Overview of naturally permeable fractured reservoirs in the central and southern Upper Rhine Graben: Insights from geothermal wells

Since the 1980's, more than 15 geothermal wells have been drilled in the Upper Rhine Graben (URG), representing more than 60 km of drill length. Although some early concepts were related to purely matrix-porosity reservoirs or Hot Dry Rock systems, most projects in the URG are currently exploiting the geothermal resources that are trapped in fracture networks at the base of the sedimentary cover and in the granitic basement. Lessons learnt from the European EGS reference site at Soultz-sous-Forets reveal highest natural permeability in the uppermost altered crystalline basement. Here, we present a compilation of related information to examine a more general validity of this hypothesis for the central URG. In this respect, 15 geothermal wells were analyzed concerning their lithologies, temperature distribution with depth, and their hydraulic yields. Among others, permeable fractures in Triassic sediments were observed among others during drilling operations at Soultz-sous-Forets, Rittershoffen, Cronenbourg (France), Landau, Insheim, Bruchsal and Briihl (Germany). The geothermal wells at Soultz-sous-Forets, Rittershoffen (France), Landau and Insheim (Germany) also intersect well-connected fracture networks in the uppermost altered granitic basement. Permeable fractures are intersected to a depth of 5 km at Soultz-sous-Forets (France) and Basel (Switzerland). The compilation of geologic, hydraulic and thermal data of 15 geothermal wells shows permeability variation among the lithologies with the maximum observed at the top of the hydrothermally altered granite. This higher permeability is likely due to the intense fracture density in the fault core of the fracture zone and the large porous and altered damage zone which allow connection with the reservoir.