Major geochemical characteristics of geothermal brines from the Upper Rhine Graben granitic basement with constraints on temperature and circulation

This paper is the first to present the main geochemical characteristics of the native brines collected from all the geothermal wells penetrating the granite basement underlying the sedimentary cover, in the Upper Rhine Graben. These deep wells (from 2580 to 5000m) were only drilled in four geotherma...

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Bibliographic Details
Published in:Chemical geology 2016-06, Vol.428, p.27-47
Main Authors: Sanjuan, B., Millot, R., Innocent, Ch, Dezayes, Ch, Scheiber, J., Brach, M.
Format: Article
Language:English
Subjects:
Brines
Chemical composition
Earth Sciences
Fluid circulation
Fluid dynamics
Fluid flow
Fluids
Gas
Geochemistry
Geothermal
Geothermal fluid
Geothermometer
Granite
Granite basement
Oligocene
Reservoirs
Sciences of the Universe
Triassic
Upper Rhine Graben
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Summary:This paper is the first to present the main geochemical characteristics of the native brines collected from all the geothermal wells penetrating the granite basement underlying the sedimentary cover, in the Upper Rhine Graben. These deep wells (from 2580 to 5000m) were only drilled in four geothermal sites (Soultz-sous-Forêts and Rittershoffen in France; Landau and Insheim in Germany). The Na–Cl geothermal brine samples collected from the granite returned TDS values ranging from 99 to 107g/l with pH values close to 5, along with Cl and Br concentrations and δD, δ18O and δ34S values that indicate a multiple origin with mixing between primary brine formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and dilute meteoric water, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to the Oligocene. Chemical, isotopic and gas geothermometers indicate concordant reservoir temperatures close to 225±25°C for all the fluids, even though the maximum temperature so far measured on site is 200°C. An exhaustive literature review has indicated that only the geothermal brine from the deep Cronenbourg well (2870m) ending in the Buntsandstein has similar chemical and isotopic compositions (apart from Br and Ca) to the fluids from the granite, with an identical estimation of reservoir temperature from geothermometry. Geothermal brine from the deep Bruchsal well (2540m), drilled down to the junction of the Buntsandstein and the Saxo-Thuringian formations, has a higher TDS value (120–125g/l) with its chemical and isotopic compositions giving a lower estimation of reservoir temperature (190±25°C). By contrast, geothermal brine from the Bühl well (2655m) ending in the Buntsandstein has an even higher TDS value of about 201g/l and a lower temperature-at-depth estimation of 110±25°C, close to the temperature measured on site (115°C). The above results indicate that the geothermal fluids collected from the granite probably originate from Triassic sedimentary formations located at great depth (≥4km) with temperatures close to 225±25°C in the centre of the Rhine Graben, but that their different TDS and Cl/Br values reflect the presence of several distinct geothermal reservoirs. Many discrepancies due to high-temperature water-rock interactions are revealed on comparing the chemical and isotopic compositions of the hot brines with those of cooler brines from Bühl and the Landa
ISSN:0009-2541
1872-6836
DOI:10.1016/j.chemgeo.2016.02.021