Reservoir quality and diagenesis of limestones from the Upper Cretaceous (Beckum-Fm.) of the Münsterland Cretaceous Basin

The Upper Cretaceous Campanian limestones from the Ahlen-Fm. (Beckum-Fm. Submember) of the Münsterland Cretaceous Basin in NW Germany are former high porosity limestones, which consist mostly of detrital components. This study focuses on the petrophysical assessment of these limestones in combination with diagenetic studies to understand the potential interaction with rising mine-water as they unconformably overlie Upper Carboniferous coal-bearing strata. Therefore, outcrop analyses were carried out and samples were taken to study the heterogeneity and controlling factors, as well as diagenetic para-sequence in tight limestones. We show that diagenesis, compaction, authigenic cementation and the detrital composition affect petrophysical properties. Mechanical compaction is seen by elliptically deformed calcispheres and foraminifera at the transition to ductile clay laminae, forming compaction bands. Mechanical compaction and early diagenetic precipitation of inter- and intragranular sparry ferroan calcite reduces porosity and permeability. Porosity ranges between 1.0% to 18.7%, permeability between <0.0001 mD to 0.2 mD, and p-wave velocity ranges between 2089 m/s and 5843 m/s. Furthermore, natural fractures are filled by either ferroan calcite and/or strontianite. Thus, the studied lithologies of the Beckum-Fm. can be considered as seals for potential rising mine-water levels. Furthermore, results indicate, that they may not be potential targets for geothermal utilization. However, open fractures formed during exhumation overprinted the rocks which may enhance the reservoir quality by generating potential fluid pathways close to the present day surface.