Recognising Remobilised Deepwater Reservoir Sands: An Example from The Britannia Sandstone Formation, UK North Sea Sector

Eggenhuisen, Joris¹, William McCaffrey¹, Peter Haughton², William Hakes³, Rob Butler¹ (1) University of Leeds, Leeds, United Kingdom (2) University College Dublin, Dublin, Ireland (3) Britannia Operator Ltd, Aberdeen, United Kingdom

The Britannia gas-condensate field is located in the Outer Moray Firth, UK North Sea sector. Reservoir sands are thick turbiditic sandstones of the Lower Cretaceous Britannia Sandstone Formation.

The current reservoir model has difficulty to predict extent and connectivity of sands in some lower intervals of the reservoir. Comparison between core and the biostratigraphical model shows that some zonal boundaries are currently located in debrite matrix intervals. The remobilised nature of shale blocks in debrites makes them an especially unfit foundation for biostratigraphy.

The objective of the current study is the construction of a geological model for the troubled zones through detailed analysis of available core from 14 wells from a small area (6*6km).

An emerging spatial grouping is consistent with a model of a large mass movement, which has detached thick (10's of feet) blocks of reservoir sands and transported them to the toe of the movement, where they have frozen. The model can be characterised with five zones, from more distal to more proximal: I) In-situ unaffected substrate. II) Relatively unaffected substrate topped by debrite, the base of which consists of remobilised, stacked blocks of sandstone. III) Relatively unaffected substrate, topped by block devoid debrite. IV) Blocks of sandstone detached from the substrate by a thin zone of debrite. V) Debrite lies on older substrate.

The resulting model has grave implications for the connectivity of sandbodies in an interval over 100 feet thick. It recognises zones of remobilised reservoir sand blocks where low connectivity is a high risk. And it improves correlation between in-situ beds, which are now recognised with higher confidence levels.