Myth or Fact? Just What Role did Strike-Slip Play in Controlling Trap Formation and Gas Prospectivity in the UK Southern North Sea?

John R. Underhill
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 leading to separate deformation styles affecting the Mesozoic and Cenozoic sedimentary overburden and the sub-salt levels.
Interpretation of a 3-D seismic survey acquired by WesternGeco, 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, evaluate and demystify 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 debunk myths surrounding the tectonic evolution of this highly prospective basin and allow 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 its all-important migration history.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece