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Core and seismic observations of overpressure-related deformation within Eocene sediments of the Outer Moray Firth, UKCS

¹ Ysgol Greenhill, Heywood Lane, Tenby, Pembrokeshire SA70 8BN, UK
² Earth Sciences and Engineering, Royal School of Mines, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2BP, UK

Cores from the Eocene Alba Field of the Outer Moray Firth, UKCS contain sandstone injections and bedding-parallel fibrous ‘beef’ veins. Both of these features are associated with high fluid pressure and the process of hydraulic fracturing. The orientation of the hydraulic fractures (along which sand injection occurred, or calcite precipitated) was controlled by the interplay of the stress field and the intrinsic anisotropy of the sediments. Seismic sections indicate that sand injection occurred on a larger scale than is apparent from the cores. Interpreted dykes (400 m long by 30 m wide) emanate from the margins of the Alba channel sandstone along fault planes. An analogy is drawn between these dykes, and the peripheral dykes formed at the margins of laccoliths as a result of the flexing and subsequent fracturing of the overlying strata. ‘Decompacting’ of ptygmatically folded dykes suggests that the process of hydraulic fracturing and sandstone intrusion initiated between burial depths of 40 to 400 m below seabed.

KEYWORDS: Eocene, North Sea, hydraulic fracturing, sedimentary structure

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