Evolution of Reservoir Properties in Buntsandstein Sandstones: A Comparative Wells Analysis from the Upper Rhine Graben

In the context of the energy transition, the characterization of deep geothermal reservoirs represents a major challenge for the development of decarbonized energy sources. The Upper Rhine Graben has a significant geothermal potential and is widely underexploited. Given the significant lithium concentrations recently identified in geothermal brines, understanding reservoir properties and their spatial distribution is crucial to optimize the exploration and exploitation of these resources.

This study focuses on the comparative analysis of reservoir properties in Buntsandstein sandstone from various boreholes in the Upper Rhine Graben. Our approach compares sedimentary lithologies below the basement unconformity from the EPS1 well in eastern France with the Sexau and Heidelberg wells in western Germany. This comparison enables the investigation of the impact of different burial histories on reservoir properties and contrasts the central graben position with its margins. The methodology employed combines several complementary measures. Detailed sedimentological analysis enables the characterization of facies, deposit environments, and their distribution. The petrophysical study of porosity, permeability, density, and thermal conductivity is coupled with diagenesis analysis to understand the evolution of the pore network. Quantitative mineralogy analyses complete the characterization of diagenetic phases. Additionally, LIBS analyses were performed top map lithium distribution and identify Li-bearing minerals within the sample. Spatial mapping of lithium content reveals its distribution patterns and mineralogical associations, providing new insights into the relationships between reservoir properties and lithium occurrence in the geothermal system.

The results show the boreholes at the graben margin exhibit better-preserved reservoir properties with average porosities and permeabilities. In contrast, the EPS1 sedimentological cover shows a significant reduction in these properties due to greater mechanical compaction and more intense cementation. These findings contribute to a better understanding of sandstone reservoir evolution in graben systems and provide valuable insights for subsurface geothermal resources assessment in similar geological contexts. Furthermore, the characterization of lithium distribution, patterns open perspectives for potential co-production of geothermal energy and lithium resources.