To develop the sustainable energy in France, a new geothermal exploration project has been launched in the Upper Rhine Graben for heat or electricity production. This resource is located in clastic formation of the lower Triassic unit, namely the Buntsandstein.

The goal of this study is to make a first assessment of this sedimentary reservoir at two different scales. In a first step, we used public available data (map, borehole data…) to assess the geothermal potential of the Buntsandstein at the Upper Rhine Graben scale in order to delineate the most favourable areas for future exploration and exploitation of the geothermal resource. It appears that the northern part of the Rhine graben constitutes a more favourable area than its southern part. The top of the reservoir located at around 2000m depth is characterized by a temperature of about 150°C and a sandstone thickness around 600m. Based on Muffler & Cataldi works (1978), the computation of the heat in place gives a geothermal potential between 15 and 30GJ/m2.

In a second step, a 30km x 35km area located between Strasbourg and Obernai in France has been investigated. Based on a detailed geological study combining data derived from 13 previous oil boreholes and 143km length of seismic profiles, the main sedimentary interfaces including geological layers and faults have been interpreted between the outcropping Quaternary layers and the deeper parts made of Permo-Triassic formations. From that interpretation, 3D geological models have been yielded based on different hypotheses. These models, constructed with the Geomodeller software developed by BRGM, allow calculating the volume of modelled sedimentary formations. According to the modelling results, different reservoir volumes have been computed which impacts the estimation of the overall geothermal potential. Temperature conditions derived from BHT (Bottom Hole Temperature) data in boreholes reaching the Buntsandstein sandstones, show a high average geothermal gradient (between 50°C/km and 58°C/km), which tends to indicate a significant geothermal potential. In the investigated area, the volume of the Buntsandstein reservoir is about 300km3 and the exploitable heat quantity is around 350GW.year ± 5%.

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