Tectono-Stratigraphic Evolution of the West Orkney Basin, NE Atlantic Margin: Implications for Hydrocarbon Exploration
Bird, Peter C.; Cartwright, Joe A.
The Moine Thrust belt of the NW Highlands of Scotland is characterised as a classic example of thin-skinned tectonics. However its offshore continuation below the West Orkney rift basin is often described as thick skinned, making offshore-onshore links and understanding of basement-influenced-rifting models complicated. The West Orkney Basin is considered to have potential for a working petroleum system based on the occurrence of Devonian lacustrine source rocks within the "Devonian Orcadian Basin". These source rocks have been proven to have charged the Beatrice Field (Inner Moray Firth) and to have contributed to the Clair Field (West of Shetlands). However, the only two wells to have been drilled in the basin encountered Permo-Triassic rocks, with no evidence for the presence of a Devonian section or a mature Devonian source rock.
A reappraisal on the tectono-stratigraphic evolution of the basin was carried out on a recently reprocessed 2-D seismic dataset. We argue against a crustal-scale thick skinned basement deformational model, and argue here that the recognition of different levels of detachment within the basement instead supports a thin skinned model for the offshore continuation of the Moine Thrust Belt. The geometry and location of post Caledonian rifting can be shown to have been partially controlled by the basement thrust belt. Evidence for the presence of Devonian rifting and deposition is provided by the apparent structural continuity of a syn-rift sequence recognised on seismic data and from Devonian outcrops onshore. Further evidence is given by the identification of an unconformity bounding the top of the syn-rift package, which we relate to late Carboniferous inversion of the Great Glen Fault. Rifting continued from the late Permian to early Tertiary intermittently with periods of uplift and erosion. Integrating the new basin tectono-strat model with outcrop studies of Devonian depositional and petroleum systems, has allowed for mapping and prediction of Devonian source, reservoir and seal distribution in the offshore areas of the basin.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013