Coupling heat conductivity and lithofacies 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 subsurface coal mining over the past centuries. With the final closure of the last coalfield in 2018, mine flooding was initiated and may result in induced ground movements. However, abandoned coal mines also provide the possibility for post-mining geothermal utilisation. To improve the understanding of rock properties with respect to flow properties and heat flux in the Ruhr Basin, three drill cores of clastic sedimentary rocks of the Westphalian A (Langsettian) and Westphalian B (Duckmantian) were characterised and the facies analysed. The studied 270 m core material show 4th order sequences of coarsening- and fining-upward cycles in an overall shallowing upward trend. Up to 17 m thick sandstone beds are associated with the delta front, mudstones and siltstones are related to the lower delta plain and up to 4 m thick coal seams linked to wetlands of the lower delta plain. Rock properties show low compressional velocities with a minimum of 2,886 m/s in sandstones of the delta front, which are negatively correlated with porosities of up to 15.6%. Likewise, porosity is a major control on thermal conductivity with a minimum of 2.3 W/(m×K) for high porosity sandstones. For less porous samples, thermal conductivity becomes less dependent on porosity but more strongly on mineralogy. Comparing our results with accessible thermal conductivity data from North Rhine-Westphalia (NRW), the high values of up to 5.3 W/(m×K) seem promising for potential geothermal applications. Furthermore, the results of this study imply that the classification based on facies associations is less suitable than the classification based on grain sizes in order to assess the geothermal potential. Overall, the Upper Carboniferous (Pennsylvanian) samples of this study show low porosity (mean 5.6%) and permeability (mean 0.1 mD). Thus fluid flow during mine flooding and potential future geothermal applications will primarily rely on permeable faults and fracture systems around the abandoned mine galleries as well as the galleries themselves. [ABSTRACT FROM AUTHOR]