Understanding the interplay of depositional rock types, mineralogy, and diagenesis on reservoir properties of the coal-bearing Langsettian and Duckmantian strata (Bashkirian, Pennsylvanian) of the Ruhr Area, NW Germany

Abstract

Deltaic siltstones and sandstones from the Pennsylvanian (Upper Carboniferous) in the Ruhr Basin are currently being examined for post-mining applications (e.g., geothermal) but are also an important tight-gas reservoir analog in NW Germany. Core material from two wells in the eastern Ruhr Basin, comprising Bashkirian delta deposits of the Langsettian and Duckmantian substages (Westphalian A and B), were studied using petrographic and petrophysical data to assess their reservoir properties and factors controlling these. The samples have low porosities and permeabilities (mean porosity 5.5% well Bork-1 and 5.1% well Haidberg-1; mean permeability 0.41 mD and 0.16 mD, respectively). Grain size and detrital mineralogy are the main factors affecting reservoir properties. The change in mineralogy from litharenites to lithic subarkoses corresponds to an increase in grain size from silt to sandstone and is associated with a general increase in porosity and permeability. Dissolution porosity largely contributes (up to 6%) to measured plug porosity. The dissolution porosity mostly is caused by the break down of detrital K-feldspar and plagioclase grains and affects low present-day feldspar contents (6.0 to 6.8%). Ductile rock fragments, such as shales and phyllites, cause porosity reduction due to facilitated mechanical compaction and are especially present in siltstones (ICOMPACT > 0.99). The study also used SiO₂ and Al₂O₃ contents from XRF analyses as proxies for estimating reservoir properties and distinguishing between sandstones and siltstones. These findings help identify sections with better reservoir properties for potential exploration and production strategies in similar geological settings.

Graphical Abstract