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

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 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

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

“…In recent related work, Holdsworth et al (2019a;2019b) report structures and textures from offshore fracture fills that reveal the widespread development of steeply inclined to sub-vertical, riftrelated tensile fissures in the basement lithologies of the Rona Ridge. They suggest that near-surface fissuring during rift-related faulting, as seen in modern rift systems -such as those exposed in Iceland (Kettermann et al 2019) -allowed pervasive influx of clastic sediment fills from above and hydrothermal mineral fills from below.…”

“…Conceptual model for fracture systems and their attributes developed in an uplifted basement block (see alsoHoldsworth et al 2019b). Cartoon logs A'-A' and B-B' correspond to 2 hypothetical, horizontally Revised 21/05/2020 31 deviated wells drilled through the block at different structural levels or through their onshore analogue 816 equivalents exposed in outcrop.…”

Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs

“…5c,d), and fills of younger porous sediment all have the ability to act as natural proppants that hold fractures open in the long term and counteract the tendency for the present day stress field to close open fracture networks in sub-surface reservoirs ( Fig. 5a; Holdsworth et al 2019Holdsworth et al , 2020. These fracture fills will however reduce permeability dramatically from the 'cubic law' relationships of ideal parallel-sided open fractures (Nelson, 1985, Laubach, 2003.…”

Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs

“…Laubach et al, , 2019 which describe fracture-fill characteristics (e.g. Holdsworth et al, 2019Holdsworth et al, , 2020.…”

“…scan lines and transects), fracture attributes have been investigated in different tectonic contexts and lithologies (e.g. Baecher, 1983;Gillespie et al, 1993;McCaffrey and Johnston, 1996;Knott et al, 1996;Odling et al, 1999;Bour et al, 2002;Manzocchi, 2002;Olson, 2003;Kim and Sanderson, 2005;Gomez and Laubach, 2006;Hooker et al, 2009;Torabi and Berg, 2011). In these studies, the statistically best constrained data tend to be acquired at a single scale, for example an outcrop or a well core.…”