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

¹ Grant Institute of Earth Science, School of Geosciences, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh, EH9 3JW, Scotland, United Kingdom; jru{at}staffmail.ed.ac.uk
² Grant Institute of Earth Science, School of Geosciences, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh, EH9 3JW, Scotland, United Kingdom

John Underhill holds a B.Sc. degree in geology from Bristol University and a Ph.D. from the University of Wales. He worked for Shell International before moving to the University of Edinburgh in 1989, where he holds the Chair of Stratigraphy. John has been awarded the European Association of Petroleum Geoscientists Distinguished Lecturer Award twice, has won the AAPG Matson Award, and has also been an AAPG Distinguished Lecturer. John's current primary research focus is on understanding the tectonic development and evolution of prospective sedimentary basins.

Kirsten Hunter holds a B.Sc. degree in geophysics from the University of Edinburgh and is now undergoing a Ph.D. there.

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.