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Abstract

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Implications of Fault Scarp Degradation for Brent Group Prospectivity, Ninian Field, Northern North Sea1

J. R. Underhill,2 M. J. Sawyer,3 P. Hodgson,3 M. D. Shallcross,3 and R. L. Gawthorpe4

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ABSTRACT

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


┬ęCopyright 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.

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