Geology and petroleum prospectivity of the Larne and Portpatrick basins, North Channel, offshore SW Scotland and Northern Ireland

The Larne and Portpatrick basins, located in the North Channel between SW Scotland and Northern Ireland, have been the target of a small programme of petroleum exploration activities since 1971. A total of five hydrocarbon exploration wells have been drilled within the two basins, although as of yet no commercial discoveries have been made. The presence of hydrocarbon shows alongside the discovery of two good-quality reservoir-seal couplets within Triassic and underlying Permian strata has encouraged exploration within the region. The focus of this study is to evaluate the geology and hydrocarbon pro spectivity of the Portpatrick Basin and the offshore section of the Lame Basin. This is achieved through the use of seismic reflection data, and gravity and aeromagnetic data, alongside sedimentological, petrophysical and additional available datasets from both onshore and offshore wells, boreholes and previously published studies. The primary reservoir interval, the Lower-Middle Triassic Sherwood Sandstone Group (c. 600-900 m gross thickness), is distributed across both basins and shows good to excellent porosity (10-25%) and permeability (10-1000 mD) within the Larne Basin. The Middle-Late Triassic Mercia Mudstone Group should provide an excellent top seal where present due to the presence of thick regionally extensive halite deposits, although differential erosion has removed this seal from the margins of the Larne and Portpatrick basins. The Carboniferous, which has been postulated to contain organic-rich source-rock horizons, as inferred from their presence in adjacent basins, has not yet been penetrated within the depocentre of either basin. There is, therefore, some degree of uncertainty regarding the quality and distribution of a potential source rock. The interpretation of seismic reflection profiles presented here, alongside the occurrence of hydrocarbon shows, indicates the presence of organic-rich pre-Permian sedimentary rocks within both basins. 1D petroleum system modelling of the Larne-2 borehole shows that the timing of hydrocarbon generation and migration within the basins is a significant risk, with many traps post-dating the primary hydrocarbon charge. Well-failure analysis has revealed that trap breach associated with kilometrescale uplift events, and the drilling of wells off-structure due to a lack of good-quality subsurface data, have contributed to the lack of discoveries. While the Larne and Portpatrick basins have many elements required for a working petroleum system, along with supporting hydrocarbon shows, the high risks coupled with the small scale of potential discoveries makes the Portpatrick Basin and offshore section of the Larne Basin poorly prospective for oil and gas discovery.