Data from recent exploration campaigns by Elf Exploration UK plc and partners in the Peel and Solway Basins (East Irish Sea) have provided significant new information on the geology and hydrocarbon potential of these previously unexplored areas. In this paper, I present a regional geological framework for the basins based on outcrop studies, potential field data and regional seismic lines. The dominant NE‐SW and NW‐SE structural trends are probably inherited from the Caledonian Orogeny and show evidence of repeated reactivation. Seismic interpretation indicates that the NE‐SW trending faults were important in controlling Early Carboniferous basin development, and that transpressional movement along these faults in response to N‐S compression led to basin inversion during the Variscan Orogeny. New well data indicate that both the Peel and Solway Basins were subjected to major erosion at this time, causing the entire Upper Carboniferous stratigraphy to be removed. Preserved Dinantian facies comprise shallow‐marine sandstones, shales and carbonates in the NE, passing into shallow‐water, high energy carbonates in the SW. A change to E‐W extension during the Permo‐Triassic led to renewed extensional motion on the underlying Caledonian faults, and to the creation of a new system of faults oriented N‐S. The Permian and Triassic sections present in the wells in the Peel and Solway Basins are very similar to those in the numerous exploration wells in the East Irish Sea Basin to the SE. Liassic rocks are partly preserved in both the Peel and Solway Basins. The intra‐basinal highs which separate the basins in this region are thought to have been initiated during Late Cimmerian uplift, and to have been enhanced by Laramide and Alpine phases of uplift. Laramide uplift was also accompanied by the emplacement of WNW‐ESE trending dykes. Regional stratigraphic information has been combined with apatite fission‐track data and vitrinite reflectance data to constrain these phases of uplift. These data suggest that at the present day, the Peel and Solway Basins are isolated remnants of a previously more extensive cover of Mesozoic rocks. The three exploration wells drilled in the Peel and Solway Basins by Elf Exploration UK plc tested valid Triassic fault‐block traps, but failed to encounter any significant hydrocarbon shows. Variable, but generally good quality reservoirs exist at the top of the Sherwood Sandstone Group. The main control on reservoir quality is the primary sedimentological character of the sandstones, with cementation and compaction locally reducing quality. Over most of the area, evaporites and shales in the Mercia Mudstone Group form a viable top and lateral seal. The main factor preventing development of a viable petroleum system appears to be the absence of significant Carboniferous source rocks.

Compared with the prolific success of the Triassic play in the East Irish Sea Basin (EISB) the lack of hydrocarbon discovery in neighbouring Permo-Triassic basins of the Irish Sea has been an enigma. However, recent exploration of the Peel, Solway and Central Irish Sea basins has provided new insights into the geology of these basins and the controls upon hydrocarbon prospectivity in the Irish Sea area. Regional seismic interpretation suggests that 12 of the 15 exploration wells drilled in the basins adjacent to the EISB tested valid structural closures at top Triassic reservoir level. Re-evaluation of the Irish Sea petrolemn system reveals that, although effective reservoirs occur in the Lower-Middle Triassic Ormskirk Sandstone Formation, and evaporites in the Middle-Upper Triassic Mercia Mudstone Group provide a regional top seal, the major factor controlling hydrocarbon prospectivity is the limited presence of effective source rocks in the underlying Carboniferous section. A further control upon prospectivity is the timing of hydrocarbon migration, from those areas where Carboniferous source rocks were deposited and preserved. The Namurian basinal marine oil-and gas-prone shales, which form the principal source of hydrocarbons for the Triassic play in the EISB, are restricted to an east-west fairway extending from the EISB into the Kish Bank Basin. Rocks of this age are absent from the Peel and Solway basins as a result of Variscan uplift and erosion. However, palaeogeographical reconstructions based on well and outcrop data suggest that, even if preserved, the depositional environment was not conducive to the formation of marine oiland gas-prone source rocks. Well and seismic data suggest that rocks of Namurian age were not deposited in the Central

No abstract