A New Appraisal of the Western Approaches Basins: Mesozoic Epeirogeny Modulated by Crustal Inheritance, and its Implications for Hydrocarbon Prospectivity, Offshore SW Britain

NAWAB (New Appraisal of the Western Approaches Basins) is one of three Oil & Gas Authority (OGA) Frontier Basin Research Programme postdoctoral projects. NAWAB investigated the hydrocarbon prospectivity of the South Celtic Sea and Western Approaches basins. The NAWAB study area (Fig. 1) separates Irish and French waters that include the North Celtic Sea and Brittany basins, respectively. The project adopted a “geology without frontiers” philosophy involving collaboration with the Irish Petroleum Affairs Division and – via OGA – the French authorities, in order to compile a dataset that transcends political boundaries. The project ran between June 2016 and June 2018, coinciding with the release of the OGA’s SW Britain seismic dataset. The NAWAB consortium includes academic and industry partners, comprising the universities of Durham, Keele and Leicester with Akeman Exploration Ltd, APT (UK) Ltd, Geospatial Research Ltd and Trace Editors Ltd. The consortium’s diverse expertise includes basin analysis, seismic interpretation, geodynamics, petroleum systems analysis, sedimentology, stratigraphy, structural geology, tectonics, GIS, and the regional geology of SW Britain. In addition to the main postdoctoral research project, members of the NAWAB team co-supervised four associated Masters-level students and an ERASMUS+ Trainee.

The NAWAB area is approximately 92,000 km2 (similar to the Southern North Sea), but has less than 30 well penetrations and does not include any fields or discoveries. Fig. 2 shows a chronostratigraphic correlation panel for the Melville Basin (part of the Western Approaches). There are multiple unconformities, with the largest stratigraphic gap being associated with the Early Cretaceous (“Cimmerian”) unconformity. This stratigraphic hiatus diminishes to the north and south of the Cornubian Massif. A correlative conformity of Berriasian (earliest Cretaceous) age is recognised within the North Celtic Sea and Brittany basins (McMahon & Turner 1998). The largest stratigraphic gap occurs in the Goban Spur, where Devonian rocks subcrop the unconformity. In the Western Approaches, the “Cimmerian” unconformity is clearly visible in the OGA seismic reflection data as an angular unconformity that truncates the mainly Permo-Triassic to Lower Jurassic (Liassic) subcrop. In the Melville Basin, seismic 19 reflectors in the lowermost part of the Liassic succession display a characteristically “stripy”, typically tramline character, consistent with deposition in a uniformly-subsiding basin. The uppermost part of the Liassic succession is preserved in the South Celtic Sea Basin, where it appears to be sub-parallel with the overlying Middle Jurassic succession. The latter thickens across major faults, consistent with active rifting in the South Celtic Sea Basin at this time.

Legacy geochemistry reports and new pyrolysis data show that the Lias includes some high quality source rock intervals; however, vitrinite reflectance analyses indicate that it is generally immature to marginally mature where penetrated. Thus, the principal risks for hydrocarbon exploration in the Western Approaches are the preservation and maturity of the Lower Jurassic source rocks, and the timing of generation with respect to trap formation. The impact of timing is perhaps demonstrated by the dry Molly Malone prospect (Irish well 49/30-1), which we interpret as having tested a Cenozoic inversion structure. Assessing these risks requires knowledge of the geological and geodynamical processes that gave rise to Early Cretaceous (“Cimmerian”) exhumation in the Western Approaches area.