A STRATIGRAPHIC REVIEW OF THE LATE CALLOVIAN TO OXFORDIAN INTERVAL, FISHER BANK BASIN AREA (UK SECTOR, CENTRAL NORTH SEA)

S. Duxbury1* and M. Vieira2*

1 Duxbury Stratigraphic Consultants, Church Lane, Chester CH2 1DJ.

2 Shell UK Ltd, 1 Altens Farm Road, Nigg, Aberdeen, AB12 3FY.

* Authors for correspondence: Duxburydsc@aol.com, Manuel.Vieira@Shell.com

The Middle to Upper Jurassic succession of six wells in the Fisher Bank Basin area has been reviewed within a consistent biostratigraphic framework. Significant, but age-equivalent, vertical and lateral facies changes across the area have been demonstrated and a number of controlling factors are discussed. Several candidate Maximum Flooding Surfaces (MFSs) have been recognised, and various approaches to sequence stratigraphy in the Central North Sea are assessed. This has been particularly focused on the J40/J50 sequence boundary (top Middle Oxfordian) of Rattey and Hayward (1993) and its position with respect to the MFS picks of various authors. New analyses across the Port-an-Righ Ironstone MFS from an onshore outcrop and its correlation to the northern Moray Firth indicate that this may not be represented by the high gamma feature widely correlated throughout the Central North Sea, as the latter seems to occur at a slightly older level; it might occur as a more subdued feature, which is difficult to pick on logs in more heterogeneous settings.

Sporomorph EcoGroup (SEG) analysis has allowed discussion of miospore assemblage fluctuations in response to various environmental factors such as climate change or sea level/base level fluctuations. Using marine palynomorphs whose distribution is essentially independent of lithology, detailed and consistent sub-division and correlation of important Fulmar Formation reservoir sands allows direct comparison with the Heather and Kimmeridge Clay Formations in more distal settings. The transition from Fulmar Formation sands (reservoir) to Heather Formation shales (caprock) within the Armada Complex and SW Seymour gas fields is shown to be equivalent to an intra-Kimmeridge Clay event in more distal wells. The current study emphasises the importance of having in place a dependable and well-tested regional biostratigraphic framework as a basis for stratigraphic interpretation.

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