Flow Heterogeneity within the Paleocene Megabed of the Central North Sea, UK

Megabeds are defined as thick, laterally extensive, event beds that differ lithologically from their host rock and are emplaced by a single particulate gravity event. There are several limitations with the current definition: 1) it is based on contextual rather than process characteristics, such that deposits of the same process may be categorized differently; 2) the role of confining topography is not defined; 3) trends in characteristics of the megabeds within an unconfined setting are not well established. Megabeds are geologically and economically significant as they are proven reservoirs, with good lateral-connectivity and act as excellent chonostratigraphic markers.

This study focuses on Paleocene megabeds within four oilfields of the Central North Sea, UK (Stella, Judy/Joanne, Orion and Flyndre); on a NW-SE flow parallel transect some 160-260 km from the inferred source. In total 1550 ft of logged core, from 13 wells was used to develop a lithofacies scheme with primary divisions of gravel-rich sediments, sandstones, siltstones, claystones and chaotic facies, subdivided as appropriate. A typical megabed exhibits an overall fining-upwards sequence: a crudely stratified basal gravel grades into laminated and banded sandstones, with minor ripples and cross-lamination, then into a dewatering zone with inclined cross-cutting water-escape pipes, and finally into an upper zone of discontinuous stringers/sand-balls that grade from fine sand to silt. Both proportions of sub-facies and the transitions between them vary spatially. Within the proximal fields banded and laminated deposits develop above gradational contacts with underlying gravel-rich facies, and comprise > 70% of the megabed thickness. In distal fields banding and lamination comprise < 40% of the megabed and there are sharp basal contacts.

Megabeds are interpreted to be emplaced by large-scale flows that progressively evolve in their rheology. Gravel-rich sediments are interpreted as debris-flows and gradational contacts reflect progressive debris-flow to turbidity current transformation. Sharp basal contacts are more common in the distal region, due to the turbidity current out-running the underlying debris-flow. Banding is common throughout the megabed and attributed to alternations between laminar and turbulent flow regimes. Predictions can be made to classify length scales over which characteristic facies changes occur.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California