Geothermal potential of karstified Devonian carbonates in NW Germany

The exploration of suitable reservoir rocks for deep geothermal energy in NW Germany focuses on karstified carbonates of Carboniferous and Devonian age. Using a case example from the city of Iserlohn, we demonstrate that these rocks exhibit high thermal yields due to the secondary porosity caused by meteoric (and potentially hypogenic) karstification. Karstified carbonates, if structurally interconnected on a reservoir scale, have the potential to provide significantly higher permeability. We conducted thermo-triaxial cell experiments for depths exceeding 2 km. Our findings indicate that dolostones offer enhanced mechanical integrity and exhibit reduced porosity loss with increasing depth. Visual inspections of µCT scans confirm that karstification results in large, interconnected pores that have been leached. The effective porosity of karstified carbonates reaches 14 %, with measured permeability values in the order of 10⁻¹⁶m². In terms of porosity and permeability, karstified units can outperform non-karstified ones by up to two orders of magnitude. We conducted heat-in-place modelling using petrophysical laboratory data as input parameters for Monte Carlo simulations. An initial map was created to illustrate the potential geothermal energy stored and to identify areas with the highest energy yield. The estimated potential energy stored in the subsurface is greatest in the southern part of the Iserlohn study area, where the thickness of the Devonian carbonates is at its maximum. This deeper occurrence in the subsurface leads to a significant increase in reservoir temperature. The modelling results indicate a potential energy range for the entire distribution of Devonian carbonates between 87 TJ (P90) and 110 TJ (P10).