A new interpretation for the Pliensbachian Cook Formation (northern North Sea) as north–south-prograding tidal deltas and shelf ridges in the Early Jurassic Seaway: new model of linkage to the Norwegian Sea

Abstract

Abstract The Lower Jurassic Cook Formation reservoir is a hydrocarbon-prolific unit that produces from several fields in the northern North Sea. For 40 years this formation has been interpreted as a westward-prograding deltaic unit sourced from Norway. Despite numerous discoveries, exploration targeting this unit has been hampered by well failures with lack of reservoir sand, discouraging companies from further exploration of this play. During a current re-evaluation of the process sedimentology of the Norwegian offshore basins, the Cook Formation is now interpreted as the middle to distal reaches of a very large, north-to-south-oriented delta system, variably confined within the Early Jurassic Seaway running from the Norwegian Sea into the northern North Sea. The Cook Formation is a subaqueous delta built southward during regression, whereas several internal transgressive phases produced sands that were reworked as north–south-oriented, shelf tidal ridges. The tidal ridges of the Cook Formation constitute some of the best reservoirs and are elongated with stacked, well-sorted, cross-bedded sandstone sets with mudstone drapes. Both the elongate tidal sand-ridges and intervening mudstone-rich, inter-ridge zones are proven by numerous well observations and illustrated by seismic amplitudes. In contrast to earlier eastern derivation models, these new results for the depositional system of the Cook Formation better explain the Cook well successes and failures in the northern North Sea. This work also strongly suggests that the tide-dominated subaqueous delta to transgressive-ridge system of the Cook Formation is spatially linked with the time-equivalent shorelines, subaerial tidal deltas and estuaries of the Tilje Formation in the Haltenbanken region to the north. The Tilje Formation deltas built into the Early Jurassic Seaway due to rift-initiation and rift-shoulder uplift, drained southwards and spilled eventually into the northern North Sea, becoming the entirely subaqueous Cook Formation. The relatively narrow seaway enhanced the tidal currents and suppressed wave activity, resulting in Cook subaqueous delta lobes and ridges without any delta-top facies. Overall, this elongate and extensive, Pliensbachian deltaic to estuarine system of the Early Jurassic Seaway off Norway competes in scale with some of Earth's largest present-day deltas.