Abstract

The Dunlin Group in the northern North Sea, consisting of the Johansen, Amundsen, Burton, Cook, and Drake formations of late Sinemurian-Toarcian 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 confined within a broad incised valley at the base of the group. During a later stage of relative sea level rise, the finer grained Amundsen and Burton formations were deposited. The overlying Cook Formation consists of four sandstone tongues, each of which is characterized by a lower zone of sharp-based, upward-coarsening, thinly bedded shoreface sandstones and siltstones (reflecting forced regression during falling relative 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 valleys that truncate the underlying shoreface deposits; (2) regressive surfaces of marine erosion occur at the base of Cook Formation units and truncate the underlying Burton and Drake shales, siltstones, and mudstones; (3) transgressive tidal channel (tidal ravinement) surfaces within the Cook Formation underlie the estuarine sandstones of the incised valley fills; (4) wave ravinement surfaces truncate the tops of estuarine sandstones and are overlain by thin transgressive lags that grade upward into the overlying black shales. Three-dimensional (3-D) models, based on structure-contour maps of sequence boundaries, unveil a paleotopography that controls the characteristics and distribution of the Dunlin Group reservoir sandstones.

You do not currently have access to this article.