Toward a better understanding of the fluid circulation in the Rhine Graben for a better geothermal exploration of the deep basins.

Abstract : In the Upper Rhine Graben, geothermal projects are strongly under development, particularly for the exploitation of fluid in the deeper part of the basin. This type of reservoirs constitutes a fractured dominated system. However, the hydraulic behaviour of the fracture netwo rk is poorly known and its knowledge constitutes an important way to better target the exploration works. Combining data from structural analysis, mineralogical analysis of fracture filling from outcrops and drilling cores and flui d geochemical analysis f rom deep boreholes, we propose a new model of fluid circulation in the deeper part of the Rhine Graben. The present - day fluids collected from the basement and the Buntsanstein sandstones in the deepest wells have the same origin and are totally different t o those from Tertiary sediments. These deep fluids have reacted with deep sedimentary rocks at about 4 or 5 km depth before migrate through the graben and probably diluted at small proportions with meteoritic freshwaters. These fluids circulate through a f racture network constitutes by different sets linked to the tectonic history of the Rhine Graben. In the basement, the Hercynian fractures, oriented NE - SW and NW - SE, show two successive stages of fillings showing two fluid circulation pulses: 1) a sheared/ cataclased phase associated with illite and quartz , which is prior to the graben opening ; 2) p recipitation of dolomite in tension fractures linked to the early stage of the graben formation . An other main direction fracture set is present in the basement, probably reactivated, and in the Permo - Triassic sandstones with a N - S direction. On outcrops, this fracture set have no macroscopic mineralogical fillings , but in boreholes and for some large shear zone in the sandstones, a cataclasis phase associated to q uartz and illite, and radial illite in tension fracture in the granite, and barite in the Buntsandstein sanstones is present . This study suggests that the reactivation of old Hercynian structures, oriented NE - SW and NW - SE, in relation with the Tertiary tec tonic history of the graben formation develops the fluid/basement interaction in deep temperature conditions. In the contrary , the N - S large structures favor recent circulation system and rather constitute a recharge drain
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Chrystel Dezayes, Catherine Lerouge, Bernard Sanjuan, Claire Ramboz, Michel Brach. Toward a better understanding of the fluid circulation in the Rhine Graben for a better geothermal exploration of the deep basins.. World Geothermal Congress 2015, Apr 2015, Melbourne, Australia. ⟨hal-00936646⟩

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