In this issue…

Abstract

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.The authors make use of downhole pressure data from a number of wells to give a first indication of Mercia Mudstone Group seal integrity. Pore pressures at the top of the oil column are seen to increase markedly, as would be expected, if the overlying sealing unit is halite‐dominated. The pressure build‐up is more variable where mudstones overlie the reservoir interval. In a second stage of the investigation, cores of the sealing interval from two wells and two onshore boreholes were logged in detail. Mercury‐injection porosimetry was used to assess the pore framework of selected core‐plug samples. The results indicate that variations in capillary entry pressures are the most important control on seal integrity in the mudstone‐dominated units. The authors also briefly assess other controls, including the presence of fractures in the seal unit, variable hydrocarbon saturation in the mudstones' pores, and variations in seal thickness across the basin.