Geochemistry of organic-rich Cretaceous and Jurassic mudstones in the West Lewis and West Flannan basins, Offshore north-west Scotland: Implications for source rock presence in the north-east Rockall Trough

The West Lewis and West Flannan basins serve as important calibration sites for understanding the source rock distribution and quality in the north-eastern part of the Rockall Trough. In the West Lewis Basin, two shallow boreholes penetrated organic-rich, oil-prone Bathonian mudstones which were deposited in a fresh to brackish water environment. These mudstones will generate oils rich in paraffins upon maturation. Organic-rich, oil-prone Ryazanian mudstones were discovered in both the West Lewis and West Flannan basins. They are offshore marine mudstones with a predominance of algal organic matter, akin to the Kimmeridge Clay in the North Sea and basins west of Shetland. The Bathonian mudstones will generate their hydrocarbon products at higher temperatures than the Ryazanian mudstones. The presence of lower Cretaceous and middle Jurassic oil-prone mudstones in basins on the Hebrides Shelf is intriguing and suggests that similar source rocks may be present along the north-eastern margin of the Rockall Trough.