Sedimentology, diagenesis and reservoir characterisation of the Brent Group in the North Viking Graben-East Shetland Basin, northern North Sea
The Middle Jurassic Brent Group was deposited within northward prograding wave dominated delta system which developed in the North Viking Graben-East Shetland Basin. Five formations are readily distinguished in core and on wireline logs, each representing a distinctly separate depositional environment. Poorly sorted, subarkosic sandstones comprise the Broom Formation at the base of the Brent Group and are interpreted as having been deposited within an alluvial to shallow marine fan delta system, from proximal to distal gravity flows. In contrast to other formations of the Brent Group, the Broom Formation thins towards the centre of the Viking Graben and its rarely of reservoir quality. The Rannoch Formation comprises of a series of stacked upwards coarsening very fine to fine grained micaceous sandstones which in the north of the Brent Province are underlain by a thick sequence of prodelta shales. Planar laminated and hummocky cross stratified sandstones of the Upper Rannoch Formation were deposited within a storm dominated shoreface environment during a period of rapid delta progradation. Shoreface sandstones of the Rannoch Formation form laterally continuous sheet sandbodies, and have good reservoir potential. Tidal channel and beach/foreshore sandstones of the Etive Formation form some of the main reservoir intervals within the Brent Group having a clean well sorted nature and good lateral and vertical connectivity. Both the Rannoch and Etive Formations thicken northeastwards towards the northern limits of the Brent Delta. Interbedded sandstones, siltstones and shales of the Ness Formation respresent the establishment of an extensive delta plain across the East Shetland Basin during Upper Brent Group times. Interdistributary bay fill deposits comprise the majority of the Lower Ness Formation and are typically of poor reservoir quality. In contrast, the sand rich Upper Ness Formation is dominated by distributary channel, mouth bar and lagoonal beach shoreline sandstones which form important reservoir units in the Ness Formation. The Tarbert Formation represents the final stages of Brent Group deposition. Facies and thickness variations reflect the gradual retreat and subsequent drowning of the Brent Delta. Reservoir quality is highly variable as a consequence of interdigitation between barrier shoreline and washover sandstones with back barrier lagoonal shales.