Upper Rhine Graben: Deciphering a geothermal fluid system and its raw material potential

In the context of the energy and mobility transition, operators of geothermal power plants and associated industries spotlight the potential to combine the production of sustainable energy with the extraction of strategic raw materials (SRM) (e.g., lithium) from geothermal fluids. Brine mining of SRM requires a comprehensive understanding of the related geothermal fluid system.

In southwest Germany, the Upper Rhine Graben (URG) represents a reservoir with great geothermal and SRM potential (thermal gradient up to 120°C/km; Li ~170mg/L). Data about the reservoir rocks and the thermal fluid at different depths is provided by ~1300 oil and geothermal wells as well as thermal water wells. Chemical data of sampled fluids indicate variations in the trace element distribution that are related to the Cl/Br-ratio, and trends of stable isotope systematics of the water.

The results show that in the northern URG evaporites are dissolved (Cl/Br>1200), while in the central URG the thermal waters have almost seawater composition (Cl/Br=290). Independent of the Cl/Br ratio, Li concentrations in the thermal waters vary between 3-43 mg/L. At nearly the same Cl/Br ratio of ~300 the geothermal fluids have ~170 mg/L Li. Stable isotope systematics imply that all thermal fluids have a meteoric component. The results indicate a modification of the fluid chemistry through time and depth, resulting from fluid mixing and water-rock interaction with various reservoir rocks. To investigate the fluid evolution, additional element tracers to determine the origin and raw material potential of Li and other trace elements, are established.