Clay mineral quantification in the Upper Cretaceous Emscher Formation – evaluating a potential hydraulic barrier during mine water rebound in the Ruhr District

The sealing potential of geological formations (“cap rocks”) plays an important role in long-term industry projects associated with mine water rebound, carbon sequestration and nuclear waste disposal. It is an important criterion in risk management and monitoring concepts. Clay content and mineralogy were used as screening parameters in order to estimate permeability in the Upper Cretaceous Emscher Formation. The Emscher Formation consists of monotonous grey to greenish alternating clay-, silt- and marlstones with a high amount of micrite. Distribution of clays was quantified on core and cuttings material using combined X-ray diffraction and Rietveld refinement. An important task was to evaluate content and changes in clay mineralogy with respect to expandable clays. The aim is to assess its self-sealing potential. Core sections and cuttings were retrieved from groundwater monitoring wells drilled in the northern part of the Ruhr District. The aim of these wells is to record hydraulic potential in the overburden section during mine water rebound in the Ruhr District. Classical lithological and sedimentological analyses of core section were conducted. Bulk geochemistry included total inorganic and organic carbon measurements. Elemental analyses on selected samples was determined using X-ray fluorescence spectroscopy. Clay mineral identification and quantification was performed on (a) bulk rock, (b) <2µm random powder and (c) <2µm oriented samples. For expandable clays, ethylene glycol and heating to 550°C was applied.