Anhydrite-dissolution porosity in the Upper Muschelkalk carbonate aquifer, NE-Switzerland: implications for geothermal energy and geological storage of gas

The Upper Muschelkalk carbonate aquifer within the Swiss Molasse Basin is currently being investigated for its potential for geothermal energy exploitation and geological storage of gas. Porosities of up to 25% are locally observed where early diagenetic (Triassic) anhydrite has been dissolved by groundwater. However, the dimensions and the spatial distribution of this type of cavernous porosity are not well known, as the Basin is underexplored. The present study reconstructs the genesis and evolution of these pores from drill-core studies, thus providing conceptual understanding to support ongoing exploration in the deeper, hotter southern regions of the Basin. The reconstruction is based on petrographic investigations, analyses of stable and radiogenic isotopes and fluid inclusion studies. The results show that the anhydrite dissolved during the Neogene upon influx of chemically modified meteoric runoff from uplifted crystalline domes in the Black Forest area of southern Germany. As this hydrogeological system is spatially restricted to the shallow, northern margin of the aquifer, we conclude that zones of anhydrite-dissolution porosity are unlikely to extend deeper into the Basin. Exploration in the deeper realms of the Basin should therefore target other types of porosity and permeability, e.g. sucrosic dolomite facies and fracture networks. (C) 2017 The Authors. Published by Elsevier B.V.