oa The Cormorant Formation (late Triassic) of the Tern Field, Northern North Sea

The Tern Field is located on the Tern-Eider Ridge at the western margin of the East Shetland Basin in the UK Northern North Sea, approximately 250 miles NE of Aberdeen. The late Triassic-aged Cormorant Formation oil reservoir was discovered by well 210/25-1 and the later (1977) Exploration well 210/25-3 recovered ~250ft of core from the Cormorant Formation in the northwest of the field. We have chosen to display the basal 120 ft of the cored interval, covering the main productive unit of the reservoir that lies at the top of the Lower Cormorant Formation and that is currently being developed. The base of the interval consists of a sequence of red siltstones and shales that display textures indicative of pedogenic modification. The sequence is sharply truncated by an amalgamated package of coarse-grained, poorly sorted sandstones. The sandstones contain abundant pebble-granule sized exotic clasts in addition to reworked, intraformational calcrete and clay material. Although lacking well-defined barform structures, some grading in grainsize may be observed. These sandstones form the main net-pay zone of the Tern Triassic reservoir; the sequence is approximately 30 ft thick and can be correlated across the field using formation pressure data acquired whilst drilling subsequent development wells. A sequence of variably mottled silts and shales containing calretised root traces and calcite nodules overlies the sandstone sequence, forming a seismically-definable marker horizon across the field. The sequence then evolves, with overlying micaceous sands being generally finer-grained, better sorted and characterised by abundant planar, low-angle planar and sub-ordinate climbing ripple lamination. Bioturbation fabrics and more elaborate root traces are also more abundant towards the top of the displayed interval. In the Tern Field, these sands are considered non-net, although in offset fields equivalent sands are on production. The Cormorant Formation in the Tern Field area is interpreted as being deposited within a continental fluvial distributary system. The displayed interval marks the transition point between two distinct continental fluvial styles. The lower interval was deposited in broad, low relief, coarse-grained fluvial distributary channels separated by sediment-starved and pedogenically modified floodplain intervals. The upper part of the interval heralds the onset of more unconfined, sheet-like fluvial deposition with colonisation of opportunistic fauna and evidence of more elaborate plant growth in the floodplain areas.