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Geological Controls on the Distribution and Preservation of Shallow and Deep Marine Sandstones in the Upper Jurassic of the Salt-Influenced Central North Sea Rift Basin, UK


The Central North Sea (CNS) Basin on the UK Continental Shelf (UKCS) is characterised by a complex structural framework and stratigraphic architecture as a result of (1) multi-phase rifting, halokinesis and basin inversion regimes, and (2) a gross change in depositional environments from continental (Triassic to Middle Jurassic), through shallow marine (early Late Jurassic) and into deep marine (Late Jurassic to Early Cretaceous). The deposition of Triassic and Jurassic sediments occurred when rifting and halokinetic processes were most active. Added to this complexity was the interaction between local uplift and subsidence and fluctuations in relative sea-level. A regional study of the CNS Basin covering most of the UKCS, involving 3D seismic, wireline well logs, biostratigraphy and conventional core data, has been undertaken in order to understand the influence of the aforementioned controlling factors on the deposition and distribution of the Upper Jurassic sediment. This has involved unravelling the interaction between coastal plain (Pentland Formation), shallow marine (Fulmar Formation) and deep marine (Heather and Kimmeridge Clay formations) depositional systems at regional-, sub-regional- and field-scales. Tectono-stratigraphic models to explain the evolution of the basin are underpinned by maps of the main, seismically-defined surfaces that bound the Mesozoic sections within a 3D seismic volume covering an area of more than 22,000 km2. Depth and isochron maps define a highly compartmentalised basin that can be divided into several tectono-stratigraphic domains, each displaying abrupt changes in the relative interaction between rift-dominated and salt-dominated tectonics. In the Jurassic this is manifested by changes in sedimentary facies, vertical stacking patterns and 3D stratigraphic architecture. Sedimentary facies analysis, based on integrated core and wireline log data, have help to classify the different tectono-stratigraphic domains in terms of their different coastal-shallow marine stratigraphic stacking patterns. Results show that the various domains can be related, initially, to the underlying Permian structural grain, and, subsequently, to both the style of salt deformation and proximity and displacement behaviour of Mesozoic extensional faults. The domain classification offers a framework that can aid the prediction of shallow marine reservoir occurrence within a structurally- and stratigraphically-complex basin.