Geological storage of CO2 in heterogenous Lower Freshwarter Molasse successions of NE Switzerland: Uncertainties and Opportunities.

The Swiss Molasse Plateau is the most prominent near-surface geological body across theentire northern Alpine Foreland and hosts most of Switzerland’s population, cities, andinfrastructure. Molasse sedimentary assemblages are composed of mixed continental andmarine siliciclastic units subdivided stratigraphically into Lower Marine Molasse, LowerFreshwarter Molasse , Upper Marine Molasse and Upper Freshwarter Molasse . TheMolasse has been extensively used for road and tunnel construction and is a preferentialtarget for major underground infrastructures, such as CERN’s Future Circular Collider. Inparticular, the presumed lateral continuity of sand bodies with good reservoir qualities makesthe Lower Freshwater Molasse (LFM) a potential target for geo-energy storage, such asheat, and anthropogenic CO2 sequestration. The success of these projects depends ondetailed rock characterisation, from a thorough understanding of the tectono-stratigraphicevolution of sedimentary units at basin scale down to sediment composition and reservoirproperty distribution. However, understanding and predicting fluid flow in Molasse units is notstraightforward, due to the occurrence of subsurface heterogeneities in the form of variablegeometry and architecture of depositional elements, variable textural and fabric properties ofsediments, and the anisotropy of key reservoir parameters (e.g. porosity, permeability).Therefore, a comprehensive analysis of borehole records, lithofacies, facies associations,petrophysical analysis and interpretation of 2D seismic (where possible) was performed inthis study to assess reservoir heterogeneities and structural complexities. This studyintegrates multiple datasets, including borehole records (Weiach, Bassersdorf, Rheinau, andfour QHAB wells), outcrops (including the Fisibach Quarry) and 2D seismic datasets with afocus on the Lower Freshwarter Molasse (LFM) in north-eastern Switzerland. The studydemonstrates how the distribution, dimensions, and stacking patterns of sedimentaryarchitectural elements vary abruptly across small spatial scale, and how theseheterogeneities can be explained by the original complex pattern of fluvial depositionalenvironments. The results also emphasize the necessity of bridging regional depositionalmodels with site-specific characterization to reduce uncertainties in rock-property predictionand to optimize project planning in complex, heterogeneous fluvio-lacustrine deposits of theLFM.