Seismic expression of paleokarst morphologies and associated Siderolithic infill in the Geneva Basin, Switzerland: Implications for geothermal exploration

The Geneva Basin is witnessing intense geothermal exploration campaigns arising from the Cantonal and Swiss federal energy transition strategies towards an ultimate net-zero future by 2050. However, a major challenge to successful geothermal exploration in the basin are uncertainties regarding the tectonostratigraphic framework of the basin, especially in relation to subsurface morphologies of significance to geothermal prospectivity. This was demonstrated when the newly drilled GEo-02 geothermal exploration borehole surprisingly penetrated a c. 158 m Siderolithic reservoir unit known to infill paleokarst structures in the Molasse Basin. Therefore, we aim to understand the seismic character diagnostic of these paleokarst features and their infill in subsurface geophysical dataset. This was achieved by interpreting industrial 2D seismic reflection and borehole dataset integrated with satellite imagery and outcrop analogues. The seismic dataset reveals these paleokarsts as depressions along the Cretaceous-Paleogene transition (K-Pg) and is characterized by three distinct end-member morphologies: (1) fault-controlled type, (2) doline type associated with dissolution/collapse and (3) complex type, atop the Lower Cretaceous carbonates. However, the karst infill is characterized by different seismic facies exhibiting either a higher or lower seismic reflectivity depending on the karst-end member type. This is related to differences in the acoustic properties of the karst infill, overlying lower molasse and the Lower Cretaceous carbonates hosting the karst. The GEo-02 borehole reveals that the Siderolithic infill consists of a variety of lithologies including quartzrich sandstone, polygenic breccias and conglomerates, terra rossa and lateritic deposits. The evolution of the paleokarst features in the study area is influenced by the structural re-organization of the Molasse Basin in relation to the Alpine orogeny. This resulted in its emergence and change in base-level promoting the dissolution of the carbonate substratum by meteoritic water subsequently generating the paleokarst morphologies. Considering the regional geothermal gradient, the foredeep of the Geneva Basin that exhibit subsurface manifestations of this Siderolithic reservoirs may present new opportunities for an underground thermal energy storage. However, the presence of karstified intervals may pose geohazard risks to geothermal exploration during drilling operations targeting deeper structures in the basin.