The Neoarchean Uyea Gneiss Complex, Shetland: an onshore fragment of the Rae Craton on the European Plate

Trice²,

Hardman

et al. 2020

JGS

Self Cite

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 Neoarchean (c. 2700–2740 Ma) charnockitic basement is cut by deeply penetrating oil-, mineral- and sediment-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 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 2 m or more, thereby highlighting a new mechanism for generating and preserving fracture permeability in sub-unconformity fractured basement reservoirs worldwide.Supplementary material: Analytical methods and isotopic compositions and ages are available at https://doi.org/10.6084/m9.figshare.c.4763237Thematic Collection: This article is part of the Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Section: Discussionsupporting

confidence: 75%

Section: Discussionmentioning

confidence: 87%

Section: ;mentioning

confidence: 98%

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

Trice²,

Hardman

et al. 2020

JGS

Self Cite

“…Neoarchaean orthogneisses of broadly similar composition and age extend north of the Scottish mainland at least as far as the northernmost tip of Shetland (Holdsworth et al 2018;Kinny et al 2019). Equivalent units underlie much of the Faroe-Shetland basin and the ca 200km Rona Ridge, as shown by analyses of basement-penetrating offshore cores (Fig 1; see Ritchie et al 2011).…”

Section: Location and Regional Structurementioning

confidence: 93%

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

McCaffrey

Pless

et al. 2020

JGS

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Upfaulted ridges of Neoarchean crystalline basement rocks formed in the Faeroe-Shetland basin as a consequence of Mesozoic rift processes and are an active target for oil exploration. We carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in NW Scotland (Lewisian Gneiss Complex) as an analogue for the offshore oil and gas reservoirs of the uplifted Rona Ridge basement high. Our analysis shows a power-law distribution for fracture sizes (aperture and length), with random to clustered spacing and high connectivity indices. Regional variations between the Scottish mainland and the Outer Hebrides are recognized that compare directly with variations observed along the Rona Ridge in the Faeroe-Shetland basin. Here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity. More generally, the combination of size, spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective, well-plumbed reservoir systems in their own right.Supplementary material: Additional methods and results are available at: https://doi.org/10.6084/m9.figshare.c.5017139Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

“…Lewisian Gneiss Complex of the Hebridean Terrane of mainland Scotland (Pringle 1970;Flinn et al 1979;Flinn 1988; however, see also Holdsworth et al 2019;Kinny et al 2019). Structurally overlying the Neoarchean Uyea Gneiss Complex is the metasedimentary Sand Voe Group that has been correlated with the early Neoproterozoic Moine Supergroup of the Northern Highlands Terrane (Flinn 1988).…”

Section: Accepted Manuscriptmentioning

confidence: 99%

Late Carboniferous dextral transpressional reactivation of the crustal-scale Walls Boundary Fault, Shetland: the role of pre-existing structures and lithological heterogeneities

Armitage

Watts

et al. 2020

JGS

The Walls Boundary Fault in Shetland, Scotland, formed during the Ordovician–Devonian Caledonian orogeny and underwent dextral reactivation in the Late Carboniferous. In a well-exposed section at Ollaberry, westerly verging, gently plunging regional folds in the Neoproterozoic Queyfirth Group on the western side of the Walls Boundary Fault are overprinted by faults and steeply plunging Z-shaped brittle–ductile folds that indicate contemporaneous right-lateral and top-to-the-west reverse displacement. East of the Walls Boundary Fault, the Early Silurian Graven granodiorite complex exhibits fault-parallel fractures with Riedel, P and conjugate shears indicating north–south-striking dextral deformation and an additional contemporaneous component of east–west shortening. In the Queyfirth Group, the structures are arranged in geometrically and kinematically distinct fault-bounded domains that are interpreted to result from two superimposed tectonic events, the youngest of which displays evidence for bulk dextral transpressional strain partitioning into end-member wrench and contractional strain domains. During dextral transpressional deformation, strain was focused into pelite horizons and favourably aligned pre-existing structures, leaving relicts of older deformation in more competent lithologies. This study highlights the importance of pre-existing structures and lithological heterogeneity during reactivation and suggests the development of a regional transpressional tectonic environment during the Late Carboniferous on the Shetland Platform.