In this issue…

WE LEAD THIS ISSUE with an interesting assessment (by J. K. Seedhouse and A. Racey of British Gas) of the sealing quality of the Triassic Mercia Mudstone Group in the East Irish Sea Basin. Hydrocarbons here are reservoired in the fluvial and aeolian sandstones of the Lower Triassic Sherwood Sandstone Group, a syn‐rift succession up to about one km thick. Both natural gas and crude oil are sourced from underlying Namurian shales, and are produced commercially at a number of relatively small fields located offshore NW England. A regional top‐seal is provided by the post‐rift Mercia Mudstones; however, the quality of this seal is very variable across the basin. In terms of general stratigraphy, the Mercia Mudstone Group is composed of alternating halite‐and mudstone‐rich units, which together reach a maximum thickness of over 3,000 m. Where a halite‐dominated unit directly overlies the reservoir sandstones, a very efficient seal is formed. However, halite‐rich units are limited in their areal distribution; where they are absent, mudstone‐dominated intervals may overlie the reservoir section, providing a seal of very variable quality. This paper reports on the factors which control the variation in seal integrity of the mudstone‐dominated units.