regime compared to the normal regime. Together with the observed band distributions, capillary pressure data show evidence that efficient membrane seals are expected for extension, whereas pervasive permeability anisotropy is expected for contraction. Such a basic new rule concerning tectonic regime is very useful for assessment of reservoir properties where deformation bands are common but below seismic resolution

Grain Deformation Processes in Porous Quartz Sandstones – Insight from the Clusters of Cataclastic Deformation Bands

Porous sandstones represent classical reservoirs for water or hydrocarbons. Deformation in such granular material is due to tectonic events and occurs through the process of cataclasis, implying the crushing of the grains to a diversity of smaller clasts. Cataclasis is generally accompanied by porosity and permeability decrease. Although it is known that cataclastic deformation localizes to form individual bands and clusters of bands, the parameters controlling the distribution of this deformation were not well understood until recently. We used scanline measurements to show a favoured localization and clustering of the deformation on the case of normal-fault stress regime and potentially in strike-slip fault regime. The reverse regime favours the formation of distributed networks of conjugate deformation bands. At the scale of a cluster, field data reveals that the minimum modal grain size value of the host sandstone(s) controls the band density. Finally, microscopic cathodoluminescence analysis reveals enhanced quartz cementation for high degree of cataclasis. Hence, because band clustering, high degree of cataclasis and band cementation are favoured in normal-fault stress regime, tectonic extension appears to be favourable conditions for the formation of efficient barriers to fluid-flow in porous sandstone reservoirs

Clusters of cataclastic deformation bands in porous

International audienceClusters of cataclastic deformation bands represent potential barriers or baffles to reservoir fluid flow, and their processes of formation remain debated. In this work, we rely on an integrated field study at seven sites to describe the extent of clusters, their morphology and their density of deformation as a function of several parameters: the tectonic loading, the burial depth of deformation, the Andersonian stress regime and the lithology of the sandstone. We perform porosity, sorting and grain shape analyses of the deformed material to improve the understanding of microscopic process of cluster development. In agreement with previous works on cataclastic deformation bands, our results reveal that the tectonic loading constrains the extent and the morphology of the clusters. Extensional tectonics favors the formation in normal-fault Andersonian regime of series of hundreds of meter long, rather thin and dense clusters, forming kilometer long networks associated with faults. We find that the formation of major slip-surfaces (faulting) can occur at any stage of cluster thickening. Contractional tectonics favors the formation of strike-slip clusters of medium-thickness, and more rarely the formation of tens of meter long, low-density, thick thrust clusters. The clusters formed in contractional tectonic settings are sparse. Since 15%–79% of the bands/slip-surface of the clusters have permeabilities close to or lower than 1 × 101 mD, and because clusters can have significant dimensions, they may be significant baffles, particularly in extensional tectonics. Our analysis of the porosity and shape evolution of the clasts with increasing deformation corroborates the hypothesis of strain hardening of the band by combined increase of cohesion and friction induced by the cataclasis. We propose that the variability of hardening associated with the variable packing and related cementation in specific parts of the deformed material controls the development and the morphology of the clusters as a function of the tectonic loading

Relations between shallow cataclastic faulting and cementation in porous sandstones: First insight from a groundwater environmental context

International audienceThe interplay between fault zone cataclasis and cementation is important since both processes can drastically reduce the permeability of faults in porous sandstones. Yet the prediction of fault cementation in high-porosity sandstone reservoirs remains elusive. Nevertheless, this process has rarely been investigated in shallowly buried faults (<2 km; T°<80 °C) where its sealing capacity could be acquired early in the geological history of a reservoir. In this paper, the macro- and microscopic analysis of a fault zone in the porous Cenomanian quartz arenite sands of Provence (France) shows that silica diagenesis occurs in the most intensely-deformed cataclastic parts of the fault zone. This fault zone shows 19–48% of its total thickness occupied by low-porosity quartz-cemented cataclastic shear bands whose porosities range from 0 – ca. 5%. The analysis of the weathering profile around the fault zone reveals the presence of groundwater silcretes in the form of tabular, tightly silicified concretions cross-cut by the fault. Detailed transmitted light, cold-cathodoluminescence and scanning electron microscopy analyses of the silica cements (from the fault and the silcrete) reveal that all the silica cements originate from groundwater diagenetic processes. This study therefore shows that silica cementation can occur specifically in fault zones and as groundwater silcrete in the shallow context of a groundwater system, generated at the vicinity of an erosional unconformity

Fault surface development and fault rock juxtaposition along deformation band clusters in porous sandstones series

International audienceOur field study examines two sites revealing the processes responsible for fault surface development and fault rock juxtaposition along normal faults in porous-sandstone–dominated formations. At the first site, we show that a cluster of cataclastic deformation bands made in an initially poorly consolidated sandstone localizes overprinting of a fault slip surface and brecciation during a subsequent tectonic episode, after a significant diagenesis of the formation induced by burial. Because the permeability of the clusters ranges between 6 × 100 and less than 5 × 10−1 md and because the breccia was highly dilatant, we deduce that the fault behaved as a baffle for cross-fault fluid flow at an early age of the formation and as a conduit after significant diagenetic evolution and subsequent fault surface development. At the second site, we show that the presence of clay-rich layers as thin as 80 cm (31 in.) are responsible for the initiation of a major fault slip surface in the underlying and overlying sandstone. The propagation of the fault prevents further cataclastic deformation and cluster development in these sandstones. Fault displacement juxtaposes fault surfaces, clusters of cataclastic deformation bands, clay-rich gouges, and different sedimentary units. Because both fault rocks have low permeability, their spatial juxtaposition provides a continuous baffle for cross-fault fluid flow. Our study shows that fault surface localization is related to an increase in the contrast of mechanical behavior between the cluster and the adjacent material (diagenetic hardening of the cluster or softening of the clay-rich gouge). Lithological contrasts and diagenesis are favorable conditions for localizing faulting and fault rock juxtaposition, allowing significant three-dimensional anisotropy of permeability during and/or after deformation. These processes must, therefore, be considered for fault-seal analyses in sandstone reservoirs