Faults in Enhanced Geothermal Systems : Soultz-sous-Forêts Site and its Analogue in the Upper Rhine Graben

Deep geothermal exploits crystalline reservoirs in which natural brines circulate in fault and fracture networks. The Soultz-sous-Forêts’ Enhanced Geothermal System (Soultz EGS) is located in France, in the Upper Rhine Graben. The 4000m to 5000m deep reservoir is accessed only by drilling. Rock chips recovered during drilling operation were thoroughly analysed for signs of hydrothermal alteration which result from water-rock interaction and hence fluid circulation, either fossil or still active. These results are correlated with well log data to identify present-day fluid flow. However, rock chips provide only scarce information on fluid pathways. To improve the knowledge about fluid flow within fractured and faulted zones, an analogue was chosen in the former Schauinsland’s silver mine on the eastern border of the URG, in order to remain in the same geological setting. This coupled approach (EGS site and analogue) provides a set of data that can be integrated in numerical models of fossil or present-day fluid circulation.