Clastic Reservoirs in the Rhine Graben: Geothermal Potential of the Triassic Sandstones Based on Seismic Profiles and Deep Boreholes

Abstract

To develop the sustainable energy in France, new geothermal exploration projects have been launched in clastic reservoir of the Upper Rhine Graben. In this framework, a 30km x 32km area located between Strasbourg and Obernai has been investigated. At the scale of the Rhine Graben, this detailed area corresponds to a local geothermal anomaly and is characterized by a temperature of 100?C at 1500 m depth. The Lower Triassic layers, namely Buntsandstein formation, made of clastic sandstones represent the deep seated formations of the Rhine Graben and thus, potential geothermal reservoirs. 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 modelling sedimentary formations. According to the modelling results, different reservoir volumes have been computed which impacts the estimation of the geothermal potential. Temperature conditions derived from BHT 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. However, the orders of magnitude of the flow rate (40l/min to 300l/min in the Buntsandstein) in the boreholes are rather low suggesting a low permeability at depth. But, numerous faults cross cut the Buntsandstein, which constitute a fractured reservoir. Consequently, flow rate could be very different from a borehole to another. Moreover, this type of reservoir could be stimulated. It is obvious that a good knowledge of the geology of the reservoir seriously impacts the assessment of geothermal potential, in terms of reservoir volume and reservoir type. In the investigated area, the volume of the Buntsandstein reservoir is about 300km3 and the heat potential is around 300GW/year ? 10%.