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The Devonian Orcadian Basin in northern Scotland belongs to a regionally linked system of post-Caledonian continental basins extending northwards to western Norway and eastern Greenland. Extensional fault systems that cut the Orcadian Basin sequences are commonly assumed to be Devonian, with some limited inversion and reactivation proposed during the Carboniferous and later times. We present a detailed structural study of the regionally recognized fault systems exposed in the Dounreay area of Caithness, which host significant amounts of authigenic mineralization (carbonate, base metal sulphides, bitumen). Structural and microstructural analyses combined with Re–Os geochronology have been used to date syndeformational fault infills (pyrite) suggesting that faulting, brecciation and fluid flow events are likely to have occurred during the Permian (267.5 ± 3.4 [3.5] Ma). Stress inversion of fault slickenline data associated with mineralization suggest NW–SE regional rifting, an episode also recognized farther west in Sutherland. Thus a dominant set of Permian age brittle faults is now recognized along the entire north coast of Scotland, forming part of the regional-scale North Coast Transfer Zone located on the southern margin of the offshore West Orkney Basin. Supplementary material: Onshore and offshore fault and fracture lineament data are available at https://doi.org/10.6084/m9.figshare.c.2182433 .

Abstract. Fracture attribute scaling and connectivity datasets from analogue systems are widely used to inform sub-surface fractured reservoir models in a range of geological settings. However, significant uncertainties are associated with the determination of reliable scaling parameters in surface outcrops. This has limited our ability to upscale key parameters that control fluid flow at reservoir to basin scales. In this study, we present nine 1D-transect (scanline) fault and fracture attribute datasets from Middle Devonian sandstones in Caithness (Scotland) that are used as an onshore analogue for nearby sub-surface reservoirs such as the Clair field, west of Shetland. By taking account of truncation and censoring effects in individual datasets, our multiscale analysis shows a preference for power-law scaling of fracture length over 8 orders of magnitude (10−4 to 104 m) and kinematic aperture over 4 orders of magnitude (10−6 to 10−2 m). Our assessment of the spatial organization (clustering and topology) provides a new basis for up-scaling fracture attributes collected in outcrop- to regional-scale analogues. We show how these relationships may inform knowledge of geologically equivalent sub-surface fractured reservoirs.

The Devonian Orcadian Basin in Scotland hosts extensional fault systems assumed to be related to the initial formation of the basin, with only limited post-Devonian inversion and reactivation. However, a recent detailed structural study across Caithness, underpinned by published Re–Os geochronology, shows that three phases of deformation are present. North–south- and NW–SE-trending Group 1 faults are related to Devonian ENE–WSW transtension associated with sinistral shear along the Great Glen Fault during the formation of the Orcadian Basin. Metre- to kilometre-scale north–south-trending Group 2 folds and thrusts are developed close to earlier sub-basin-bounding faults and reflect late Carboniferous–early Permian east–west inversion associated with dextral reactivation of the Great Glen Fault. The dominant Group 3 structures are dextral oblique NE–SW-trending and sinistral east–west-trending faults with widespread syndeformational carbonate mineralization (± pyrite and bitumen) and are dated using Re–Os geochronology as Permian (c. 267 Ma). Regional Permian NW–SE extension related to the development of the offshore West Orkney Basin was superimposed over pre-existing fault networks, leading to local oblique reactivation of Group 1 faults in complex localized zones of transtensional folding, faulting and inversion. The structural complexity in surface outcrops onshore therefore reflects both the local reactivation of pre-existing faults and the superimposition of obliquely oriented rifting episodes during basin development in the adjacent offshore areas.Supplementary material: Stereographic projections of compiled structural data from individual fieldwork localities are available at https://doi.org/10.6084/m9.figshare.c.5115228