Geothermal exploitation in the Upper Rhine Graben increasingly targets the interface between the granitic basement and the overlying Buntsandstein unit. Results from deformation experiments are combined with structural assessments to provide reservoir-scale wet and dry strength and elastic modulus profiles for the Buntsandstein at Soultz-sous-Forêts (France). Our analysis finds five zones characterised by low strength and elastic modulus. The strength and elastic modulus of “massive” zones are lower when the rock is wet (i.e. water-saturated), highlighting the importance of performing wet deformation experiments for geothermal rock mass assessments. These data and methods can be used to provide assessments of other geothermal sites within the region to assist prospection, stimulation, and optimisation strategies.