3D Basin modelling of the northern Upper Rhine Graben : insights on geothermal fluid pathways

The area of the Upper Rhine Graben (URG) is known for its geothermal potential. However, the recent interest for lithium co-production from geothermal brines raises questions about the quantification and dynamics of fluid flow paths in the geothermal system at the basin scale. This study aims to better understand the impact of the fluid circulation on the temperature field evolution and on the fluid recharge by performing a 3D thermal basin modeling. The Buntsandstein group sandstones, constituted by early Triassic fluvial to playa-lake deposits are one of the targeted reservoir layers for geothermal and lithium co-production. They overlay permo-carboniferous deposits and the the crystalline variscan basement. The basement inherited structural network plays an important role on heat distribution and flow pathways on the graben shoulders and within the basement. Faults control also the lateral and vertical reservoir connections and fluid mixing, and thus need to be integrated into the burial model. In this study, the focus is made on the northern part of the URG between the cities of Haguenau (France) and Frankfurt (Germany). A new structural model and the geometry of twelve sedimentary layers are implemented in TemisFlow® software. The thermal simulation included both conductive and advective heat transfer. The model integrates the Tertiary rift event from 46 Ma to 23 Ma, by coupling the lithosphere with the depositional evolution. The influence of permeability heterogeneity in the crystalline basement, the role of the main graben border faults, and some selected internal faults on the fluid flow were also investigated. The model is calibrated with the available temperature measurement data, vitrinite reflectance data and temperature maps at different depths or horizons. As a result, the simulations show that the thermal structure of the Eastern part of the URG is mainly controlled by conductive heat transfer, and directly related to the burial. Modeling outputs also highlight the impact of the basement heterogeneity on hydrothermal circulation and the temperature field of the Western part of the URG.