Did Strike-Slip or Tectonic Inversion Control Trap Formation and Gas Prospectivity in the UK Southern North Sea?

Underhill, John R.¹ (1) School of Geosciences, The University of Edinburgh, Edinburgh, United Kingdom

A degree of uncertainty has always surrounded the nature and timing of trap-forming events in the highly prospective Southern North Sea Basin (SNS). The long-held view has been that the structural complexity found at the highly prospective Upper Palaeozoic (Carboniferous and Lower Permian (Rotliegend)) levels primarily resulted from strike-slip motion with traps occurring as integral parts of flower structures. Despite the blanket 3-D seismic coverage in the SNS, structural interpretation has proved extremely difficult with imaging of structures at reservoir levels suffering as a result of the presence of a highly mobile Upper Permian (Zechstein Group) evaporites and the resultant detachment of deformation styles within the Mesozoic and Cenozoic sedimentary overburden and the sub-salt levels.

Interpretation of a newly-acquired 3-D seismic survey, strategically located outside the area of Zechstein Group evaporite deposition (i.e. where the deformation between the reservoir levels and overburden is coupled), has provided a unique opportunity to assess and evaluate outstanding structural questions in the basin. Significantly, it can now be shown that tectonic inversion played the predominant role in the structural development of the SNS rather than strike-slip deformation. Furthermore, the contractional reactivation can be shown to have begun in the Late Cretaceous and continued in discrete episodes during the Cenozoic rather than occurring in one phase. The new data allows a unifying structural model to be produced for the SNS, which not only explains the nature of trap formation and the timing of maturation but also provides insights into the basin's all-important migration history.