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Role of Supra-Salt Decoupling on Mesozoic Graben Formation in the UK Southern North Sea

Abstract

The UK Southern North Sea (SNS) basin is a mature and prolific gas province. The main play involves Permian (Rotliegend) clastic reservoirs sealed by a thick Upper Permian sequence of cyclic evaporite and carbonate platform-basinal sequences (Zechstein Group). The ductile Zechstein salt acts as a major structural detachment across the more distal (axial) parts of the basin causing decoupling between block faulted Carboniferous and Rotliegend sub-salt sequences and the locally complex syn-sedimentary extensional grabens in the Mesozoic (supra-salt) section. Towards the margin of the basin, a major contrast in structural styles is seen to affect the respective pre- and post-Zechstein sections as evaporites are replaced by platform carbonate successions leading to structural coupling. Access to, interpretation and depth conversion of 5430 km2 (2097 sq. miles) well-calibrated, high-fidelity, 3D post-stack time-migrated (PSTM) seismic data along the southwestern margin of the basin, in UK Quadrants 47 and 48, has permitted a more regional scale analysis of the relationship between underlying basement structure, Zechstein architecture, and overlying overburden deformation. It can now be shown that a c. 15 km (9 miles) wide NW-SE striking extensional graben system transects the area, defined by a series of large, overlapping, en-echelon listric growth faults, with oblique secondary planar faults that sole onto multiple detachment layers in the Zechstein Group and the Middle Triassic Röt Halite Member. Further mapping of the Zechstein internal and external architecture has revealed the evolution of the extensional system to be influenced by a bisecting a thick ESE-WNW striking anhydrite-carbonate shelf margin. The mapping and structural analysis enable us to propose a hybrid, syn-sedimentary, thick- and thin-skinned tectonic model for the formation and evolution of the narrow graben systems. Whilst their initial formation was related to punctuated Mesozoic rifting and extension within the basin, they also display a significant contractional overprint resulting from regional compression and basement inversion during the Tertiary. The results have significant impact for understanding prospectivity and delineation of buried traps since the narrow grabens contain thick, low velocity sediments, the accurate depth-conversion of which leads to the redefinition of underlying Rotliegend prospects.