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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.; Trice, R.; Hardman, K.; McCaffrey, K.J.W.; Morton, A.; Frei, D.; Dempsey, E.; Bird, A.; Rogers, S.

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


Authors

R. Trice

K. Hardman

A. Morton

D. Frei

E. Dempsey

A. Bird

S. Rogers



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.

Citation

Holdsworth, R., Trice, R., Hardman, K., McCaffrey, K., Morton, A., Frei, D., …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), 1057-1073. https://doi.org/10.1144/jgs2019-142

Journal Article Type Article
Acceptance Date Nov 22, 2019
Online Publication Date Jan 30, 2020
Publication Date 2020-09
Deposit Date Dec 6, 2019
Publicly Available Date Mar 28, 2024
Journal Journal of the Geological Society
Print ISSN 0016-7649
Electronic ISSN 2041-479X
Publisher The Geological Society
Peer Reviewed Peer Reviewed
Volume 177
Issue 5
Pages 1057-1073
DOI https://doi.org/10.1144/jgs2019-142

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