Main Geochemical Characteristics of the Deep Geothermal Brine at Vendenheim (France) with Constraints on Temperature and Fluid Circulation
Bernard SANJUAN, Gabrielle NEGREL, Morgan LE LOUS, Ewan POULMARCH, Frederick GAL, Pierre-Clément DAMY
The first analytical results relative to the native geothermal brine discharged from the two deep wells drilled at Vendenheim, in the Rhine Graben, in Alsace (France), obtained within the framework of the Vendenheim FONROCHE geothermal project and the H2020 European DEEP-EGS project, suggest the geochemical composition of this brine is very similar to that of the fluid which was discharged from the neighboring Cronenbourg deep well, in the past. It is also close to that of the brines discharged from the other deep wells located in more northern areas of the Rhine Graben, such as Soultz-sous-Forêts, Rittershoffen, in France, and Landau, Insheim, in Germany (Sanjuan et al., 2016). This Na-Cl brine has a TDS value around 100 g/l and a pH value close to 5, before cooling and degassing. Except for the calcium and strontium concentration values, which are much lower than those in the Soultz-sous-Forêts, Rittershoffen, Landau and Insheim brines, the other concentration values of major and trace species are comparable. Given the similarity of the geochemical composition of all these waters, their origin is probably similar but also multiple, because it results from processes of mixing between primary brines formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and meteoric freshwaters, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to Oligocene (Sanjuan et al., 2016). As for the other deep brines, the main solute cation geothermometers give estimations of reservoir temperature close to 225 ± 25°C for the Vendenheim native brine. This estimation probably corresponds to the temperature of equilibrium at which this brine acquires its chemical composition by interaction with the reservoir rocks. The mineralogical assemblage in equilibrium with the brine at this temperature was described in Sanjuan et al. (2016). According to all these results, the deep brines discharged from the granite basement could probably originate from Triassic sedimentary formations (Buntsandstein, for example) located at great depth ( ≥ 4 km), in the centre of the Rhine Graben, in which they acquire their high salinity and chemical composition at temperatures close to 225 ± 25°C. These hot brines would then migrate through a complex, but still poorly defined system of deep faults (probably NE-SW but also NW-SE faults), from the centre of the Rhine Graben to the granite-fractured basement and the Graben’s NW borders. References: Sanjuan B., Millot R., Innocent Ch., Dezayes Ch., Scheiber J., Brach M. (2016) - Main geochemical characteristics of geothermal brines collected from the granite basement in the Upper Rhine Graben and constraints on their deep temperature and circulation. Chemical Geology, 428, 27-47.
|        Topic: EGS - Enhanced Geothermal Systems||Paper Number: 31005|