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Chemistry, Morphology, and Distribution of Illites from Morecambe Gas Field, Irish Sea, Offshore United Kingdom

Louis Macchi, Alan Levison, Charles D. Curtis, Karen Woodward, Colin R. Hughes

The Morecambe gas field, discovered and operated by the British Gas Corporation, is centered on United Kingdom Irish Sea license block 110/2,26 mi off the west coast of England, and has recoverable reserves of 5 tcf. Production is from Triassic Keuper sandstones at an average depth of 3,000 ft (9,000 ft less than maximum burial), but it is constrained by the occurrence of a coarse-textured (crystals often > 20 µm) platy illite phase. Thin-section and scanning electron microscopy studies have shown this phase to have a well-developed boxwork texture that bridges all but the larger pores, and indicates a depth zonation in which platy illite occurs below a productive illite-free interval and above a zone of fibrous illite. Top platy illite cannot be predicted from pr sent day structure. To improve the existing diagenetic model, analytical transmission electron microscopy was used to define as precisely as possible the chemistry of the illite phases. It has shown all morphological types (grain tangential, replacement, boxwork, and fibrous) to have a composition very close to that of phengitic muscovite. A minor increase in Al/Mg, Si/Mg, and K/Mg ratios (consistently higher than in "classic" sedimentary illites) corresponds to the morphological sequence tangential ^rarr fibrous ^rarr platy, suggesting the latter represents the optimum equilibrium phase. Geochemical considerations indicate adequate K+ sources, and show that one possible illite-producing reaction involving brines (2K+ + 2Cl- + 3Al2Si2 O5(OH)4 ^rarr 2KAl3Si3O10(OH)2 + 2H+ + 3H2O + 2Cl-) would increase H+ activity in the pore fluids and could only proceed where a compensating reaction (e.g., carbonate dissolution or FeIII reduction) also operated.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.