Abstract
Spring water geochemistry is applied here to evaluate the geothermal potential in Rhenohercynian fold and thrust belt around the deepest borehole in Belgium (Havelange borehole: 5648 m MD). Fifty springs and (few) wells around Havelange borehole were chosen according to a multicriteria approach including the hydrothermal source of “Chaudfontaine” (T ≈ 36 °C) taken as a reference for the area. The waters sampled, except Chaudfontaine present an in-situ T range of 3.66–14.04 °C (mean 9.83 °C) and a TDS (dry residue) salinity range of 46–498 mg/L. The processing methods applied to the results are: hierarchical clustering, Piper and Stiff diagrams, TIS, heat map, boxplots, and geothermometry. Seven clusters are found and allow us to define three main water types. The first type, locally called “pouhon”, is rich in Fe and Mn. The second type contains an interesting concentration of the geothermal indicators: Li, Sr, Rb. Chaudfontaine and Moressée (≈5 km East from the borehole) belong to this group. This last locality is identified as a geothermal target for further investigations. The third group represents superficial waters with frequently high NO3 concentration. The application of conventional geothermometers in this context indicates very different reservoir temperatures. The field of applications of these geothermometers need to be review in these geological conditions.
Highlights
Geothermal exploration, as any subsurface resource evaluation, requires a multidisciplinary approach and usually follows a downscaling approach starting from a regional scale toward a shortlist of potential sites for more detailed investigations [1,2]
It allows us to investigate the geothermal potential of an unconventional reservoir, which consists of fracture tight metamorphic units in a fossil fold-and-thrust belt and in a zone where the presence of post-Variscan extensional fractures is absent or very-limited
The geochemical composition analysis of spring water is applied as a surface tool for deep geothermal potential exploration
Summary
Geothermal exploration, as any subsurface resource evaluation, requires a multidisciplinary approach (e.g., geological mapping, geophysics, geochemistry) and usually follows a downscaling approach starting from a regional scale toward a shortlist of potential sites for more detailed investigations [1,2] These sites usually show surface indicators of the presence of a geothermal resource at depth. These indicators can even take the shape of biological manifestations as bacterial mats (e.g., [3,4,5], presence of bacterial community on travertine deposits [6] or even saline water tolerant plant and animal concentrations (e.g., eels) [7]) Even if such indicators are interesting clues in evaluating the geothermal fields, not all of them show obvious indicators at ground surface of their presence. A common denominator of these blind geothermal systems is the presence of a cap formation, which hides many indicators of the presence of the geothermal resource
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