Fault permeability in carbonate-marl multilayers: implications for faulted CO2 storage site assessment

This study investigates the petrophysical and structural properties of faulted marl-carbonate multilayers to better understand their potential to act as barriers or conduits to fluid migration. The research focuses on the potential Smeaheia CO₂ storage site in the Norwegian North Sea, complemented by analogues from the Gubbio and Mt. Gorzano Fault zones in Italy. Utilizing petrophysical measurements, optical microscopy, and structural analysis, we evaluate the deformation mechanisms and their impact on fault rock permeability. Our findings reveal significant variability in fault rock permeability, governed by lithological heterogeneity. Marl-rich fault rocks exhibit permeability that is lower in comparison to that measured within the undeformed host rock, due to clay-rich pressure solution seams and veining, enhancing their sealing capacity. Conversely, carbonate-rich units display increased permeability, associated with brecciation, cataclasis, and open fracturing, which may compromise sealing integrity. Carbonate-marl mixed scenarios show intense brecciation, cementation, veining and pressure solution that results in an average permeability that is similar to that measured within the undeformed host rock, despite the variety of deformation microstructures. At Smeaheia, the dominance of marl within the overburden may suggest a potential for effective containment when juxtaposed against the reservoir unit, due to pressure solution being the likely main active mechanism, that may lower the transmissibility and transmissibility multipliers of the fault. By refining our understanding of fault-sealing mechanisms in carbonate-marl multilayers, these insights are useful for site selection and risk assessment for CO₂ storage where these lithologies are common in the subsurface, contributing to global efforts in carbon management.