The Upper Triassic caprock of the Ketzin CO₂ storage site, North German Basin: Implications for geological hydrogen storage

Porous rocks are considered for underground hydrogen storage (UHS) to reduce greenhouse gas emissions and meet global energy demands. For effective geological storage, caprocks are essential to prevent hydrogen (H₂) migration. The Lower Triassic Stuttgart Formation sandstones were successfully tested for CO₂ storage at the Ketzin pilot site (North German Basin) and are now being evaluated for UHS. The overlaying Middle Triassic Weser Formation acts as a multi-barrier seal. This paper presents the lithological, mineralogical, and petrophysical properties of the Weser Formation and assesses its quality as caprock for UHS. This unit comprises mudstones interbedded with evaporites and dolomite. Mudstones are composed of quartz and feldspar grains within a clay-rich illite matrix with pore-filling cements of illite, anhydrite, and dolomite. Porosity ranges from 5.3 % to 15.5 %, gas permeability from 0.002 to 0.018 mD, and liquid permeability from 30 to 47 nD. The Weser Formation's thickness (∼80 m), high clay-mineral content (∼50 %), and petrophysical properties suggest its potential as an effective seal for UHS. Caprocks may hold H₂ column heights of ∼800–1300 m, though H₂ wettability studies are needed to accurately assess sealing capacity. Mineralogical analysis suggests that the caprock is unlikely to react significantly with H₂ due to dominant non-reactive minerals like illite, quartz, and feldspars. However, further studies including laboratory batch experiments and geochemical modeling are required to investigate fluid-rock interactions. This work provides valuable insights into the geological characterization of the Ketzin caprock, delivering transferable knowledge for UHS projects in the North German Basin.