The Interaction of Shelf Accommodation, Sediment Supply and Sea Level in Controlling the Facies, Architecture and Sequence Stacking Patterns of the Tay and Forties/Sele Basin-Floor Fans, Central North Sea
Published:December 01, 2000
David C. Jennette, Timothy R. Garfield, David C. Mohrig, Glenn T. Cayley, 2000. "The Interaction of Shelf Accommodation, Sediment Supply and Sea Level in Controlling the Facies, Architecture and Sequence Stacking Patterns of the Tay and Forties/Sele Basin-Floor Fans, Central North Sea", Deep-Water Reservoirs of the World, Paul Weimer
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A unique perspective of sand-rich, basin-floor fan deposition was gained through an integrated study of the Paleogene deepwater reservoirs in the UK Central North Sea. The data set included contiguous 3D seismic (8400 km2), 2D seismic (11,100 km), well logs (350 wells), high-resolution palynology (180 wells), and core (30 wells). The study provided new insight into downfan changes in reservoir facies and architecture and a framework to understand fan evolution through both low-and high-frequency depositional cycles.
The Paleogene in this part of the basin is subdivided into four low-frequency (1-3 MY) composite sequences. Major basin floor fan cycles include the Maureen Formation, the Andrew-Lista units, the Forties-Sele-Balder units, and the Tay-Chestnut units. These low-order successions exhibit large-scale compensational stacking behavior and, in contrast to classical fan models, maintain channel-form patterns to their distal pinchouts. Sheet geometries are surprisingly rare. Sandstone body geometries, coupled with core-based lithofacies suggest that both turbidity currents and semi-cohesive sandy debris flows were active during reservoir deposition.
Each of the major sand-rich fan units (e.g., Forties, Tay) is composed of higher frequency depositional sequences that display important changes in lithofacies and architecture through time. The early sequences are heterolithic consisting of variable proportions of mud-rich debrites and sandstones that are commonly thin and arranged into broadly channelized bodies (high aspect ratios). The later sequences are much sandier. The sandstone bodies show slightly sinuous to linear channel-form patterns (lower aspect ratios). Although less laterally extensive, the youngest sequences have high quality reservoirs that locally have strongly mounded cross-sectional geometries.
The vertical change in reservoir character reflects a progressive change in the composition and relative volume of the sediment gravity flows being triggered at the shelf edge and then delivered into the basin. Following initial phases of muddy debris flows and sandy turbidity currents, sediment gravity flows became progressively sandier and confined in discrete channels. Latest-stage sequences are locally dominated by sandy debrites. This pattern records the evolution of the lowstand shelf-margin system as it became progressively sandier and increasingly prone to large, sand-rich failures that maintained a semi-cohesive rheology as they flowed onto the basin floor. Superimposed on this changing sediment composition is a progressive decrease in sediment volume as more sediment is trapped on the shelf in response to the low-order rise in sea level.
This integrated seismic, lithofacies, and stratigraphic analysis leads to an improved regional (play) to local (field-scale) stratigraphic correlation and reservoir mapping methodology. This analysis also addresses variations in reservoir quality, channel geometry, and lateral and down-fan facies changes.