THE SEDIMENTOLOGY OF THE BRENT GROUP IN THE CORMORANT BLOCK IV OILFIELD

Abstract

Cormorant Block IV is one of four, large‐scale fault‐bounded accumulations which together comprise the Cormorant oilfield. The field lies some 150 kms NE of the Shetland Islands, in the East Shetland Basin. Oil is trapped in the Middle Jurassic (Aalenian‐Bathonian) Brent Group. Block IV is a separate accumulation in a downthrown area formed by a large synsedimentary listric fault, which produced a dome‐shaped structure with further internal fault‐block compartmentalisation. A concerted data‐gathering effort on the accumulation included the coring of 43% of the reservoir section penetrated by wells.The Brent Group in Block IV fits into the “classical” overall regressive/transgressive, wavedominated delta model widely accepted for these sediments. However, the synsedimentary boundary fault has had some influence on the shallow‐marine Broom and Tarbert Formations and the coastal‐plain Upper Ness sediments, in each case preserving a unique and thicker sequence on the downthrown eastern side. The lowermost Broom Formation includes a shallow‐marine, Gilbert‐delta type deposit that infilled a topographic feature formed by the fault, and is genetically distinct from the rest of the Group. The Rannoch and Etive delta‐shoreface sediments, the main resrvoir interval, appear to have prograded through the area towards the NNE so rapidly that no fault influence was felt. Lateral variability at this level can be explained by the location of distributaries in the upper shoreface, controlling the grade of sediment supply and progradation rates of the entire system. The barrier‐attached coastal‐plain sequence of the lower Ness is chronostratigraphically equivalent to the Etive. and thins rapidly towards the north. The upper Ness is strongly layered by repeated periods of emergent coastal‐plain development separated by at least block‐wide lagoonal drowning events. Erosion at the base of the shallow‐marine Tarbert Formation, and the marked variability of sediment type and thickness in this horizon, suggests that it formed over a considerable time‐period at the end of Brent Group deposition, as the structure of the block was still developing.