DUNLIN GROUP SEQUENCE STRATIGRAPHY IN THE NORTHERN NORTH-SEA - A MODEL FOR COOK SANDSTONE DEPOSITION

The Dunlin Group in the northern North Sea, consisting of the Johansen , Amundsen, Burton, Cook, and Drake formations of late Sinemurian-Toar cian age, hosts important hydrocarbon reservoirs in the Cook Formation sandstones. The Johansen Formation is associated with a relative fall of sea level and is interpreted to be a large sandstone delta confine d within a broad incised valley at the base of the group. During a lat er stage of relative sea level rise, the finer grained Amundsen and Bu rton formations were deposited. The overlying Cook Formation consists of four sandstone tongues, each of which is characterized by a lower z one of sharp-based, upward-coarsening, thinly bedded shoreface sandsto nes and siltstones (reflecting forced regression during falling relati ve sea level) and an erosively based upper zone of thin tidal flat and thick deltaic/estuarine sandstones (reflecting lowstand incision, as well as initial progradation and subsequent transgressive backfill of estuaries during relative sea level rise). The Drake Formation shales were deposited during continued relative sea level rise. Several types of erosional surfaces are recognized within the studied succession: ( 1) sequence boundaries occur at the base of the Johansen Formation and within the Cook Formation, and represent the bottoms of incised valle ys that truncate the underlying shoreface deposits; (2) regressive sur faces of marine erosion occur at the base of Cook Formation units and truncate the underlying Burton and Drake shales, siltstones, and mudst ones; (3) transgressive tidal channel (tidal ravinement) surfaces with in the Cook Formation underlie the estuarine sandstones of the incised valley fills; (4) wave ravinement surfaces truncate the tops of estua rine sandstones and are overlain by thin transgressive lags that grade upward into the overlying black shales. Three-dimensional (3-D) model s, based on structure-contour maps of sequence boundaries, unveil a pa leotopography that controls the characteristics and distribution of th e Dunlin Group reservoir sandstones.