Chemostratigraphy of a Turbidite System: The Britannia Formation (Lower Cretaceous), of the Brodgar Field, Outer Moray Firth, North Sea

Ken Ratcliffe¹, Tim Pearce², Cristian Scheie², and Steve Tucker^3
1Chemostrat Inc, Houston, TX
² Chemostrat Limited, Llanfyllin, United Kingdom
³ ConocoPhillips (UK) Ltd, Aberdeen, United Kingdom

Over recent years chemostratigraphy has become an accepted stratigraphic tool in terrestrial sequences that are essentially barren of fauna and flora. However, there still remains scepticism of how useful the technique is in marine sequences where biostratigraphic data commonly remain the primary correlation tool. The lower Cretaceous Britannia Formation in the Brodgar Field of the Outer Moray Firth is in many ways a typical deepwater turbidite system where detailed biostratigraphic correlation can be challenging. The problem with using biostratigraphic data in such sequences lies in the highly episodic and rapid nature of turbidite sandstone deposition when compared to the slow, continuous deposition of hemipelgaic claystones. Additionally, the sandstones contain mostly reworked fossils, which when combined with the potential for mixing due to caving, exacerbates problems when using biostratigraphy.

This paper demonstrates how chemostratigraphy is used to define a series of regional chemostratigraphic packages and higher resolution, locally correlative chemostratigraphic units in the Brodgar Field and surrounding areas, thereby providing an enhanced correlation framework. The whole rock geochemical variations that allow the framework to be erected are shown to be controlled by changes in sediment provenance, both laterally and vertically. In addition to stratigraphic interpretation, the whole rock geochemical data are used to aid with modelling mineralogical variations that impact upon reservoir quality within the Britannia A reservoir unit and on a more regional scale, extending into the Britannia Field to the north, the data are used to interpret sediment dispersal patterns.