MESOZOIC ROCKS FROM THE HEBRIDES SHELF AND IMPLICATIONS FOR HYDROCARBON PROSPECTIVITY IN THE NORTHERN ROCKALL TROUGH

Shallow boreholes drilled in the West Lewis and West Flannan Basins (o uter Hebrides Shelf) have proved rocks of probable latest Bajocian-Bat honian to Barremian age, where previously a mainly Permo-Triassic infi ll has been assumed, The full geological succession of the basins rema ins unproved. Although the Permo-Triassic history of the basins may be similar, the rocks encountered suggest that the basins may have dissi milar geological histories during the Jurassic and Early Cretaceous. L ithologies are mainly fine grained, poorly sorted sandstones or organi c-rich mudstones. Some correlation can be made with the' Isle of Skye and the West Shetland area. The sandstones are poor hydrocarbon reserv oirs but the Middle Jurassic and Ryazanian mudstones, although immatur e where drilled, have excellent oil source rock potential and may have generated large amounts of hydrocarbons if they are present at depth in the adjacent northern Rockall Trough.