Creation of Secondary Porosity in Dolostones by Upwelling Basement Water in the Foreland of the Alpine Orogen

Abstract

Creation of secondary dissolution porosity in carbonate rocks during deep burial has the potential to improve reservoir properties for hydrocarbons, gas storage, and geothermal applications. However, the occurrence and mechanisms of such porosity enhancement are controversial. Here, we present compelling evidence for generation of deep burial porosity from the Swiss Molasse Basin, where dissolution of eogenetic anhydrite nodules in dolostones of the Middle Triassic Muschelkalk increased the matrix porosity by up to 15 vol.%. We reconstruct the genesis and evolution of the anhydrite-dissolution porosity based on petrography, porosity determinations, analyses of stable and radiogenic isotopes (δ2H, δ18O, δ34S, and 87Sr/86Sr), fluid inclusion studies, and laser U-Pb geochronology of secondary calcite. The results show that modified meteoric waters derived from the Variscan crystalline basement ascended via basement-cover cross-formational faults into the overlying Muschelkalk, where they dissolved the anhydrite nodules throughout an area of at least 55 km2 at 700-2300 m depth and 40-160°C. Secondary calcite in anhydrite moulds yields Late Eocene to Middle Miocene U-Pb ages, which coincide with the timing of basement uplift in the foreland bulge of the Swiss Alpine Orogen. This uplift provided the hydraulic gradients to drive meteoric water deep into the adjacent Molasse basin. Similar enhancement of reservoir properties can be expected in dolostones in other foreland basins that are bordered by a foreland bulge in which fractured basement rocks are exhumed.