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A new lithostratigraphy and depositional model for the Upper Rotliegend of the UK Sector of the Southern North Sea

Abstract The lack of a pragmatic basin-wide lithostratigraphic subdivision for the Upper Rotliegend of the Southern North Sea Basin has presented numerous problems when attempting to correlate wells or implement reservoir zonation schemes within this important gas province. This deficiency has also made it extremely difficult to predict facies distribution, and thus reservoir quality, when generating or assessing new prospects. With this in mind, a detailed study of the cores and logs from 126 released wells from the UK sector of the gas province was undertaken. The results of this study suggest that it is possible to subdivide the Upper Rotliegend into five lithostratigraphic units (Units 1–5 from oldest to youngest) based on the recognition of drying-upward cycles. These drying-upward cycles are developed throughout the basin in both the Leman Sandstone and Silverpit Claystone Formations. They are considered to reflect long term fluctuations in the Rotliegend climate and thus they have important chronostratigraphic implications. In any given well the numbers of drying-upward cycles (or alternatively lithostratigraphic units) actually developed, the nature of their bounding surfaces and their component lithofacies are controlled by the position of that well within the depositional basin. Utilizing the new subdivision as a stratigraphic framework it has been possible to construct detailed lithofacies maps for the five units. These maps have revealed the complex tectonic and sedimentary evolution of the Rotliegend basin and they have also suggested a new depositional model for the fluvial facies association. The tectonic evolution of the basin has been traced by analysing the spatial and temporal distribution of the various units and their component facies. In areas such as the Sole Pit, where the onset of sedimentation was contemporary with early basin rifting, all five units (Units 1–5) can be recognized. However, in those areas where sedimentation was delayed due to progressive tectonic evolution (e.g. back-stepping of the Dowsing Fault Zone), one or more of the earlier units may be absent. The later post-rift sequences (Units 4 & 5) display the greatest areal distribution within the basin.