Reservoir Characterization of the Coal-Bearing Upper Carboniferous Clastic Succession, Ruhr area, Germany

With the successive closure of coal mining in the northwestern German coalfields including associated cessation of active mine water drainage, heterogeneous ground movements are related to the rising mine water table. The Ruhr district is one of the world’s best documented subsurface 3D rocks volume. It thus offers to better understand the change of reservoir properties at varying effective stresses in a reservoir-scale. In the scope of the interdisciplinary FloodRisk project, reservoir properties of Upper Carboniferous (Westphalian A and B) drill cores are investigated for selected German mining areas in the eastern Ruhr area as their characterization is crucial to process understanding. The cyclic succession of the clastic Westphalian units was deposited under fluvio-deltaic (paralic) conditions in the late Carboniferous Ruhr Basin. In the Westphalian, the interplay of rapid basin subsidence, sedimentation rate, and glacio-eustatic sea-level changes controlled the sedimentation pattern and promoted paralic coal-forming environments. Drilled cores from three exploration wells in the study area stratigraphically cover the Witten, Bochum, and Essen Formation of the Westphalian A and B. Cyclically occurring sandstones, siltstones, and mudstones were categorized into different delta facies by their lithology and sedimentary structures. In all wells, the Westphalian A and B deposits consist of fining-upwards cycles of delta front (medium-grained planar laminated) sandstones at the basis to siltstones with intercalated lower delta plain (planar/wavy laminated to bioturbated/rooted) mudstones and coal seams at the top, locally overlain by clayey prodelta deposits. Preliminary results from petrophysical measurements on core plugs indicate that most samples are generally tight (mean permeability: 0.26 mD; mean porosity: 6.4%) but reservoir properties vary by grain size and facies. Reservoir properties are higher in sandstones (up to 3 mD; mean porosity: 8%) than in siltstones (permeability: <0.01 mD; mean porosity: 4%) and mudstones (permeability: <0.2; mean porosity: 2%). Similarly, reservoir properties in delta front deposits are higher (up to 3 mD; mean porosity: 7%) compared to lower delta plain (permeability: < 0.02 mD; mean porosity: 6%) and lower delta plain (wetland) deposits (permeability: <0.1 mD; mean porosity: 4%), respectively. Future geological and petrophysical parameters will include fractures and subsurface galleries to better understand subsurface flow and to consider the fluid distribution for potential geothermal energy use.