Bleaching Mechanisms and Reservoir Quality in Buntsandstein Sandstones: Limitations of Outcrop Analogue Use

Understanding and predicting heterogeneous reservoir properties of Lower Triassic sandstones is crucial for sustainable resource management. This study investigates the petrographical and petrophysical characteristics of red and bleached sandstones to address bleaching mechanisms and their implications on reservoir quality in the context of gas storage and geothermal exploration. Fieldwork and laboratory observations reveal three distinct bleaching patterns, i.e., stratiform bleached sandstones (SBS), lamina-bound bleached sandstones (LBBS), and patchy bleached sandstones (PBS). Each represents very different diagenetic processes across geological timescales. Diagenetic cement dissolution is much more pronounced in outcrop samples than in their subsurface equivalents. Multiple analytical techniques provide a comprehensive understanding of rock and pore types, and overall pore network architecture, enabling reservoir characterisation from the microscale to the macroscale. While bleaching processes intrinsically improve porosity, the associated (silica) overgrowths negatively impact pore interconnectivity. Telodiagenetic dissolution significantly enhances the petrophysical properties in outcrop samples. Therefore, it is essential to correct these modifications for the accurate prediction of subsurface reservoir properties. By improving the reliability of subsurface reservoir assessment, these findings can support geothermal energy development and underground gas storage in Buntsandstein reservoirs and contribute to the advancement of the UN Sustainable Development Goals.