Coupling heat conductivity, lithofacies and reservoir quality of the coal-bearing Upper Carboniferous in the eastern Ruhr Basin, NW Germany

The Ruhr Basin in Germany is one of the most extensively studied 3D rock volumes due to decades of subsurface coal mining. Mine flooding associated with the closure of the coalfields may lead to local stress perturbation but also may enable post-mining geothermal applications. However, important geothermal parameters of the Upper Carboniferous clastics are poorly constrained with respect to flow and thermal properties. To improve the understanding of the subsurface heat flux, a dataset of petrophysical parameters is presented for three drill cores of the Westphalian A and B. The studied core material shows fourth order sequences of coarsening/fining upwards cycles in an overall shallowing upward trend. Up to 17 m thick sandstones are associated with a delta front, mudstones and siltstones with a lower delta plain with up to 4 m thick coal seams with a lower delta plain (wetlands) environment. Low compressional wave velocities of 2886 m/s in sandstones are related to higher porosities of up to 15.6%. Likewise, porosity is a major control of thermal conductivity with up to 5.3 W/(m*K) in sandstones. In less porous samples thermal conductivity depends rather on the mineralogy. The low thermal conductivity (mean 3.0 W/(m*K)) of mud and silt-dominated deposits is due to the presence of organic matter or sheet silicates. Generally, low porosity (mean 5.6%) and permeability (mean 0.1 mD) of the rock matrix indicate that fluid flow during mine flooding and potential future geothermal applications will primarily rely galleries and on permeable faults and fracture systems.