Rock characteristics and reservoir properties of Upper Carboniferous (Stephanian A–B) tight siliciclastic rocks from the Saar–Nahe basin (SW Germany)

Abstract

The environmental consequences of mine flooding in the Saar hard coal district, post-mining re-use concepts in the course of the energy transition, and the potential of coalbed methane production require an understanding of subsurface rock properties on the microscale. In this study, mineralogy, microtexture, microstructure, porosity, permeability, and geochemistry of an Upper Carboniferous (Stephanian A–B) drill core recovered in the Saar–Nahe basin are quantified. Based on these data, the diagenetic history and reservoir quality are analyzed regarding mine flooding and coalbed methane potential. The feldspar-poor and igneous rock fragment-free siliciclastic rock succession shows multiple fining upward sequences deposited in a fluvial environment during the pre-volcanic syn-rift phase of the Variscan intramontane Saar–Nahe basin. Intercalated small-scale coarsening upward sequences are related to the floodplain where near-surface soft-sediment deformation and paleosol formation took place. Porosity (< 7%) of the tight siliciclastic rocks is mainly controlled by an interplay of authigenic microporous kaolinite, dissolution porosity, and quartz cement, whereas permeability (< 0.05 mD) shows no systematic variation with petrography. During burial, quartz cements preserved porosity by stabilizing the granular framework against mechanical compaction, while phyllosilicates were ductilely deformed reducing reservoir quality. Relative phyllosilicates and quartz contents and mean grain size are reliably inferred from SiO2/Al2O3 ratios (1.8–28.8), Ba (0.0108–0.0653 wt%), Rb (0.0024–0.0181 wt%), and Sr (0.0013–0.0086 wt%) concentrations measured with a portable x-ray fluorescence analyzer. Regarding coalbed methane production and mine flooding, sealing of cleats and heterogeneous subsurface rock properties due to dynamically changing depositional settings during the Late Carboniferous need to be considered. Graphical abstract