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Clay Minerals; March 2000; v. 35; no. 1; p. 135-149
© 2000 Mineralogical Society of Great Britain and Ireland
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Research Paper

Fracturing, overpressure release and carbonate cementation in the Everest Complex, North Sea

D. M. CONYBEARE1,* and H. F. SHAW2

1 Baker Atlas Geoscience, Kettock Lodge, Campus 2, Science and Technology Park, Balgownie Drive, Aberdeen AB22 8GU, and 2 T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, London SW7 2BP, UK

* E-mail: david.conybeare{at}bakeratlas.com

(Received 16 July 1998; revised 18 March 1999)

The Palaeocene Everest Complex, located in the Central Graben of the North Sea, comprises interbedded sandstones and mudrocks deposited in a submarine fan environment and overlies Cretaceous limestones. Depositional facies are the main influence on reservoir quality, although the latter is reduced significantly locally by ferroan calcite cementation, particularly in the lower part of the complex. Carbon and O stable isotope analyses have been made from cements in sandstones and mudrocks and from shear fractures and injection features within mudrock intervals. These indicate a marine source of detrital carbonate becoming mixed with HCO3– from decarboxylation higher in the succession. Petrographic observation and microprobe analyses indicate that dissolution of coccoliths and foraminifera in limestones are the principal sources of cementation. The similar composition of cements in sandstones, fractures and injection features suggests a common formation. Burial history modelling shows cement precipitation is synchronous with periods of excess pressure and therefore excess pressure release associated with rock failure is proposed as a prominent mechanism for initiating cementation.

KEYWORDS: overpressure, Everest Complex, North Sea, carbonate cement






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