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The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland.

Holdsworth, R.E. and Trice, R. and Hardman, K. and McCaffrey, K.J.W. and Morton, A. and Frei, D. and Dempsey, E. and Bird, A. and Rogers, S. (2020) 'The nature and age of basement host rocks and fissure fills in the Lancaster field fractured reservoir, West of Shetland.', Journal of the Geological Society., 177 (5). pp. 1057-1073.

Abstract

Hosting up to 3.3 billion barrels of oil in place, the upfaulted Precambrian crystalline rocks of the Lancaster Field, offshore west of Shetland, give key insights into how fractured hydrocarbon reservoirs can form in such old rocks. The Neoarchaean (ca 2700-2740 Ma) charnockitic basement is cut by deeply penetrating oil-, mineral-and sediments-filled fissure systems seen in geophysical and production logs, and thin sections of core. Mineral textures and fluid inclusion geothermometry suggest that a low temperature (<200 °C) near-surface hydrothermal system is associated with these fissures. The fills help to permanently prop open fissures in the basement, permitting the ingress of hydrocarbons into an extensive well-connected oil saturated fracture networks. U-Pb dating of calcite mineral fills constrains the onset of mineralization and contemporaneous oil charge to the mid-Cretaceous and later from Jurassic source rocks flanking the upfaulted ridge. Late Cretaceous subsidence and deposition of mudstones sealed the ridge, and was followed by buoyancy-driven migration of oil into the pre-existing propped fracture systems. These new observations provide an explanation for the preservation of intra-reservoir fractures (‘joints’) with effective apertures of two meters or more, thereby highlighting a new mechanism for generating and preserving fracture permeability in sub-unconformity fractured basement reservoirs worldwide.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1144/jgs2019-142
Publisher statement:© 2019 The Author(s). Published by The Geological Society of London http://creativecommons.org/licenses/by/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/)
Date accepted:22 November 2019
Date deposited:06 December 2019
Date of first online publication:30 January 2020
Date first made open access:07 February 2020

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