Geology and petroleum prospectivity of the Sea of Hebrides Basin and Minch Basin, offshore NW Scotland

Abstract

The Sea of Hebrides Basin and Minch Basin are late Paleozoic–Mesozoic rift basins located to the NW of the Scottish mainland. The basins were the target of small-scale petroleum exploration from the late 1960s to the early 1990s, with a total of three wells drilled within the two basins between 1989 and 1991. Although no commercially viable petroleum discoveries were made, numerous petroleum shows were identified within both basins, including a gas show within the Upper Glen 1 well in Lower Jurassic limestones. Organic-rich shales have been identified throughout the Jurassic succession within the Sea of Hebrides Basin, with one Middle Jurassic (Bajocian–Bathonian) shale exhibiting a total organic carbon content of up to 15 wt%. The focus of this study is to review the historical petroleum exploration within these basins, and to evaluate whether the conclusions drawn in the early 1990s of a lack of prospectivity remains the case. This was undertaken by analysis of seismic reflection data, gravity and aeromagnetic data, and sedimentological data from both onshore and offshore wells, boreholes and previously published studies. The key findings from our study suggest that there is a low probability of commercially sized petroleum accumulations within either the Sea of Hebrides Basin or the Minch Basin. Ineffective source rocks, likely to be due to low maturities (due to lack of burial) and the fact that the encountered Jurassic and Permian–Triassic reservoirs are of poor quality (low porosity and permeability), has led to our interpretation of future exploration being high risk, with any potential accumulations being small in size. While petroleum accumulations are unlikely within the basin, applying the knowledge obtained from this study could provide additional datasets and insight into petroleum exploration within other NE Atlantic margin basins, such as the Rockall Trough and the Faroe–Shetland Basin.