Reservoir quality controls in the Lower Buntsandstein and their relation to detrital grain size, URG, SW-Germany

Reservoir quality of the Lower Triassic Buntsandstein in the Upper Rhine Graben (URG) is currently being explored to better assess the possibility of geothermal production and associated lithium brine production. Previous studies highlight a generally positive effect of increased detrital grain size and blocky cement content on elevated porosity and permeability, while outlining a generally negative influence of compactional processes, which are enhanced by tangential illite grain coatings and to a lesser extent by ductile rock fragments. This overall assessment can be supported by the studied samples from a research well (Kraichgau-1002) on the western graben shoulder, penetrating the coarser-grained Lower Buntsandstein at present-day depths between 500 and 710 m. The overall porosity and permeability is low (2.9 to 16.0 %, <0.0001 to 7.8 mD) in the studied core section. Tangential illite grain coatings enhance the effect of chemical compaction, whereas blocky cements (quartz, K-feldspar, carbonates, anhydrite) stabilize the grain framework against mechanical compaction. In samples with the same porosity in this study, higher permeability is found in samples with a coarser grain size. However, when also including other available Buntsandstein core samples, the overall correlation between permeability and grain size is poor (R²=0.05). Furthermore, a reported correlation between clay mineral grain coating characteristics (grain-to-IGV and grain-to-grain coating coverage) and detrital grain size could not be established (R²<0.01 and R²=0.19, respectively) for neither the studied samples, nor the combined Buntsandstein sample series from core material. Therefore, the detrital grain size by itself is an unsuitable criterion to assess Buntsandstein reservoir quality in the marginal basin facies. However, the extension of available datasets, which assess reservoir quality controls in the Buntsandstein in the area surrounding the URG highlights, that the previously defined reservoir quality controls are affecting larger areas of the marginal facies of the Buntsandstein present in NE France and SW Germany.