Abstract

Upper Permian (Zechstein Supergroup) evaporites have a major control on structural styles and prospectivity in the UK Southern North Sea (SNS). They form the regional super-seal for the main Rotliegend Group (Leman Sandstone Formation) reservoir play fairway immediately beneath. The evaporites have highly variable thicknesses due to the syn-depositional basin architecture, differential loading and post-depositional deformation through diapirism and salt withdrawal. The halokinetic activity leads to touchdown (welding) of the supra-salt section onto the sub-salt strata and the development of narrow (up to 15 km-wide) graben systems. The interpretation and depth conversion of well-calibrated, high-quality, 3D post-stack time-migrated (PSTM) seismic data along the southwestern margin of the basin show that a NW-SE striking elongate extensional Dowsing Graben System transects the area. The graben is defined by a series of large, overlapping, en-échelon listric growth faults, with oblique secondary planar faults, which sole-out on two main (deep and shallow) decollement levels in the Zechstein Supergroup and the Middle Triassic, Röt Halite Member. Whilst its initial formation was related to Mesozoic extension, the graben system also displays a contractional overprint resulting from regional compression and structural inversion during the Cenozoic. Detailed mapping of the Zechstein Supergroup has revealed that the evolution of the extensional system was influenced by the ESE-WNW striking anhydrite-carbonate Zechstein shelf-margin. The occurrence of variable thickness, low-velocity sediments within the graben impacts seismic imaging and depth conversion leading to prospective structures being overlooked, something that has implications for prospectivity in SNS and other evaporite basins where similar graben occur.

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