A reappraisal and 3D characterisation of fracture systems within the Devonian Orcadian Basin and its underlying basement: an onshore analogue for the Clair Group

Abstract

The Orcadian Basin is a Devonian (Old Red Sandstone) sedimentary basin formed as a result of extensional tectonics after the end of the Caledonian Orogeny in onshore-offshore northeast Scotland. The Clair oil field lies in a smaller basin with similar types of continental sedimentation and it represents the largest remaining oilfield in the UKCS. Oil is found within Devonian-Carboniferous red beds of the Clair Group directly overlying crystalline basement rocks of the Rona Ridge. Recent work has shown that Lewisian Complex exposed in NW Scotland is excellent for assessing the nature and importance ofthe Rona Ridge basement fracture network. Here, geological evidence suggests that the Devonian rocks of the Orcadian Basin are also a suitable analogue for the Clair cover sequences to some degree and that a reappraisal of the deformation history of the Orcadian Basin is necessary to enhance the understanding of spatial and temporal characterization of
structures in the subsurface.
Faults, fault rocks, associated mineralization and deformation allow differentiation of three groups of structures within the Devonian Orcadian Basin: i. N-S, NW-SE, WNW-ESE trending faults showing little or no carbonate mineralisation (Group 1); ii. Metre- to kilometre-scale N-S to NE-SW trending folds and thrusts related to a highly heterogeneous regional inversion event, recognized locally throughout the field area, but especially on Orkney (Group 2); iii. dextral oblique NE-SW trending faults and sinistral E-W trending faults with widespread syn-deformational carbonate mineralisation (± base metal sulphides and bitumen) both along faults and in associated
mineral veins (Group 3). Localized folds are associated with Group 3 structures due to reactivation of pre-existing faults. Crucially, these later folds are synchronous with carbonate and associated mineralisation events. Re-Os model ages of syn-deformational pyrite in two faults in the Caithness area (Dounreay) are 268.4 ± 4.9 & 266.4 ± 5.2 Ma (Permian). This is consistent with the field observation that Group 3 deformation is synchronous with the emplacement of ENE-WSW-trending lamprophyres of the Orkney Dyke Swarm (ca. 250 Ma based on K-Ar dating). Thus we suggest that Group 3 structures are synchronous with Permian rifting (NW-SE extension) which new faults and locally reactivated earlier structures. It appears that the Devonian rocks of the Orcadian basin were taken through the oil window at this time during a thermal event associated with regional alkaline basic igneous activity in Northern Britain.
Scalability studies from 1D transect analysis show that fracture attributes (length and aperture)in the Devonian in Caithness (Scotland) are well described by a power-law distribution over 8 and 4 orders of magnitude. Also, 2D fracture connectivity is highly variable in the system and
appears to be mainly associated with corridor structures at a large scale and on longer structures at the mesoscale. The onshore dataset shows sub-vertical fault intersections (3D) suggesting that horizontal drill orientation is favoured to vertical if these rocks were to be drilled as a reservoir.