Morecambe field is centered on Block 110/2, 26 mi (42 km) off the northwestern coast of England. The gas-bearing structure occurs in Triassic sandstones with the top of the reservoir at an average depth of 3,000 ft (915 m), about 6,500 ft (2 km) less than maximum burial. Production is constrained by the presence of coarse-textured (crystals often >20 micrometers) platy illite. Thin-section and scanning electron microscopy have shown this phase to have a well developed, pore-bridging boxwork texture and indicates a depth zonation in which platy illite occurs below a productive illite-free interval and above a zone of fibrous illite. The top of the platy illite zone forms a mappable datum but cannot be predicted from present structure. Illite also is present throughout t e reservoir as grain-tangential rims and as a replacement of unstable framework grains, matrix clays, and neoformed kaolinite cement.

Analytical transmission electron microscopy, used to define the chemistry of the illites, has shown that all morphological types (grain tangential, replacement, platy, and fibrous) have compositions close to phengitic muscovite. A calculated maximum paleotemperature of 270 degrees F (132 degrees C) indicates that phengite can form as a lower temperature mineral than hitherto has been recognized.

Excluding replacement illite, which has no specific association with respect to the sandstone pore system, minor increases in Al/Si and K/Mg + Fe ratios (0.69 to 0.72 and 1.86 to 2.26, respectively) correspond to the morphological change from tangential/fibrous to platy. This suggests that platy illite represents the closest approach to ideal muscovite. Geochemistry indicates that either K-feldspar dissolution or brines could have provided K for illitization. The latter reaction (smectite + KCl changes to illite) would result in pore-water acidification and is unlikely to have occurred unless accompanied by acid-consuming reactions such as carbonate dissolution or iron reduction.

The presence of a mappable illite datum in Morecambe supports a neoformational origin for platy illite and suggests that Gulf Coast data are not directly applicable to this field. This consideration should not be overlooked in development of exploration strategies for other comparable reservoir targets.