Effect of Zechstein Supergroup (Z1 cycle) Werrahalit pods on prospectivity in the southern North Sea

Abstract

Mapping of the Werraanhydrit Formation, a component part of the Upper Permian Zechstein Supergroup in the southern North Sea, highlights the profound effect that large, isolated, and hitherto poorly documented halite pods have on the assessment of gas prospects and existing fields in the basin. Seismic interpretation of well-calibrated three-dimensional volumes demonstrates that the pods, ascribed to the Werrahalit Member, occur throughout an approximately 20-km (12-mi)-wide, west-northwest–east-southeast–striking belt that transects the basin. They commonly have a pronounced lensoid cross sectional profile and a northwest–southeast–striking elongate lozenge shape in plan view. Integration of the geometrical evidence with facies and thickness data suggests that the pods resulted from original deposition in an evaporating salina setting. Their occurrence appears to have been influenced in part by topography generated by the effect of compaction above fault systems that had affected the Rotliegend Group deposition. Direct calibration of the halite pods demonstrates that they have a significantly lower velocity than the anhydrite-dominated host in which they are encased. The velocity effect has led to major discrepancies between prognosed and actual depth for deeper, highly prospective, Rotliegend Group, Leman Sandstone Formation gas reservoir targets. Significantly, the identification of, and allowance for, differential velocities has a potential to improve the seismic imaging and the definition of sub-Zechstein traps in the area. The results provide a new-found basis on which to reassess the gas volumes in producing fields (e.g., Leman and Tristan), the economic viability of undeveloped discoveries (e.g., Tristan North West), and the validity of both tested and undrilled prospects.