JENNETTE, DAVID C and TIMOTHY R. GARFIELD, Exxon Production Research Company

Abstract: The Interaction of Shelf Accommodation, Sediment Supply and Sea Level in Controlling the Facies, Architecture and Distribution of the Forties and Tay Basin-Floor Fans, Central North Sea

A high-quality data set consisting of contiguous 3D seismic (8400 km² ), 2D seismic (11,100 km), well logs (350), biostratigraphy (180 wells) and core provided new insights into down-fan and vertical changes in facies and architecture of sand-rich basin floor fans.

The main fan cycles in the study area are the Forties-Sele-Balder fan cycle and the Tay-Chestnut fan cycle.These fan cycles are subdivided into 4 to 5 high-frequency sequences (0.1- 0.3 MY) which stack to show distinct changes in reservoir properties. Basal sequences have moderate to low sand content. Sand bodies are broadly channelized to sheet-like, and muddy mass-transported complexes are locally abundant. Middle sequences are sandier and strongly channelized and extend farthest into the basin. The upper sequences have the highest quality reservoirs and stack in a backstepping pattern.The upper sequences locally exhibit strongly mounded and non-erosional cross-sectional geometries implying a cohesive (debrite) rather than fluidal (turbidite) origin.

The vertical change in reservoir character reflects an evolution in the composition and relative volume of the sediment gravity flows entering the basin.The increasing sand content and shift toward large, cohesive sediment gravity flows can be tied to an evolving shelf-edge system which becomes increasingly sand-rich and prone to large failures.