Structural and Geophysical Characterisation of the Crystalline Basement in the Northern Upper Rhine Graben

In the northern Upper Rhine Graben (URG), the crystalline basement constitutes an attractive target for deep geothermal exploitation due to the favourable reservoir temperatures and abundance of natural fractures and large-scale faults. Consequently, especially the upper, hydrothermally altered part is already successfully used for heat and power generation at several locations (Insheim, Landau, Rittershoffen and Soultz-sous-Forêts). Nevertheless, because of the small number of very deep boreholes drilled into the crystalline basement, little is known about its structure and composition. An interdisciplinary multi-scale approach was applied to gain new insights into the properties of the crystalline crust. By building on existing geological models of the URG, a detailed 3D model of the crystalline basement was developed. Additional information was provided by high-resolution gravity and magnetic data, which served as input for a stochastic joint inversion. Inverted density and susceptibility models allowed to identify the predominant rock types below the sedimentary cover. The Tromm granite in the southern Odenwald was chosen as an outcrop analogue to further analyse the hydraulic properties of the crystalline reservoir. By examining the lineaments on the regional scale and the fractures in a total of 5 outcrops, statistical parameters describing the fracture network were extracted. These were then used to create discrete fracture network (DFN) models, in order to calculate the equivalent porous media permeabilities of the bedrock at reservoir depth. In addition, gravity and radon measurements were carried out, which enabled more precise localisation of naturally permeable fault zones. The combination of structural geological and geophysical methods results in a more advanced characterisation of the crystalline basement, that can in future studies be used for more realistic potential assessments and a reduction of exploration risks for geothermal projects.