The Permeability and Petrophysical Properties of the Mercia Mudstone Group: A Heterogeneous Caprock

Summary The Mercia Mudstone Group (MMG) has been cited as a potential caprock to CO2 storage, and is being considered as a potential host rock for the UK’s first Geological Disposal Facility. 18 samples of core have been collected throughout a continuous succession of MMG, and have undergone a range of experimental analyses in order to obtain controlling factors on permeability, permeability anisotropy and pressure sensitivity. X-ray diffraction (XRD) and petrographic analysis has shown a complex suite of detrital and early diagenetic minerals, resulting from an arid, continental environment of deposition. Mercury intrusion porosimetry (MIP) revealed a diverse range of mean pore throat sizes within the meso- to macroporous range, and porosity values ranging from 3.2% to 13.4%. These samples of MMG show a range in measured permeabilities, spanning 6 orders of magnitude. Permeability anisotropy within individual samples reach up to 3 orders of magnitude. More than 600 individual gas permeability measurements have been undertaken on these MMG samples, revealing the sealing potential of the different stratigraphic horizons present at depth. This extensive dataset will provide useful inputs for upscaling and predicating fluid migration under varying stress conditions.