A reassessment of the brittle deformation history, age and attribute analysis from the Orcadian Basin, Scotland: implications for offshore Devonian fractured reservoirs
The onshore Devonian sedimentary rocks of the Orcadian basin host significant localized zones of fracturing, faulting and some folding on all scales. New field and microstructural analyses carried out within the Devonian cover sequences in Scotland and Orkney reveal 3 main groups of structures based on orientation, kinematics and infill.
Group 1 faults trend N-S and NW-SE and display predominantly sinistral strike-slip to dip-slip extensional movements. They form the dominant structures in the eastern part of Caithness, and to a lesser extent on Orkney. Gouges/breccias associated with these faults display little or no mineralization or veining. It is suggested that these structures are related to Devonian ENE-WSW transtension associated with sinistral shear along the Great Glen Fault (GGF) during Orcadian and proto-West Orkney basin formation.
Group 2 structures are closely associated systems of metre- to kilometre-scale N-S trending folds and thrusts related to a highly heterogenous regional inversion event recognized locally throughout the field area, but especially on Orkney. Once again, fault rocks associated with these structures display little or no mineralization or veining. Group 2 features are likely related to late Carboniferous – early Permian E-W shortening associated with dextral reactivation of the GGF.
Group 3 structures are dextral oblique NE-SW trending faults and sinistral E-W trending faults with widespread syn-deformational carbonate mineralisation (± pyrite and bitumen) both along faults and in associated mineral veins. In a few localities (e.g. Brough, Scarfskerry, East Scapa faults) oblique reactivation of large pre-existing Group 1 faults has led to complex zones of localized transpression or transtensional folding, faulting and inversion synchronous with the carbonate and associated mineralisation events. Re-Os model ages of syn-deformational fault hosted pyrite in Caithness yield Permian ages (ca. 267Ma). This is consistent with the field observation that Group 3 deformation is synchronous with the emplacement of ENE-WSW-trending lamprophyres east of Thurso (ca 268-249 based on K-Ar dating). Stress inversion of fault slickenline data associated with mineralization suggest NW-SE regional rifting, an episode also recognized farther west in the Caledonian basement of Sutherland. Thus from St John’s Point to Cape Wrath, Permian age brittle faults dominate the north coast of Scotland, forming part of a regional-scale North Coast Transfer Zone translating extension from the offshore West Orkney Basin westwards into the North Minch Basin.
We investigated fault size scaling in Caithness across 8 orders of magnitude using a combination of 1D and 2D methods. Fracture length was quantified from 10-4 to 104 m scales using remote sensing, outcrop and thin section datasets. The lengths are well described by a power-law distribution with an average exponent value of ~ -1. Results are consistent with previous fracture analyses in similar lithologies from Norway (mesoscale) and offshore datasets in the North Sea (seismic/regional scale). Fracture aperture/width, constrained from 10-6 to 10-2 m scales, shows a similar -1 scaling exponent. Results from 2D box counting and topology analysis on regional to mesoscales confirm self-similarity and reveal locally high fracture connectivity at fault intersections.