La localisation de la déformation dans les grès poreux : caractérisation d'un analogue de réservoir gréseux et faillé dans le Bassin du Sud-Est, Provence, France

Fluid circulation in the crust and in particular hydrocarbon migration in reservoirs is highly dependant on fault geometrical and hydromechanical properties. Understanding the evolution of these properties during fault growth and network development is of major importance in fluid prediction. We made a multi-scale study of a faulted reservoir analogue in cretaceous high-porosity sands and sandstones from Provence, South-East France, studying a wide range of structures and their distribution from the smallest deformation bands to decametric-offset ultracataclastic faults zones. This study shows three main axes : i) A detailed field study, based on 700 m of structural logging ; ii) A microstructural study, based on SEM photomicrograph analyses including a statistic porosity and grain size analysis, complemented by laser grain size distribution measurements ; iii) Permeability measurements based on the same large range of samples. The main results of our different methods of analysis can be summarized as follows : i) A 250 m long outcrop recorded a persistent high density of deformation bands which did not appear to cluster around any mapped faults ; ii) For two study areas, a moderate, undulating background density of CDBs was recorded, which became focussed into clusters in places. Later, larger ultracataclastic faults and discrete slip planes are found localised within or at the edges of some of the CDB clusters ; iii) The petrophysical studies shows : i) the important role played by host rock properties in the deformation processes ; ii) the grain-size and the porosity reduction evolve with increasing displacement ; iii) permeability measurements show that CDBs do not have a large influence on single phase fluid migration. On the other hand, a small number of the larger ultracataclastic faults can form barriers to fluid migration resulting in reservoir compartmentalization.La compréhension de l’organisation géométrique et hydromécanique des failles qui affectent les réservoirs est tout à fait essentielle pour contraindre les circulations de fluides et d’hydrocarbures. Cette problématique est d’autant plus importante dans les réservoirs de grès de forte porosité. Nous avons réalisé une étude multi-échelle, depuis les BDs jusqu’aux failles majeures dans plusieurs analogues de réservoirs gréseux et faillés situés dans les terrains du Crétacé Supérieur du Bassin du Sid-Est, en Provence (France). Nous avons choisi de mener cette étude selon trois axes principaux : i) Une étude de terrain détaillée, avec la réalisation de plus de 700 mètres de relevés linéaires de la déformation ; ii) Une étude microstructurale en laboratoire, basée sur l’analyse de clichés MEB puis sur des mesures en porosité et granulométrie à partir d’un logiciel de comptage automatique puis d’un granulomètre à diffractométrie laser ; iii) Une étude de perméabilité au gaz sous pression de confinement des échantillons de roche saine et déformée, prélevés selon deux techniques d’échantillonnage adaptées. L’ensemble des données a permis l’obtention de résultats essentiels à la compréhension de l’évolution de la déformation des réservoirs gréseux et de l’incidence sur les migrations de fluides : i) Sur un affleurement de 250 mètres de long, on observe la présence d’une forte densité de déformation se traduisant par des BDs créées en dehors de tout cluster ou de toute faille majeure ; ii) Deux autres sites d’études présentent une déformation modérée avec une localisation de la déformation sur quelques zones de clusters, au sein desquels il est possible de générer des zones de glissement ; iii) les analyses microstructurale montrent : i) le rôle essentiel de la lithologie sur les microorganismes de la déformation ; ii) la relation étroite entre la taille de la structure déformée et la réduction de porosité et de perméabilité associée ; iii) les mesures de perméabilité montrent que les BDs affectent peu la perméabilité globale des réservoirs, alors que les larges failles ultracataclastiques sont susceptibles de former des barrières à l’écoulement des fluides

Stress path (Q–P’) maps and Andersonian faulting in porous rocks

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Evolution of brittle deformation and fault growth in high porosity sandstone: the Bassin du Sud-Est, Provence, France

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Wear your mask, but think about deaf students

International audienceMasks are vital to contain the spread of coronavirus, but lecturers and universities must find ways to be inclusive, say Olivier Pourret and Elodie Saillet We are a married pair of associate professors-one a geochemist and the other a structural geologist-living in France. One of our three children is deaf. Both of us will soon be teaching in lecture theatres, and wearing masks in line with French regulations. We support this policy, but we also know how difficult our son would find our lectures, if he were there. Our son, like many deaf people, is a talented lip reader. What he can't pick up aurally, from what's sometimes called 'residual hearing', he regularly picks up from reading lips. Most masks prevent this mode of understanding among deaf students, who already work very hard to participate and learn in lecture theatres and in schools. Experts estimate that only about 30-40% of spoken English-and French, our first language-is distinguishable to a talented lip reader in favourable circumstances. Most people with hearing impairments can and do speak using their natural speech organs, which they have learned to use through speech therapy. But many cannot track or automatically adjust the tone and volume of their voice, so their voice can initially be difficult for people to understand. Awareness grows as one becomes more familiar with the deaf student's speech pattern. By taking simple action, the scientific community can help deaf students. A few years ago, a deaf student asked us to wear a microphone to amplify and clarify our voices. We have als

Mechanics of fault-zone localization in high-porosity sandstones and impact on flow efficiency

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Faulted reservoirs in high-porosity sandstones: structural analogues from the Cretaceous of the Bassin du Sud-Est, Provence, France

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Evolution of brittle deformation and fault growth in high porosity sandstone: the Bassin du Sud-Est, Provence, France

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Fault growth in high porosity sandstones: Field and petrophysical study in the “Bassin du Sud-Est”, Provence, France

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Mechanics of fault-zone localization in high-porosity sandstones and impact on flow efficiency

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Permeability and flow impact of faults and deformation bands in high-porosity sand reservoirs: Southeast Basin, France, analog

International audienceOutcrops of the Cretaceous high-porosity sandstone of the Southeast Basin, France, show two main types of deformation structures: a large number of small-offset, shear-enhanced cataclastic deformation bands (DBs); and a small number of large (meters to decameters)-offset ultracataclastic fault zones. Microstructural analyses of the cataclastic DBs show that fragmentation produces strands of cataclastic fragment-supported matrix, separated by weakly fractured host rock, which cluster to form the DBs. The ultracataclastic fault zones, however, are composed of a matrix-supported ultracataclasite material. Permeability data show that the DBs reduce host-rock permeability by 0.5 to 2 orders of magnitude, whereas the ultracataclasites reduce permeability by approximately 4 orders. Simple calculations considering the structural frequency, thickness, and permeability of these faults suggest that, although the DBs may have an impact on single-phase flow, it is most likely to be less than a 50% reduction in flow rate in extensional contexts, but it may be more severe in the most extreme cases of structural density in tectonic shortening contexts. The larger ultracataclastic faults, however, despite their much lower frequency, will have a more significant reduction in flow rate, probably of approximately 90 to 95%. Hence, although they are commonly at or below the limit of seismic resolution, the detection and/or prediction of such ultracataclastic faults is likely to be more important for single-phase flow problems than DBs (although important two-phase questions remain). The study also suggests that it is inappropriate to use the petrophysical properties of core-scale DB structures as analogs to larger seismic-scale faults