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 the
entire northern Alpine Foreland and hosts most of Switzerland’s population, cities, and
infrastructure. Molasse sedimentary assemblages are composed of mixed continental and
marine siliciclastic units subdivided stratigraphically into Lower Marine Molasse, Lower
Freshwarter Molasse , Upper Marine Molasse and Upper Freshwarter Molasse . The
Molasse has been extensively used for road and tunnel construction and is a preferential
target for major underground infrastructures, such as CERN’s Future Circular Collider. In
particular, the presumed lateral continuity of sand bodies with good reservoir qualities makes
the Lower Freshwater Molasse (LFM) a potential target for geo-energy storage, such as
heat, and anthropogenic CO₂ sequestration. The success of these projects depends on
detailed rock characterisation, from a thorough understanding of the tectono-stratigraphic
evolution of sedimentary units at basin scale down to sediment composition and reservoir
property distribution. However, understanding and predicting fluid flow in Molasse units is not
straightforward, due to the occurrence of subsurface heterogeneities in the form of variable
geometry and architecture of depositional elements, variable textural and fabric properties of
sediments, 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 in
this study to assess reservoir heterogeneities and structural complexities. This study
integrates multiple datasets, including borehole records (Weiach, Bassersdorf, Rheinau, and
four QHAB wells), outcrops (including the Fisibach Quarry) and 2D seismic datasets with a
focus on the Lower Freshwarter Molasse (LFM) in north-eastern Switzerland. The study
demonstrates how the distribution, dimensions, and stacking patterns of sedimentary
architectural elements vary abruptly across small spatial scale, and how these
heterogeneities can be explained by the original complex pattern of fluvial depositional
environments. The results also emphasize the necessity of bridging regional depositional
models with site-specific characterization to reduce uncertainties in rock-property prediction
and to optimize project planning in complex, heterogeneous fluvio-lacustrine deposits of the
LFM.