Facies and petrographic assessment of Upper Devonian outcrops, Dunnet Head and Orkney, northern Scotland

Abstract

Upper Devonian strata occur in the Central and Northern North Sea and crop out in coastal exposures in northeastern Scotland. The strata comprise continental sandstones and intercalated conglomerates, siltstones and mudstones. The offshore Upper Devonian unit (Buchan Formation) has been proven as a locally important hydrocarbon reservoir; however, the limited core coverage in the North Sea means that the architecture of the Buchan Formation is poorly understood. This study looks at two localities in Caithness and Orkney with excellent Upper Devonian exposures, which are chronologically equivalent to the Buchan Formation. The main aims are to describe the facies present and to investigate the mineralogical compositions and porosity variations using petrographic analyses. The results indicate that the studied outcrops were formed by braided fluvial and aeolian dune deposition. The aeolian sandstones have higher compositional/textural maturity and porosity than the fluvial sandstones. The main control on porosity is facies variation that results in differences in sedimentary structures, grain size and abundance of rock fragments. With similar palaeoclimate, depositional environments, lithologies and petrographies, these outcrops can be considered excellent analogues for the Buchan Formation reservoirs in the North Sea.