Porosity microstructures of a sandstone affected by a normal fault

Abstract

Dans un système de failles normales de la bordure du fossé rhénan, les interactions eaux-roches de part et d'autre de ces failles peuvent contrôler les conditions des circulations fluides. L'objectif de ce travail est de caractériser les structures du réseau poreux dans la zone endommagée autour d'une de ces failles. Il est intéressant d'étudier la relation entre porosité et perméabilité dans cette zone. Des études pétrographiques et pétrophysiques, des mesures microthermométriques sur des inclusions fluides et la composition isotopique de l'oxygène ont permis de caractériser les structures de porosité des roches et notamment des ciments primaires et secondaires.Le couplage de ces approches montre qu'une faille normale peut à la fois jouer le rôle de drain et de barrière à la circulation des fluides. En fonction de la direction de circulation, la faille joue le rôle de drain en laissant remonter les fluides parallèlement au plan de faille et le rôle de barrière, en focalisant les circulations dans le toit. L'anisotropie, notamment des propriétés de transfert héritées des conditions de dépôts fluviatiles, est profondément modifiée par les transferts subits dans le matériau. Ainsi les modifications des transferts dépendent des modifications du réseau poreux : l'hétérogénéité de la structure du réseau et l'anisotropie d'orientation ou de connectivité. Ce modèle de circulation est contrôlé par une interaction entre les modifications des structures du réseau poreux et les circulations fluides, entraînant des modifications de l'anisotropie de certaines propriétés du matériau autour de la faille.Abstract. -Introduction -Normal faults are part of the elements that control fluid flows in sedimentary basins. They can play the role of a barrier or a drain [Hippler, 1993]. These pathways are anisotropic. The aim of this study is to determine the fluid pathways and to characterise the pore network and its role in the transfer properties.Petrophysics, petrographics, geochemical and fluid inclusion studies allow us to characterise a Buntsandstein sandstone affected by a normal fault. This sandstone has a fluviatile origin, field evidenced by fluviatile channels, but also by some clay layers. The fault is located in the north east of France, in the Rhine Graben. The vertical displacement is about 3 meters, and the dip is 70 o east. The fractured zone is composed of three compartments (the hanging wall and the footwall separated by a gouge) divided by three main faults ( fig. 1). Oriented samples were taken from the three blocks and were studied following the procedure figure 2. Results -The petrographical and mineralogical composition of the three compartments were different. The gouge and the footwall were characterised by quartz overgrowths, authigenic kaolinite (30 to 40 % of the clay fraction) and diagenetic illite (40 to 60 % of the clay fraction). The hanging wall was characterised by 70 to 80 % diagenetic illite of the clay fraction ( fig. 3).The isotopic composition of the footwall quartz overgrowth ( fig. 4) was δ 18 O ...

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Grès d'Annot, southeastern France)

“…However, in deeply buried sandstones, the activation of pressure solution and cementation of quartz at temperatures higher than 80°C (Renard and Ortoleva, 1997;Lander et al, 2008;Becker et al, 2010) tends to seal the fractures that then become ineffective conduits for fluid flow (Laubach, 2003). Structural and petrophysical studies in various examples of deeply buried quartz-rich sandstones confirm the reduction of permeability in the core zone caused by grain-size reduction and/or quartz cementation (Fisher et al, 2003;Surma et al, 2003;Balsamo et al, 2010). Hence, both the fault core and damage zones are likely to act as barriers to fluid flow in deeply buried sandstones.…”

“…For studying fault zone petrophysical properties, most authors use fault strike or bedding plane to choose test plug orientations (Faulkner and Rutter, 1998;Surma et al, 2003;Richard and Sizun, 2011;Leclère et al, 2012). In this study, we aimed to investigate the anisotropy of the matrix petrophysical properties of the foliated fault rocks and thus chose the drilling directions of the plugs considering the sample microstructure.…”

“…With the exception of some works that document a potential capillarity effect of cataclastic deformation bands (Sigda and Wilson, 2003;Cavailhes et al, 2009), the literature generally describes a permeability reduction in these structures from 1 to 6 orders of magnitude relative to the host rock (e.g., . By contrast, relations between faulting and fluid flow in lowporosity sandstones buried to deep diagenesis conditions (150-250°C) have been much less studied (Fisher et al, 2003;Surma et al, 2003;Balsamo et al, 2010;Laubach et al, 2010). In these conditions, which are those of the deeply buried reservoirs, in particular, the tight-gas sandstones, mechanical and chemical processes are intimately linked in faulting mechanisms.…”

The south-western Black Forest and the Upper Rhine Graben Main Border Fault: thermal history and hydrothermal fluid flow

“…1;Lüders 1994;Surma et al 2003). These hot fluid migrations are on the one hand dated as Cenozoic (Lüders 1994) and on the other hand a Mesozoic age is also possible (Surma et al 2003).…”

“…Additionally, some fluid inclusion data from the eastern and western URG Main Border Fault display fluid flow with temperatures of up to 225°C related to tectonic activity (Fig. 1;Lüders 1994;Surma et al 2003). These hot fluid migrations are on the one hand dated as Cenozoic (Lüders 1994) and on the other hand a Mesozoic age is also possible (Surma et al 2003).…”