The geology of the North Morecambe Gas Field, East Irish Sea Basin

Abstract

The North Morecambe Gas Field lies in the East Irish Sea Basin close to the producing South Morecambe Field, from which it is separated by a very deep WSW–ENE graben. It is a shallow structure, crest slightly above −3000 ft, with bounding faults to the south and east, but dip-closed to the north and west. The GWC is at −3950 ft. The Triassic Sherwood Sandstone Group reservoir was deposited under semi-arid continental conditions and comprises a complex interplay of channel-fill sandstones, non-channelized sheetflood sandstones and high-permeability aeolian and reworked aeolian sandstones. The vertical alternation of these facies forms the basis for differentiating nine facies-defined reservoir units. Palaeocurrent analysis indicates flow from NE to SW. A complex diagenetic history is recognized, with several phases of dolomite and quartz cementation. Differential compaction is also a significant control on the disposition of reservoir properties. The greatest control on permeability (but not porosity) is platy illite which precipitated beneath a palaeo-gas/water contact at an early stage in the growth of the structure. This gives rise to a diagenetic layering of the reservoir into a high-permeability illite-free layer and a deeper, low-permeability, illite-affected layer. The Mercia Mudstone Group which provides the seal, comprises four mudstone–halite cycles. Thickening of these cycles on the north flank of the structure indicates very early development of the northerly component of dip. The imposition of northerly dip entirely pre-dates the growth of platy illite, and this conclusion has a major impact on the manner in which the surface which separates the illite layers has been mapped on the north flank. The gas, sourced from Westphalian/Namurian sediments, is enriched in CO 2 compared with South Morecambe. This, together with contrasts in vertical cement distribution between the two fields, indicates that the breaching and loss of the first gas charge which is believed to have occurred on South Morecambe did not occur on North Morecambe. Furthermore, although separated from South Morecambe by only a narrow graben, the aquifer pressure is 140 psia less than that of South Morecambe, indicating that the major basin faults are full seals. Proved reserves are 1.08 TCF. The field is currently being developed for first gas in 1994.