Abstract: Facies, Architecture and Sequence Stratigraphy of Sand-Rich Basin Floor Fans: Examples from the Paleogene, UK Central North Sea

Jennette, David C. & Timothy R. Garfield - Exxon Production Research Co.; Geoffrey W. Farquharson & Glen T. Cayley - Esso E&P UK

A unique perspective of sand-rich, basin-floor fan deposition was gained through an integrated study of the Paleogene deep-water reservoirs of the Central North Sea. The proprietary data set included contiguous 3D seismic (8400 km²), 2D seismic (11,100 km), well logs (350 wells), high-resolution biostratigraphy (180 wells) and core (30 wells). The study provides 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. Development of these sequences is linked to underplating and uplift of Scotland with superimposed eustasy. Major 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-like reservoirs are rare. Sand body geometries coupled with core-based lithofacies suggest that both turbidity and non-cohesive sandy debris flow processes were active during reservoir deposition.

Each of the major sand-rich fan units (e.g. Andrew, Forties, Tay) are composed of higher-frequency cycles or depositional sequences which exhibit distinct vertical stacking patterns. The basal sequences contain an abundance of mud-rich mass-transported debrites. Sandstones are commonly thin and arranged into broadly channelized bodies (high aspect ratios). Overlying sequences are sandier with sand bodies exhibiting strongly linear channel-form patterns (lower aspect ratios). Although less laterally extensive, these later stage channel complexes have higher quality reservoirs and locally have strongly mounded cross-sectional geometries. Field studies show this vertical pattern is often repeated at the scale of an individual sequence. These sequence-based patterns improve regional (play) to local (field-scale) stratigraphic correlation, reservoir mapping and begins to explain variations in reservoir quality, channel geometry, lateral and downfan facies changes and well-performance anomalies.

The vertical change in reservoir character reflects a progressive change in the composition and relative volume of the sediment gravity flows being delivered to the basin during cycles of relative change in sea level. Following initial phases of mass-transported, cohesive debris flows, flows became progressively sandier and associated with discrete channels. Late-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. Superimposed on this changing sediment composition was a late stage decrease in sediment influx driven by the low-order rise in sea level.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil