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

Abstract

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

Data acquisition by digitizing 2-D fracture networks and topographic lineaments in geographic information systems: further development and applications

“…These methods have been used for advanced fracture network analysis and modelling for applications related to fluid flow, gas migration and engineering/construction (e.g. Bisdom et al, 2017;Menegoni et al, 2019;Strijker et al, 2012;Tavani et al, 2016). These 3-D scanning techniques require sophisticated hardware, proprietary software and training that potentially limits their applicability.…”

“…1732 R. Palamakumbura et al: Data acquisition by digitizing 2-D fracture networks rameters at regional scales requires sampling at multiple sites (e.g. McCaffrey et al, 2020). Practical constraints on data collection are therefore critical factors.…”

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

“…Thus in our view it is not possible to separate brittle structures into separate sets of simple tensile and shear fractures. This practical approach ensures comparability with sub-surface structures in Clair cover sequences and related fractured basement studies where similar interlinked mesh systems are dominant (see McCaffrey et al, 2020). One reason for the development of such mesh networks is that many Group 3 structures reactivate earlier (Group 1 and 2) brittle structures and therefore display a variety of hybrid opening modes (Dempsey et al, 2014).…”

“…Fracture attribute distributions are thought to display three main types of statistical distribution (Fig. 4, Bonnet et al, 2001;Gillespie et al, 1993;Zeeb et al, 2013): (a) exponential, random or Poisson distributions are characteristic of a system with one randomized variable (Gillespie et al, 1993); (b) log-normal distributions are generally produced by systems with a characteristic length scale where mechanical stratigraphic boundaries control joint spacing for example (Narr, 1991 andOlson, 2007); and (c) power-law distributions lack a characteristic length scale in the fracture growth process (Zeeb et al, 2013), and the data exhibit scale-invariant fractal geometries ( Fig. 4c bottom).…”

“…However, it is generally accepted that power-law distributions and fractal geometry provide a widely applicable descriptive tool for fracture size attributes such as aperture and length (e.g. Bonnet et al, 2001;Olson, 2007;Hooker et al, 2014;McCaffrey et al, 2020).…”