The Triassic Sherwood Sandstone Group sediments of the East Irish Sea Basin are over 4000 ft thick and comprise medium- to coarse-grained sandstones and rare thin mudstones. Facies models developed during production drilling on the South Morecambe Field show that deposition occurred in a braided fluvial setting with minor aeolian episodes. The major facies associations present are: (A) major channel fill; (B) ephemeral channel fill; (C) non-channelized sheetflood deposits; (D) and (E) non-reservoir fines (abandonment and playa respectively); (F) aeolian dune and sand-sheet. Volumetrically, the major channel (A) and sheetflood (C) facies dominate, and the alternation of these facies associations is correlatable across the field. The aeolian sandstones form units varying in thickness from a few grain diameters up to 3 m thick. The sand-sheet and dune deposits can be correlated over considerable distances and show pressure communication on RFT logs. Although the aeolian sandstones comprise only 5–10% of the reservoir they have very high porosities and permeabilities (up to 30% and 10 darcies) and make disproportionate contributions to flow into the wellbore. The depositional criteria used to differentiate between aeolian and fluvial deposition in the South Morecambe Field are pinstripe lamination, good sorting and lack of rounded clay clasts, but these criteria are not definitive. As a result of the dissolution of an early diagenetic cement, aeolian sandstones have very high porosities compared to fluvial sandstones. This high porosity is reflected in high sonic transit times and allows aeolian sandstones to be identified tentatively in uncored wells by use of sonic logs.

Two distinct units can be recognized in the Lower Triassic Ormskirk Sandstone Formation in the East Irish Sea Basin. Perennial fluvial channel, lacustrine and minor aeolian sandstones dominate the upper part, but the lower is characterized by thick (tens of metres) enigmatic wavy bedded deposits interbedded with aeolian and minor fluvial channel sandstones. Previous interpretations that invoked an overbank ‘sheetflood’ or ‘sandflat’ origin for the wavy bedding are difficult to reconcile with the sedimentary structures, which are more consistent with algal and evaporitic processes in modern sabkhas. These deposits contain widespread (km) ‘drying upward’ patterns 2–10 m thick, reflecting a sabkha to more aeolian dominated setting. Such thicknesses are consistent with an origin related to 23 000 year Milankovitch cyclicity. The lower, wavy bedded portion of the Ormskirk Sandstone Formation is regionally truncated by fluvial channel-belt sandstones. Their extent reflects a combination of high sediment supply, uniform subsidence and low basinal relief. Lacustrine units near the top of the formation can be correlated regionally, with one passing southward into a major aeolian unit that thickens to 60 m in the south of the basin. This transition demonstrates the diachroneity of the Mercia Mudstone Group transgression. The key to correlating the Ormskirk Sandstone Formation lies in identifying intervals dominated by fluvial, lacustrine or sabkha-aeolian deposits. Correlation of water-table-controlled ‘drying upward’ patterns within sabkha-aeolian deposits allows finer scale subdivision, and provides an estimate for the degree of fluvial truncation at the top of the unit.

Regional seismic lines from the East Irish Sea area and a 2D radial seismic survey across the Morecambe Fields have been used to assess fault pattern evolution and the history of tilting in the East Irish Sea Basin. Extension and fault-controlled sedimentation began in Permian times and continued throughout the Permo-Triassic. Later inversion and erosion has removed the post-Triassic cover in the area. The main extension direction during basin evolution was ENE–WSW to NE–SW. Under this extension, NE–SW-trending basin-bounding faults in the north and south were dominated by strike-slip and oblique displacements. Offset of the pre-Permian basement suggests the amount of post-Carboniferous net extension varies from 10–16%. The basin can be divided into two discrete structural domains which controlled the geological evolution of the East Irish Sea Basin. Basin evolution in the northern part of the basin was controlled by easterly dipping faults which induced tilting towards the southwest or west. The southeastern part of the basin is dominated by two faults which dip to the west or southwest, inducing tilting towards the east. The complex structural and tilting history of the Morecambe Fields arises from their position at the boundary of these two domains. Detailed analysis of this area illustrates how the propagation of the Keys Fault (from the north) and the Crosh Vusta Fault (from the south) competed to dominate tilting history. The faults which now bound the Morecambe Fields show only minor influence on Permo-Triassic sedimentation patterns, implying that the main displacement on these faults post-dated the deposition of the Sherwood Sandstone. The development of these boundary faults is thought to be related to the accommodation of arching between the Keys Fault and Tynwald fault system. The differing tilting histories of the North and South Morecambe Fields are indicated by the differing orientations of a previously shared palaeo-hydrocarbon–water contact. This is now preserved as the top platy-illite surface and is interpreted as arising from the development of a tilt-transfer zone during the propagation and linking of the northerly and southerly propagating fault arrays. Analysis of the tilting history of the Morecambe area has allowed the top platy-illite surface to be mapped outside well control.

Summary Upper Carboniferous sediments from the central part of the southern North Sea Basin were deposited on a broad fluvio-deltaic plain. Potential reservoir rocks occur within distributary channel facies sandstones, but porosity and permeability have been reduced by authigenic mineral growth and improved by at least two separate phases of porosity enhancement: firstly by the infiltration of meteoric waters during post-Carboniferous uplift, and secondly by the action of acidic fluids generated during burial. The depth of invasion of meteoric fluids is highly variable, ranging from a few metres to hundreds of metres, and the best porosity is preserved in sediments which have undergone both phases of porosity enhancement. Virtually all the observed porosity in Carboniferous sandstones can be interpreted as having a secondary origin. Integrating the deduced diagenetic history with a burial and temperature history for specific wells allows the timing of diagenetic events to be estimated.

Dipmeter logs have been processed in a variety of ways as an aid to understanding the geology of the North and South Morecambe Gas Fields. They have yielded high quality structural information which has contributed to understanding the structural configuration and tectonic history of these fields. Several faults have been identified, cutting well paths, and dipmeter analysis has allowed the correct orientation and direction of throw to be determined. In one well, the deviated well path has intersected each of two listric faults in two separate places, allowing unusually precise definition of these fault planes. Throughout the Ormskirk Sandstone Formation, the predominant palaeocurrent direction has been found to be towards the west or southwest. The most consistent palaeocurrent orientations were measured in intervals of planar cross-stratified channel-fill facies sandstones, but similar results, with greater scatter were observed in ephemeral channel sandstones facies. These consistent palaeocurrent results suggest that the East Irish Sea Basin formed a distinct depo-centre during Ormskirk sandstone times. The implication is that during the deposition of the Ormskirk Sandstone, palaeocurrents were controlled by a regional dip towards the Keys Fault. There are changes in palaeoflow from the base of the Ormskirk Sandstone Formation to the top (a vertical thickness of 800 ft), also changes from the southern to the northern ends of the field. In the underlying St Bees Sandstone Formation, consistent palaeocurrent directions proved more difficult to obtain, since major channel facies sandstones are rarer. No angular unconformity was observed between the St Bees and Ormskirk Sandstone formations. Within the Ormskirk Sandstone interval, however, very consistent, 1° to 2° changes in structural dip occur at the same stratigraphic levels in several wells and indicate small-scale tectonic rotations during the deposition of these sandstones. These are probably related to tectonic reactivation which has been invoked in previous literature to explain periodic rejuvenation of the depositional system during deposition of the reservoir sandstones. The fact that the observation of such small-scale adjustments can be repeated in several wells is an indication of the degree of precision of the results obtained from the dipmeter logs in these wells.