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Sheltered Tidal Deltas within from Active Fault-Block Setting. A Case Study of the Cook Formation (Pliensbachian-Toarcian) in the Northern North Sea (Norway) Based from Cores, Borehole Images and Logs for Sedimentological Interpretation

Folkestad, Atle 1; Roberts, Paul 2; Veselovsky, Zbynek 3
1 Global Exploration Technology, StatoilHydro ASA, Bergen, Norway.
2 Statoil UK Ltd, London, United Kingdom.
3 Eriksfiord ASA, Stavanger, Norway.

Well 34/11-A6 discovered gas in the Cook Formation below the main reservoir of the offshore Kvitebjørn Field, North Viking Graben. This triggered a need for a sedimentological model for the Cook Formation as input to reservoir characterisation and reserve estimates. A study was initiated using data from core, conventional logs and borehole image-logs (FMI).

Borehole image-logs can provide sedimentological descriptions and interpretations both in cored and uncored sections, particularly as the resolution of the tool (mm-scale under optimal conditions) enables identification of sedimentary structures. In order to avoid over-interpretation of the image-facies in this study, a system of descriptive, simple and robust image-facies was established. This include: horizontal lamination, low angle lamination, cross-bedding, mottled and deformed strata.

Regional studies suggest that the Early Jurassic Cook Formation was deposited in a narrow seaway. In the study area, this formation is proposed as a regressive tidal-dominated delta to transgressive wave-dominated estuary couplet bounded encased by offshore shale above and below. The coarsening upward unit shows: numerous mud-drapes, bioturbation, cross-bedding and bedsets of massive sandstones capped by current ripples and thick (max. 10cm) mud-drapes. The unit shows a general lack of wave-structures and the image logs indicate in this unit a general NW (basinward) sediment transport direction. These observations suggest a tidal delta deposited in a sheltered setting with influence of fluvial event-beds. The upper part of the Cook Fm is not cored but by interpretation of image-facies and conventional logs, this unit is interpreted as stacked cross-bedded sandstones with SE (landward) sediment transport direction and overlain by planar-laminated sandstones. This unit is interpreted as suggesting a wave-dominated estuary deposited during transgression and subsequent drowning of the Cook Formation.

The regressive part of the Cook Fm shows a westward thickening trend suggesting syn-sedimentary tectonic activity during basin infill. This activity is probably associated with the sheltering of the tidal delta with a proposed basinward barrier system attached to the top of a fault-escarpment, sourced from a documented large adjacent delta to the north by longshore drift. After basin infill and removal of basin relief, the subsequent transgression resulted in the formation of a wave-dominated estuary.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009