The gas permeability of a fine-grained siliciclastic caprock: the Middle-Upper Triassic Mercia Mudstone Group

Fine-grained siliciclastic rocks play a critical role as caprocks for subsurface energy storage and as host media for geological disposal of radioactive waste, yet their permeability structure and anisotropy remain poorly constrained at formation scale. This study presents a comprehensive experimental characterisation of gas permeability, mineralogy and microstructure through a continuous ∼ 200 m cored succession of the Middle-Upper Triassic Mercia Mudstone Group, a regionally extensive and heterogeneous caprock unit across the UK. More than 700 laboratory measurements of horizontal and vertical gas permeability were acquired from 18 samples over a wide range of effective pressures representative of burial conditions. Measured permeabilities span six orders of magnitude, reaching values as low as 10⁻²¹ m², and show pronounced pressure sensitivity and hysteresis associated with microstructural compaction. Strong permeability anisotropy is observed at both sample and formation scales, with individual k h /k v ratios exceeding 10³ at high effective pressure and an upscaled anisotropy ratio of over 5000 for the full succession, reflecting stratigraphic heterogeneity and bedding-parallel flow pathways. Multivariable statistical analysis demonstrates that connected porosity exerts the primary control on vertical permeability, while horizontal permeability is more strongly influenced by pore throat size and clay mineral content, which together govern tortuosity and flow localisation. These results provide the most detailed permeability dataset yet reported for the Mercia Mudstone Group and highlight the importance of integrating stratigraphy, diagenesis and anisotropy when evaluating the long-term containment performance of fine-grained siliciclastic caprocks for CO₂ storage, hydrogen storage and radioactive waste disposal.