| Title | Reservoir Characterization by Sr Isotopes – A Case Study from the Upper Rhine Valley, Germany |
|---|---|
| Authors | Bettina A. WIEGAND, Martin SAUTER |
| Year | 2016 |
| Conference | Stanford Geothermal Workshop |
| Keywords | geothermal reservoir, Sr isotopes, water-rock interaction, Upper Rhine Valley |
| Abstract | As part of the European Cenozoic rift system, the Upper Rhine Valley located in southern Germany forms an active tectonic graben structure with high geothermal potential. The geothermal reservoir is composed of sedimentary rocks, mainly of Permian to Tertiary age, deposited on Variscan basement. Heat anomalies in the Upper Rhine Valley are produced by upwelling of deep groundwater that provides the source for shallow and deep geothermal energy exploitation. Discharge of natural thermal springs is related to the occurrence of deep-reaching fault zones. Reservoir temperatures reach 150 °C at depths of 3000 m. The chemical composition of thermal waters in the Upper Rhine Valley is heterogeneous as a result of a variety of producing aquifers in the Permian, Triassic, and Tertiary sandstones, and Triassic and Jurassic limestones. Water compositions range from calcium and sodium dominated bicarbonate to calcium sulfate, and sodium chloride dominated waters in the deep reservoir. Sr isotopes were employed to characterize the geothermal reservoir and to study the origin and evolution of thermal waters. 87Sr/86Sr ratios of dissolved Sr2+ provide information on the source of solutes as well as the degree of mixing of thermal waters in the reservoir. The results show that the distribution of water types based on major ion concentrations does not necessarily coincide with the interpretation based on Sr isotope data, and therefore suggest a complex pattern of water-rock/mineral interaction and mixing processes in the reservoir. |