Deep-water sandstones of the Early Cretaceous Britannia reservoir are rich in ‘muddy’ material, with the development of unusual ‘slurry-flow’ deposits (sensuLowe & Guy 2000), including banded facies. The banding comprises couplets of pale sandstone containing microporous detrital chlorite and other clays, retaining substantial porosity, and dark sandstone in which biotite (now altered to chlorite) has promoted quartz pressure solution that has largely destroyed porosity. The main source of the abundant chlorite and biotite is the Jurassic Forties Igneous Province, underlying and surrounding the Britannia Field. Altered alkali basalts and other lithologies here are known to be rich in both minerals.

Grain-size distributions have been examined using image analysis. The banded facies are generally finer grained than the high-density turbidite and unbanded slurry-flow sandstones, and may be a distal equivalent. Dark bands in some cases contain no more fine clays than associated light bands, indicating that Lowe & Guy’s model for dark-band formation, invoking a cohesive sublayer, is incomplete. Gelation of chlorite-rich clays within each banded couplet is proposed as an alternative mechanism. These microporous pore-filling chlorites have restricted the precipitation of quartz overgrowths and other non-porous cements, although their presence inhibits permeability.

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