Detailed descriptions of the lower-middle Triassic and Permian formations using cores and gamma-rays from the EPS-1 exploration geothermal borehole (Soultz-sous-Forêts, Upper Rhine Graben, France)

In the 1950s, a petroleum well in Soultz-sous-Forêts (no 4589; Upper Rhine Graben, France) reached the Lettenkohle (Triassic) at 830 m depth. During the exploration phase at Soultz-sous-Forêts, the first enhanced geothermal system pilot project in the world, the well was core-deepened up to 2227 m into the granitic basement (then renamed EPS-1). The objectives of this deeper well were (i) to explore the granitic fractured reservoir and (ii) provide a precise geological characterization of the Soultz-sous-Forêts horst. This paper presents the first complete core analysis of the sedimentary formations observed in the EPS-1 cores (from the middle Muschelkalk to the Permian) in conjunction with the gamma-ray log description and field works. The detailed descriptions of the geological formations encountered in the well are presented with photo boards displaying the most characteristic facies of each formation, since the wells are rarely cored, especially in the Muschelkalk. This study also aims to describe the gamma-ray log to provide a strong baseline for future geothermal well exploration in this area (or old well reinterpretations). At this time, it is the only available reference for the complete section: Muschelkalk to Permian sedimentary formations.