In the East Irish Sea Basin of the UK Continental Shelf hydrocarbon accumulations in the Triassic Sherwood Sandstone Group (SSG) are sealed by the Triassic Mercia Mudstone Group (MMG). The MMG comprises alternating mud-and halite-dominated intervals. Where halites overlie the SSG (e.g. in the Morecambe and Millom gasfields), seal integrity is excellent; however, where the SSG is overlain by mudstones of the MMG (e.g. in the Douglas and Lennox oilfields), seal integrity is variable and is controlled by the capillary entry pressure of hydrocarbons into the pore network of the mudstones. Mercury porosimetry was used to characterise the pore network of the Mercia Mudstone samples from several localities＇ in the East Irish Sea Basin. This data was used to assess seal quality and thus to calculate theoretical hydrocarbon column heights supportable fora given seal. It appears that the sealing quality of the MMG mudstones and siltstones varies principally with the nature of the pore-throat network. Fracture filling and pore-plugging by bitumen representing an earlier oil charge may also have improved seal integrity by blocking pore throats.