The evolution of the Dowsing Graben System: implications for petroleum prospectivity in the UK Southern North Sea

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 syndepositional 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 echelon listric growth faults, with oblique secondary planar faults, which sole-out on two main (deep and shallow) décollement 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 the SNS and other evaporite basins where similar graben occur.

A Regional Geological Overview of the Upper Permian Zechstein Supergroup (Z1 to Z3) in the Sw Margin of the Southern North Sea and Onshore Eastern England

“…1) (Grant et al, 2019b). The Zechstein Supergroup however was in general not affected by this structural uplift, as the faults which controlled the structural reorganization of the Sole Pit High sole-out in the Triassic or at the very top of the Upper Permian without penetrating the Zechstein (Grant et al, 2019b).…”

“…Offshore, the Sole Pit High, a Cretaceous -Cenozoic inversion structure to the east of the Dowsing Fault Zone, has resulted in the exposure of Jurassic and Triassic strata on the seafloor to the east of the Excalibur field (Fig. 1) (Grant et al, 2019b). The Zechstein Supergroup however was in general not affected by this structural uplift, as the faults which controlled the structural reorganization of the Sole Pit High sole-out in the Triassic or at the very top of the Upper Permian without penetrating the Zechstein (Grant et al, 2019b).…”

Interactions between deep-water gravity flows and active salt tectonics

“…1; e.g., Hodgson et al 1992;Prather et al 2012;Oluboyo et al 2014). Salt-tectonic deformation influences over 120 basins globally (Hudec and Jackson 2007), including some of the world's largest petroleum-producing provinces (e.g., Booth et al 2003;Oluboyo et al 2014;Charles and Ryzhikov 2015;Rodriguez et al 2018, in press;Grant et al 2019Grant et al , 2020aGrant et al , 2020bPichel et al 2020).…”

Sedimentology, Palaeogeography and Diagenesis of the Upper Permian (Z2) Hauptdolomit Formation on the Southern Margin of the Mid North Sea High and Implications for Reservoir Prospectivity

“…The structural and tectonic history of the northern Southern North Sea (SNS) is long and complex. Structures inherited from Caledonide and later events have been reactivated many times, most notably by Variscan compression and more recent Triassic and Tertiary extension (Grant et al, 2019). There is a dramatic change in structural style between the pre-and post-Zechstein successions.…”

“…Early Tertiary sedimentation marked a change from carbonate to predominantly clastic deposition which continued until the Miocene. Alpine movements in the Oligo-Miocene produced the final phase of basin inversion, resulting in a continued reversal of faults within the pre-Zechstein section and stimulating renewed halokinesis involving Zechstein evaporites (Grant et al, 2019).…”