Role of Footwall and Hangingwall Release Faults in Accommodating Extensional Strain and Petroleum Prospectivity in the North Sea Rift System

John R. Underhill, Caroline Gill, and Aileen McLeod
The University of Edinburgh, Edinburgh, United Kingdom

Well-calibrated seismic data provides the basis for understanding the development and evolution of normal fault systems in the highly-prospective Northern North Sea. It can now be demonstrated that the Middle-Late Jurassic extensional system was characterised by progressive along-strike propagation and linkage of its component normal fault segments and by a marked basinward shift in the locus of extension through time. Detailed interpretation of fault arrays in the basin highlights the occurrence of numerous syn-sedimentary transverse faults and folds in both hangingwall and footwall locations. The strike-perpendicular normal structures, termed hereafter hangingwall and footwall release faults because of their similarity to structures defined in the Brazilian, Reconcavo Basin, show a systematic decrease in displacement and amplitude away from the master normal faults. Whilst poorly documented, the features appear to be an essential and integral part of the rift system. It is thought that the structures developed as a natural consequence (structural feedback) to normal fault propagation to accommodate variable displacement along the strike of the controlling normal fault. Importantly, the cross-faults are fundamentally different from transfer faults, which appear to play a negligible role in the development of the North Sea rift system. The occurrence of footwall release faults exerts a fundamental control on the position of structural spill points and hence, is a significant control on oilfields size in the basin. Although hitherto largely undetected, it is expected that similar release faults would occur in other rift systems, where their presence could have an equally important effect on petroleum entrapment.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005