Towards a Better Knowledge of the Clastic Lower Triassic Reservoirs in the Upper Rhine Graben (France)

Abstract

To develop the sustainable energy in France, we propose a methodology for estimation of the geothermal potential in a limited area. This method is applied to the Upper Rhine Graben, for the resource located in clastic formation of the lower Triassic unit, namely the Buntsandstein. The methodology is based on Muffler & Cataldi works (1978) and the computation of the heat in place and exploitable heat.
A 30 km x 35 km 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 143 km 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 silicoclastic 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 350 GWth.year ± 5%.
For the future, we start a characterization of the Buntsandstein formation in terms of petrophysics to better assess the quality of the geothermal resource and to define exploitation target.