Reconstructing Remobilisation of Deepwater Reservoir Sequences from Core Data: The Britannia Sandstone Formation, UK North Sea

Joris T. Eggenhuisen¹, Bill McCaffrey¹, Peter Haughton², and Rob Butler^3
1School of Earth and Environment, University of Leeds, Leeds, United Kingdom
²Department of Geology, University College Dublin, Dublin, Ireland
³School of Geosciences, University of Aberdeen, 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 correlation model often fails to predict the presence of lower reservoir sand intervals. The model is based on biostratigraphy and facies studies of the upper reservoir in which ‘layer cake’ stratigraphy makes correlation more straightforward. Inspection of core samples shows that the troubled lower intervals sometimes comprise a high proportion of debrites. The remobilised nature of shale blocks in debrites makes them unfit for biostratigraphic sampling. Not recognising the significance of remobilisation facies and over-relying on problematic biostratigraphy cause the problems in the lower half of the reservoir model.

The aim of this study is to reconstruct and understand the nature and scale of remobilisation in the lower part of the Britannia reservoir interval, by performing a remobilisation oriented sedimentological study of core from 14 wells covering a 9*5km area in the centre of the field.

Two different interpretations fit a core sequence of debris-flow matrix and apparently in-situ deposits: I) In-situ turbidite beds alternate with multiple debris flow deposits. II) Blocks of remobilised reservoir sandstone float in a larger scale mass transport deposit. Distinction between these two models is crucial for predicting lateral sand body extent and connectivity.

Spatial mapping of the remobilisation results in the recognition of a NE-SW debris flow axis with an in-situ bounding sequence in the NW. The map of the remobilisation front provides a predictive tool for risking low connectivity of reservoir sands.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas