Britannia Field, UK North Sea: petrographic constraints on Lower Cretaceous provenance, facies and the origin of slurry-flow deposits

Abstract

Deep-water sandstones of the Early Cretaceous Britannia reservoir are rich in ‘muddy’ material, with the development of unusual ‘slurry-flow’ deposits ( sensu Lowe & 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.

Character and distribution of hybrid sediment gravity flow deposits from the outer Forties Fan, Palaeocene Central North Sea, UKCS

“…5b). The banding observed within the Forties Fan is similar to the meso-or micro-banding described by Lowe and Guy (2000), Blackbourn and Thomson (2000) and Lowe et al (2003) from the Britannia Field of the Central North Sea, although its expression in the Forties core is typically more subtle than in Britannia. The light bands characteristically have planar tops and irregular or loaded bases.…”

Reconstructing large-scale remobilisation of deep-water deposits and its impact on sand-body architecture from cored wells: The Lower Cretaceous Britannia Sandstone Formation, UK North Sea

“…Provenance results reported by Blackbourn and Thomson (2000) show different clay compositions for the debris flows and the fine-grained components incorporated in the sandstone beds in the Britannia sequence. The particular clay compositions indicate that the mud fraction of the debris flows (and therefore the debris flows themselves) was not derived from the same source as the mud fraction in the flows that deposited the sandstones in the Britannia area, but from local basin slopes (Blackbourn and Thomson, 2000).…”

“…The particular clay compositions indicate that the mud fraction of the debris flows (and therefore the debris flows themselves) was not derived from the same source as the mud fraction in the flows that deposited the sandstones in the Britannia area, but from local basin slopes (Blackbourn and Thomson, 2000). Hence, erosion by axially introduced (i.e.…”

Hybrid sediment gravity flow deposits – Classification, origin and significance

“…Although the origin of the banding has been disputed (c.f. the chlorite gelation model of Blackbourn and Thomson, 2000), the idea that it forms due to processes operating during the transition between turbulent and laminar behaviour is borne out by the context of banding in hybrid event beds where it tends to span the transition from cleaner lower sand to clay-prone upper sand or sandy mud (the deposit of turbidite and debrite respectively). Even in Britannia, banding sits between clean 'S3' sandstone (interpreted as non-cohesive turbidites sensu lato) and muddy sandstone with clasts attributable to debris flow processes (Barker et al, 2008); single Britannia beds thus include elements deposited by a range of flow types (and partly overprinted by intrabed remobilisation) and to label these as the product of a single flow type obscures the rheological heterogeneity of the original flows.…”