| Title | TRACE: Detection of Permeable Deep-Reaching Fault Zone Sections in the Upper Rhine Graben, Germany, During Low-Budget Isotope-Geochemical Surface Exploration |
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| Authors | Kraml, M; Jodocy, M; Reinecker, J; Leible, D; Freundt, F; Al Najem, S; Schmidt, G; Aeschbach, W; Isenbeck-Schroeter, M |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Isotope geochemistry, fault zone permeability, Upper Rhine Graben |
| Abstract | Due to the apparent lack of surface manifestations (blind resources), current methods of geothermal surface exploration in Germany rely on expensive geophysical methods (mainly 3D reflection seismic surveys) to identify the geometry of hydrogeothermal low-temperature aquifers and permeable fault zones in depth. However, geophysics alone do not allow for an estimation of present-day’s fault permeability nor provide a characterization of the chemical properties of the local deep aquifer fluid. Both factors are important for optimising siting of geothermal wells to obtain maximum flow rates and sustainable operation of a binary power plant. The TRACE project, funded by the Federal Ministry for Economic Affairs and Energy (funding code FKZ: 0325390), presents a low-cost strategy for characteri-zing deep hydrogeothermal reservoirs using a combi-nation of methods from hydrogeochemistry and isotope geochemistry on fluid samples mainly taken from shallow groundwater wells. The main goal is to confine the area of interest for further geophysical investigation, as already done in the exploration of high-temperature resources around the world. For this purpose naturally occurring geochemical and isotopic tracers like noble gases, and radiogenic isotopes of strontium and lithium are investigated. The Upper Rhine Graben was chosen to test the multi-tracer method due to its well-studied geology and some significant pre-existing geophysical exploration data which allow for validation of the study’s findings. The project’s objective was to identify the most useful tracers of deep geothermal fluid circulation, which consecutively can be applied to other regions with less prior information. The new surface exploration data from the Northern Upper Rhine Graben close to Groß-Gerau, Germany, show promising results, indicating fault sections of increased permeability where elevated helium and strontium isotope ratios coincide with specific geochemical characteristics, fault traces and a previously known saltwater anomaly in the shallow groundwater. These findings are in accordance with independent geomechanical modelling. The modelling results display zones of enhanced structural permeability for upwelling thermal fluids as indicated by an enhanced slip and dilation tendency in the same section of the fault characterized by enhanced mantle helium and chloride concentrations. The most likely reason for deep fluid pathways are (i) the mantle shear zone (detected by refraction seismics) persisting through the lower crust and (ii) diatreme pipes etc. to channel mantle fluids to a level of enhanced structural permeability in the upper crust. |