Long-term evolution of a carbonate reservoir submitted to fresh, saline and thermal waters interactions – Jurassic carbonates in the coastal area of the Gulf of Lion margin (southern France) ; Évolution long terme d’un réservoir carbonaté soumis à des interactions entre eaux douces, salines et thermales : les carbonates du Jurassique de la zone côtière de la marge du Golfe du Lion (sud de la France)

International audience ; Securing and managing underground water resources requires a good knowledge of the structure, texture and connections of the reservoir, in order to develop realistic and reliable hydrogeological models. On the coastline of the Gulf of Lion Margin (S. France), the Balaruc-les-Bains deep karst reservoir is subjected to interactions between fresh, marine and deep thermal waters, respectively. Water resource usage for drinking, spa resort, and fish-farming raises important economic and social issues. These were addressed by an integrated research program, involving drilling of an exploratory borehole across the Jurassic carbonate reservoir. This contribution analyses the 750 m cores, in order to (i) characterise the architecture and evolution of the karst reservoir and (ii) investigate the paleo-fluids circulations, witnessed by calcite and dolomite mineralization in the fractures, karst cavities, and as cement of tectonic beccia. The structure of the reservoir is characterised by the superposition of several aquifers separated by marly intervals. At shallow level, the initial grainstone is incompletely dolomitized in metre-thick intervals, while limestone in the 210-340 m interval was completely dolomitized at an early stage. Dolomite has been subjected to penetrative extensional cataclastic deformation, while the preserved limestone is affected by normal faulting, resulting from NNE-SSW extension. Distinct types of karsts have been documented, from the top of the reservoir (paleo-lapiaz filled with Burdigalian marine marls), down to 500 m depth (paleoendokarst filled with continental silts). The upper reservoir (75-150 m) is intensely karstified, and includes 0.1 to 1 m-wide cavities, where present day water fluxes are documented. Analyses of calcite and dolomite crystallisation under natural light and cathodoluminescence indicate precipitation from distinct fluids: formation water in chemical equilibrium with the host rock, water rich in oxides and hydroxides, ascending hydrothermal fluid ...