Seismic interpretation and generation of key depth structure surfaces within the Devonian and Carboniferous of the Central North Sea, Quadrants 25 – 44 area

This report details the rationale, methodology and results of a regional seismic interpretation of the western margin of the Central North Sea (CNS) area, specifically over the Mid North Sea High area, the offshore extension of the Northumberland Trough and the Forth Approaches area. The aim of the interpretation was to create maps that show the distribution of Palaeozoic basins and highs, and where possible interpret key Devono-Carboniferous surfaces and main structural elements in order to build a tectono-stratigraphic model of the Palaeozoic geology. Some 50,000 line kilometres of predominantly 2D seismic data have been interpreted and tied to key released wells in the study area. The seismic and well data were augmented by donated reports from sponsor companies. A set of 5 depth structure maps of selected Palaeozoic horizons has been produced for the pre-Permian succession. These maps provide a key element to aid assessment of the petroleum prospectivity of the Palaeozoic within the study area. The surfaces, with a grid spacing of 5000 m, give a regional view of the topography of the horizons, and comprise:  Upper Permian Base Zechstein Group;  Lower Carboniferous near Top Scremerston Formation;  Lower Carboniferous near Top Fell Sandstone Formation;  Lower Carboniferous near Top Cementstone Formation; and  Middle Devonian near Top Kyle Limestone Group. The regional structure map of the area constructed for this report and observations made from the seismic data, have been integrated with peer reviewed published information to describe a tectonostratigraphic model for the region (Leslie et. al., 2015). A new pre-Permian subcrop map is presented here that builds on existing publications (Smith,1985a, b; Kombrink et al., 2010) and incorporates all relevant new well penetrations since the previous map was published. The well dataset has been either validated or re-interpreted before being integrated with the new seismic interpretation (Kearsey et al., 2015). Figure 10 in Section 3.3.1 below summarises the regional structures referred to in the general observations listed below. General observations on the structures defined across Quadrants 29, 30, 31, 37, 38 and 39:  The Middle-Upper Devonian basins and highs follow a NW-SE trend across Quadrants 29-30 and 37-38;  Lower Carboniferous sequences (Tournaisian and Visean) are interpreted to be present in depocentres across much of the area covered by Quadrants 29 to 38; wells, mainly drilled on the structural highs, constrain the edge of the Lower Carboniferous basins;  There is a structurally complicated area in the southernmost part of Quadrant 38 which comprises a folded Visean and probably Namurian succession. The structure can be interpreted either as an anticlinal rollover on a low-angle fault, or as a compressional anticlinal fold (see Figure 17 below). The structure trends broadly NNE-SSW, plunging northwards into Quadrant 38.