Within the framework of the European Hot Rock (HDR) Project, the natural fracture geometry of the deep granitic basement in the Upper Rhine Graben has been studied at Soultz-sous-Forets (France). The porphyritic granite is characterized by hydrothermally altered and fractured sections. Detailed structural analysis of continuous core samples and borehole wall imagery indicates that fracture zones constitute a complex network of fault segments dominated by N-S trends in accordance with the geometry of the local Soultz horst structure. Some of these fault segments still carry hydrothermal brines. The attitude of the faults is closely related to the rifting of the graben during the Tertiary. The fractures strike in a direction nearly parallel to the maximum horizontal stress. In this favorable situation, hydraulic injection will tend to reactivate natural fractures at low pressures