Facies and development of the Middle Jurassic Brent Delta near the northern limit of its progradation, UK North Sea

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Abstract
Summary: The lithofacies patterns in the Middle Jurassic Brent Group, the principal hydrocarbon reservoir in the UK northern North Sea, are examined in seven wells. The sections, from the area of the Don and Murchison oil fields, record the nature of the Brent Delta near the northern limit of its progradation. Here the deltaic deposits of the Brent Group, which regionally constitute a major regressive-transgressive clastic wedge, progressively lose the interbedded sands and muds of the delta plain which characterize the middle part of the succession further S. This leads locally to a maximum sand-to-mud ratio for the Brent Group reservoir interval. A regressive storm-wave-influenced marine succession, from offshore transition-zone silts and muds up to hummocky cross-stratified nearshore sand, is overlain in turn by stacked distributary channel sands. Distributary mouth deposits are absent. The interbedded delta-plain succession, where it occurs, exhibits some marine influence and records wave reworking of interdistributary bay or lagoonal sediments. The Brent Group succession is capped by transgressive sands which were deposited in a storm-wave-influenced nearshore environment. Delta-plain drowning is also evident in the gradual marine incursions into distributary channels at the northern limit of the study area. The character of the delta locally is compared with the nature of the wave-dominated Brent Delta elsewhere on its progradational path. High energy nearshore deposits in both regressive and transgressive phases are widely distributed, but there is a contrast between the dominance of fluvial processes in the study area and the development elsewhere of a barrier island complex in front of a coastal plain.