Porous rocks in the subsurface are now used for carbon capture and storage (CCS), to help ameliorate the effects of greenhouse gas emissions. These porous reservoir rocks require a caprock to retain the CO2. It is important to characterize caprock quality and its stability in the presence of elevated partial pressures of CO2. Lower Triassic sandstones are common in the UK and NW Europe and are being considered for future CCS projects. The caprock to these sandstones is the Middle and Upper Triassic Mercia Mudstone Group. We have studied the Mercia Mudstone using mineralogy, petrology and mercury injection porosimetry to assess its caprock quality. Detrital minerals are dominated by quartz, K-feldspar, illite and chlorite; diagenetic pore-filling minerals are dominated by calcite, dolomite and gypsum. In samples with abundant clay minerals, there are only small quantities of pore-filling diagenetic cements. Porosity is broadly uniform for both clay-rich and clay-poor samples. The cleaner (clay-poor) samples had their initial pore spaces filled with early diagenetic pore-filling carbonates and gypsum. Despite the broadly uniform porosity, mean pore throat diameter displays a strong inverse correlation with clay content whereas threshold capillary entry pressure shows a strong positive correlation with clay content. The more clay-rich samples represent much better caprock than the coarser-grained samples that contain abundant pore-filling cement. The samples could support potential column heights of supercritical CO2 between 70 and 540 m and have calculated permeabilities between 10−20 and 10−19 m2; leakage would occur only on geological time scales, assuming that these samples are representative of the whole caprock. Because caprock quality correlates with illite content and illite will be relatively immune to elevated partial pressures of CO2, the Mercia Mudstone probably represents a durable caprock for future Lower Triassic sandstone CCS projects.