Linking geothermal reservoir and lithium distribution: A handheld spectroscopy approach on deep wells in the Upper Rhine Graben

The Upper Rhine Graben (URG) hosts a deep geothermal system characterized by fluid circulation within fractured reservoirs at the sedimentary-basement interface. This study investigates cuttings from two deep geothermal wells in the Strasbourg area to assess how reservoir heterogeneity and hydrothermal circulation influence alteration and lithium distribution. Cuttings collected below 3.8 km true vertical depth cover lithologies from the Buntsandstein sandstones to the Carboniferous granitic basement. Reservoir characterization was performed using handheld SWIR (Short-Wave Infrared) and LIBS (Laser-Induced Breakdown Spectroscopy) spectrometers as rapid screening tools, complemented by laboratory analyses, to characterise alteration and lithium zonation at the well scale. The results reveal strong heterogeneity in fracture-related alteration patterns and lithium-enriched fluid circulations in the reservoir. Lithium-enriched geothermal brines, with concentrations up to ∼200 ppm, are interpreted to circulate preferentially along fault and fracture networks, promoting sustained fluid–rock interactions. Variations in lithium content correlate with distinct alteration facies, highlighting the role of clay minerals and carbonates during hydrothermal circulation. These findings support a conceptual geothermal model in which structurally controlled fluid flow governs lithium-bearing brine circulation in the URG, with implications for geothermal projects targeting the co-extraction of heat and lithium.