Although the majority of the oil reserves
contained in the Ninian field are being produced from tilted Middle Jurassic
sandstones of the Brent Group, significant volumes of oil have also been
discovered within a poorly documented, subtle structural trap that lies
in the immediate footwall to the field's controlling, eastern boundary
fault. Integration of well data and the interpretation of seismic surveys
allow the definition of an area characterized by major footwall degradation.
The data demonstrate that the 1.25-mi (2-km) wide area affected lies to
the east of the zone of Brent Group truncation, along the whole 15-mi (25-km)
length of the field. A combination of sedimentary and stratigraphic information
shows that the degradation and products obtain a thickness that locally
exceeds 450 ft (126 m). The degradation process appears to have been triggered
by accelerated extension during the Callovian-early Oxfordian deposition
of the Heather Formation. The development of pronounced fault terraces
due to footwall collapse
©Copyright
1997. The American Association of Petroleum Geologists. All rights reserved.
1Manuscript
received February 26, 1996; revised manuscript received August 12, 1996;
final acceptance December 24, 1996.
2Department
of Geology and Geophysics, The University of Edinburgh, Kings Buildings,
West Mains Road, Edinburgh, EH9 3JW Scotland; e-mail: [email protected]
3Chevron
(UK) Ltd., Ninian House, Crawpeel Road, Aberdeen, AB12 3LG Scotland.
4Department
of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13
9PL England.
We
would like to thank the Ninian Partners [Chevron UK Ltd., Murphy Petroleum
Ltd., Oryx UK Energy Company, Sands Petroleum (formerly NESTE North Sea
Ltd.), Sun Oil Britain Ltd., and Ranger Oil] for permission to publish
this article. However, the views expressed here are ours and are not necessarily
those of Chevron or the Ninian Partnership. Help was received from Rick
Davis, Robert Heming, Bryan Man, Bill Higgs, Andrew Harding, and John Connor
of Chevron. Gerry White is thanked for drafting the diagrams. Interpretations
carried out using GeoQuest ies and iesx software mounted on Edinburgh's
Seismic Workstation facility were supported by The Petroleum Science and
Technology Institute (PSTI), Norsk Hydro, Shell Expro, and Esso Exploration
and Production (UK) Ltd. The study forms part of a wider research program
studying the occurrence and effects of fault scarp degradation and subtle
stratigraphic trap formation in the Brent province currently being supported
at The University of Edinburgh by a Natural Environmental Research Council
(NERC) Realising Our Potential Award (ROPA) Grant, GR3/R9521. Finally,
we would like to acknowledge AAPG reviewers Alan Roberts and J. Brint for
helpful and constructive comments.
