Utilising borehole image logs to interpret delta to estuarine system: A case study of the subsurface Lower Jurassic Cook Formation in the Norwegian northern North Sea

The Lower Jurassic Cook Formation forms a regressive and transgressive sandstone wedge of shallow marine reservoir sandstones. It is distributed mainly in the Norwegian sector of the northern North Sea and the formation has proven to be hydrocarbon bearing. A case study of this formation from the Tampen Spur area presents a methodology for reconstructing depositional environments in areas of scarce data coverage and poor seismic quality using limited core-coverage, wire-line logs and borehole image logs. The core material and image logs are from different wells. The latter offer interesting opportunities for 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, a system of descriptive, simple and robust image facies was established for this study. These include: horizontal lamination, low-angle lamination, cross-stratification, as well as mottled and deformed strata.The Cook Formation is interpreted here as a regressive tidal-fluvial delta to transgressive wave-dominated estuary couplet with offshore shale above and below. The tidal-fluvial delta of the regressive part seems to be at odds with the regional context as the regressive part of the Cook Formation in the Tampen Spur area is interpreted as a wave-dominated delta system. Internally, the regressive part of the Cook Formation thickens westwards with 68% which is unusual for the otherwise tabular regressive Cook Formation in the Tampen Spur area. Both the thickness and depositional environment difference of the regressive part of the Cook Formation can be explained with temporary fault movement of a blind fault located basinward during basin infill. This could have created a fault-induced monocline that led to wave sheltering of the tidal-fluvial delta system. A spit-barrier system was probably associated with the monocline sourced by longshore drift from the otherwise wave-dominated coast of the regressive Cook Formation. After basin infill and removal of basin relief, the subsequent transgression resulted in the formation of a wave-dominated estuary.