| Title | Hydrothermal Alteration of a Hidden Granite in the Geothermal Context the Upper Rhine Graben |
|---|---|
| Authors | Jeanne VIDAL, Marc ULRICH, Hubert WHITECHURCH, Albert GENTER, Jean SCHMITTBUHL, Eléonore DALMAIS, Violaine GIRARD-BERTHET |
| Year | 2016 |
| Conference | Stanford Geothermal Workshop |
| Keywords | hydrothermal alteration, sediment-basement interface, Soultz-sous-Forêts, Rittershoffen, Upper Rhine Graben |
| Abstract | The thermal regime of the Upper Rhine Graben (URG) is characterized by a series of geothermal anomalies near Soultz-sous-Forêts (France), Rittershoffen (France) and Landau (Germany). It is mainly interpreted as the effect of natural brine displacement channelized within hydrothermal convective cells circulating inside a nearly vertical fracture network in the granite basement and in the Triassic fractured sediments above it. This multi-scale fracture system could act as fluid pathways or as permeability barrier in case of fracture sealing by hydrothermal minerals. A petrographic and mineralogical study on alteration products over the first 200 meters of highly altered granite (X-ray powder diffraction scanning electron microscopy or SEM) was performed with more than 30 thin sections from core samples in the Soultz-sous-Forêts exploration well EPS-1. This mineralogical study reveals the presence of veins filled by carbonates, quartz and illite. Mathematic treatments of SEM data suggest that illite is the main uranium host. The fact that there is an overconcentration of radioactive elements at the top basement could influence the thermal profile and thus heat production flux. A laser ablation might be a good perspective to constrain this theory. At less than 10 km than Soultz-sous-Forêts, a first geothermal well has been drilled at Rittershoffen in 2012 in the framework of EGS industrial project. This well reached 2.6 km TVD and the open-hole section is composed by Permo-Triassic sandstone, Paleozoic altered granite and fresh granite. Detailed structural analysis of image logs acquired in 2012, 2013 and 2015 combined with temperature and gamma ray data shows the occurrence of permeable local-scale fractured zones developing into the top of the hydrothermally altered granite. This permeable local-scale fracture zone presents temperature and gamma ray anomalies that evolve through time. The ongoing study on cuttings of GRT-1 might reveal the presence of radioactive elements adsorbed or bear into secondary minerals. |