Evolution of Upper Carboniferous tight sandstone reservoirs in the Ruhr and Lower Saxony basins (NW Germany) of the Central European Variscan foreland

Abstract

Foreland basins have been extensively explored for hydrocarbon resources, geothermal, and gas storage applications, and multidisciplinarily studied to reconstruct orogenic processes. In fact, in the case of the Late Carboniferous Central European Variscan foreland basin, numerous drill cores have been recovered by the hard coal industry and comprehensive geological data sets have been made available by research. These cores and data sets provide a continuous record of the tectono-sedimentary and reservoir evolution over time. For this purpose, we have compiled petrophysical and petrographic literature data of >450 mostly tight (porosity <10%, permeability <1 mD) siliciclastic rock samples from Variscan foreland (Ruhr and Lower Saxony basin, NW Germany) and intramontane basins (Saar-Nahe basin, SW Germany) covering the complete Late Carboniferous stratigraphy. As a result, increases in mean grain size, sorting, detrital quartz, and proportion of metamorphic to sedimentary rock fragment contents, and decreases in detrital feldspar contents along the Westphalian B–C boundary are interpreted as a response to sedimentary recycling and unroofing of the Variscan hinterland due to tectonic uplift in the course of the northwestward propagating Variscan front. Thus, tectonics have a greater influence on sandstone petrography and porosity than the depositional environment. Porosity is mainly controlled by grain size and dissolution porosity. Basin-specific paleogeothermal gradients affect authigenic quartz (up to 25.0%) and chlorite (up to 7.7%) formation. Moreover, quartz cement contents <5% may stabilize the granular framework against mechanical compaction preserving porosity. With increasing quartz cement contents >5%, however, pore space is progressively clogged and the porosity preserving effect diminishes.