The structural geology of Foula, Shetland: an onshore analogue for the top of the Clair Ridge, west of Shetland.

The Clair Field, situated to the west of Shetland, represents the largest hydrocarbon resource in the UKCS and Europe. It is comprised of fractured Devonian-Carboniferous sandstones that overlie an up-faulted ridge of fractured Precambrian metamorphic basement. The onshore, broadly extensional Orcadian Basin has long been used as an analogue for the Clair Field but it is possible that it formed in a somewhat different tectonic setting, as it lies in a separate basin to the west. On Shetland, evidence of abundant strike-slip events is preserved in both the basement and Devonian-age cover sequences and associated igneous rocks. Indeed, some of the known complexity in the sub-surface structures, and basement-cover relationships can be attributed to deformation associated with this phase of strike-slip tectonics. This suggests that they may have also played a significant role in the evolution of the Clair Basin during the Devonian-Carboniferous period.

Foula, a 13km2 island situated 25km SW of the Shetland Isles is possibly the best onshore analogue to the Clair Ridge which is the second phase development area for the Clair Field. Despite being only ~70km from the Clair Field, the island is relatively poorly studied with regards to its structural evolution. Some 1600m of Middle Devonian sandstones are spectacularly exposed in continuous, kilometre-long cliff sections up to 376m high. These rocks unconformably overlie likely Precambrian-age amphibolite facies basement gneisses and schists intruded by sheeted granites of uncertain age and affinity. Building on earlier studies carried out in the 1980’s, the initial results of an ongoing reappraisal of the structure, stratigraphy and tectonic evolution of the island and surrounding area will be presented. This is being achieved through detailed land-, sea- and aerial-(drone) based studies of exposed basement-cover contacts, and the structure and broad stratigraphic architecture of the overlying sandstone dominated sequences exposed in the continuous coastal sections. Samples of the basement have also been collected for radiometric dating using U-Pb geochronology.

Fieldwork data will be presented, supplemented by the use of photogrammetry to capture the geology of key localities and analyse structures in difficult to access areas. 3D models of the stratigraphy and structure are being created for later use in modelling of stratigraphic architecture, fracture networks, numerical simulations of fluid flow, up-scaling and reservoir quality studies. These findings will be compared to previous analogue studies and applied to the current concepts and models which underpin the stratigraphic and structural architecture of the Clair field and the wider regional tectonic setting.