Reservoir characterization of the coal-bearing Upper Carboniferous clastic succession, Ruhr area, Germany

Due to the closure of coal mining in the northwestern German coalfields, active mine water drainage becomes technically redundant. As a result, the rising mine water table affects the subsurface stress conditions and may induce heterogeneous ground movements of fault blocks. Petrophysical properties of the Upper Carboniferous (Westphalian A and B) rocks are crucial to understand subsurface behavior during mine water rise. As a part of the interdisciplinary FloodRisk project, we present a petrophysical and petrographical characterization of Westphalian A and B drill cores from the Ruhr area. Based on fining-upward cycles consisting of basal sandstones (medium-grained, planar laminated) followed by siltstones with intercalated lower (planar/wavy laminated to bioturbated/rooted) mudstones and coal seams at the top, that are locally overlain by clayey deposits, the rock succession was interpreted as fluvio-deltaic facies association. 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. Petrographic analyses indicate that sandstone cementation is dominated by ferroan carbonate (mostly siderite and Fe-calcite) and quartz overgrowth in the pore space. Locally, veins are cemented by ferroan calcite or sulfides. Feldspar-rich rock fragments are often replaced by kaolinite, reducing the secondary porosity. The outcome of this investigation will be integrated into an interdisciplinary model that involves geomechanical, geodetic and geophysical data in order to understand subsurface flow. Furthermore, data can be used to consider the fluid distribution for potential geothermal energy use.