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Exploring in Mixed Axial and Transverse Post-Rift Deep-Water Systems: The Cretaceous Lysing Formation, Offshore Norway


Successful exploration and development of deep-water post-rift plays requires placing good constraints on evolving physiographic configurations, and routeing patterns, as the influence of syn-rift structures decreases. The Halten and Donna terraces, offshore mid-Norway, comprise a series of rift-related sub-basins. Traditionally, Jurassic syn-rift hydrocarbon reservoirs have been the focus for hydrocarbon exploration, but now there is active exploration in the shallower post-rift Cretaceous successions. The Cretaceous Lysing Fm. reservoirs are up to 50 m thick and have been interpreted as slope- and basin-floor fans within a series of weakly confined post-rift sub-basins present in a 300 km by 130 km area. A deep-water setting for the Lysing Formation has been supported by seismic and facies interpretations. However, this study highlights an enigmatic facies association in the lower Lysing Formation (up to 25 m thick), which is key to refined paleogeographic reconstructions. Detailed facies classification of 380 m of 14 cored wells from the Lysing Formation across the Halten and Donna terraces have been documented. More than 50% of the cored interval comprises turbidites and hybrid beds, which dominate the upper Lysing Formation, and have a north-to-south increase in the proportion of hybrid beds. Towards the south and west, the lower Lysing Formation comprises glauconite-rich coarse-grained cross-stratified sandstones, with many indicators of erosion, and low diversity and high intensity bioturbation, which makes up 30% of the cored interval. The presence of glauconite in the cross-stratified sandstone beds indicate significant resedimentation processes. Multiple sediment input points identified along the shelf edge to the east indicate that the lower and upper intervals were supplied by different source areas. The facies of the lower interval indicates an environment with significant sediment bypass, suggesting a westward-facing slope to base-of-slope environment transverse to inherited structures, with the presence of glauconite reflecting reworking of shallow marine strata from paleoshelf areas. In contrast, the deep-water facies in the upper interval suggest a weakly confined north-to-south axial basin floor fan system. This transverse to axial change in sediment routeing of a deep-water system is interpreted to be driven by the transgression of the lateral basin margin and is vital to refine prediction of reservoir type and quality.