Devonian and Carboniferous stratigraphical correlation and interpretation in the Orcadian area, Central North Sea, Quadrants 7 - 22

This report details the stratigraphy and palaeogeography of Devonian and Carboniferous rocks of the northern sector of the UK Central North Sea (the ‘Orcadian Area’) for the 21CXRM Palaeozoic project. The work integrates the lithostratigraphic framework for the Middle Devonian to Late Carboniferous succession in the region of the Mid North Sea High developed in the first phase of the 21CXRM Palaeozoic project (Kearsey et al., 2015), with the Early Devonian to Early Carboniferous succession present in the north Central North Sea. This report describes the stratigraphical correlation of Devonian and Carboniferous strata of the Orcadian Basin and the northern extension of the Forth Approaches into the Witch Ground Graben (Quadrants 7 – 22). A lithostratigraphic framework for the region is presented, building on the work of Cameron (1993) and Marshall and Hewett (2003). For the Devonian strata, the lithostratigraphic framework developed by Marshall and Hewett (2003) is followed. Their study presented a substantial revision of the previous Devonian stratigraphy for the region, particularly related to (1) reassessment of the Devonian - Permian contact and (2) the identification of the Eday Group offshore in the Inner Moray Firth Basin. In this study, onshore outcrops, well data and seismic interpretation (cf. Arsenikos et al., 2016) verify the key elements of the framework of Marshall and Hewett (2003). In addition, well interpretations further define the extent of the stratigraphical units and form the basis of potential source and reservoir horizon palaeogeographic reconstructions for four time intervals within the Devonian succession. For example, the potential source rock of the Orcadia Formation has been interpreted to the north of the Halibut Horst and into the East Orkney Basin, significantly increasing the extent of this unit outside the Inner Moray Firth. A revised lithostratigraphic framework for the Carboniferous strata of Quadrants 14, 15, 20 and 21 is presented. This framework links the Carboniferous succession of the Orcadian Area with equivalent age strata in areas on and surrounding the Mid North Sea High (Kearsey et al., 2015). Well and seismic interpretations have been integrated to better define the extent of Carboniferous units. Regional facies variations for key time intervals in the Early Carboniferous are presented which highlight relationships between the Carboniferous basins of the Witch Ground Graben and Forth Approaches, and those of the Mid North Sea High and adjacent regions to the south. For example, fluvial channel systems have been interpreted within late Visean age coal-bearing, fluvial and lacustrine deposits of the Firth Coal Formation, that are potential feeder systems for Yoredale Formation fluvio-deltaic to marine deposits farther south. This report forms one of a series of outputs from the 21CXRM Palaeozoic project (Orcadian Area) and provides explanatory information for the associated digital datasets (spreadsheet). The Orcadian Area study follows previous work undertaken in the Mid North Sea High area of the Central North Sea (CNS area; Figure 1). Key elements of the regional petroleum geology of the Orcadian area are summarised in an accompanying synthesis report (Monaghan et al., 2016).