Basement reservoir plumbing: fracture aperture, length and topology analysis of the Lewisian Complex, NW Scotland

K. J. W. McCaffrey, R. E. Holdsworth, J. Pless, B. S. G. Franklin and K. Hardman
Journal of the Geological Society, 177, 1281-1293, 22 July 2020, https://doi.org/10.1144/jgs2019-143

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Figures

  • Fig. 1.
    Fig. 1.

    Map of the NW UK continental shelf showing location of fields, prospects, top basement depth map offshore and onshore crystalline basement exposures.

  • Fig. 2.
    Fig. 2.

    Lineament interpretation for well-exposed parts of the mainland and Hebrides basement of NW Scotland. Outcrop fracture sample sites are labelled and shown in blue (Hebrides), light green (mainland – Rhiconich Terrane) and dark green (Assynt Terrane). Summary rose diagrams of fracture orientations for the mainland and Hebrides. Inset map shows Clair Field (outline of Clair first development phase in black line) with lineaments from Pless (2012). Underlying onshore geology from BGS 1:625,000 geology map. Main units include Neoarchaean with/without Paleoproterozoic (Laxfordian) overprint: A = intermediate to granitic gneiss (Lewisian), Paleoproterozoic: Z = felsic intrusive rocks, Zm = Mafic intrusive rocks, Zs = metasedimentary rocks, M = Moine metasediments, Mesoproterozoic: S = Stoer Gp, Neoproterozoic: T = Torridonian, CO = Cambro-Ordovician sedimentary rocks, OS = Ordovcian/Silurian alkaline syenite, F = fault rocks (mylonites, cataclasites and pseudotachylytes), PT = Permo-Triassic sedimentary rocks. Major structures are labelled – KLB F = Kinlochbervie Fault.

  • Fig. 3.
    Fig. 3.

    Typical basement fracture types and fills. (a) Closely spaced laterally and vertically extensive jointing in granitic gneiss Lewisian basement, Uyea, Shetland (see Kinny et al. 2019). Note that later Devonian-age dykes have exploited these well-developed joint systems. (b) Composite carbonate veins cutting mafic gneisses, Traigh Dhail Mor, Isle of Lewis (1.2 km SW of Dail Beag, see Fig. 2). Note large open vug (V). (c) Cross-sectional view of part of a c. 30 m-wide fissure filled with chaotic millimetre- to metre-sized angular clasts of basement, and possible red sediment locally cemented by carbonate. Age of fill uncertain, but note that the contact with the wall rock has been exploited by a Cenozoic basalt dyke, suggesting that the breccia is likely Mesozoic in age. Traigh Dhail Mor, Isle of Lewis. (d) Close-up view of crudely laminated nature of the fill at Traigh Dhail Mhor suggesting an element of water-lain deposition. (e) Fissure filled with chaotic collapse breccia where the matrix is cataclasite and pseudotachylyte, Canisp Shear Zone, Achmelvich. Note that in this case, the development of the dilational cavity is thought to be related to seismogenic slip events along the well-developed foliation in the wall rocks at depths >5 km (see Hardman 2019 for details). (f) Foliated multicoloured gouges and breccias from the core of the Seaforth Fault, a major N–S Mesozoic normal fault with kilometre-scale offsets that cuts the Isle of Lewis (Fig. 2; see Franklin 2013 for details).

  • Fig. 4.
    Fig. 4.

    Fracture aperture intensity data for: (a) mainland Scotland; (b) Hebrides; and (c) Clair basement. The grey polygon highlights the same Fracture Intensity/Aperture space with a slope of −1 and is shown for comparison in each plot. Orange bars show comparative fracture intensity ranges for 10 mm aperture fractures as discussed in text. Data from locations that sample Mesozoic structures on the Hebrides include Garrabost, Memorial Cairn, Pabail, Seisadar and Tolstadh.

  • Fig. 5.
    Fig. 5.

    Two measures of fracture length intensity scaling. (a) Fracture lengths intersected in 1D samples plotted on a multi-scale diagram from mainland outcrops and the Clair top-basement seismic attribute map. (b) The intensity of fractures per unit area (m) is shown for 2D length data from window samples taken across mainland, Hebrides and Clair seismic attribute and topographic maps.

  • Fig. 6.
    Fig. 6.

    Plot of Coefficient of variation (Cv) v. Fracture Intensity for outcrop, mesoscale (virtual model) and regional (lineament maps) datasets from mainland (Assynt and Rhiconich), Hebrides and Clair.

  • Fig. 7.
    Fig. 7.

    Fracture topology results from Clair drill core samples, the greater Rona Ridge, and the Assynt Terrane (outcrops and regional lineament samples). Examples of the three scales sampled are shown: regional scale; outcrop scale; and core scale.

  • Fig. 8.
    Fig. 8.

    Conceptual model for fracture systems and their attributes developed in an uplifted basement block (see also Holdsworth et al. 2020a). Cartoon logs A’-A’ and B-B’ correspond to two hypothetical, horizontally deviated wells drilled through the block at different structural levels or through their onshore analogue equivalents exposed in outcrop.

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