Oil and gas, Mesozoic of Wales

From Earthwise
Jump to navigation Jump to search

From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Since the early 1970s, the Mesozoic sequence in the Welsh offshore basins has been the target for hydrocarbon exploration but, as yet, no significant discoveries have been made. The generation and entrapment of hydrocarbons depends on the sedimentation of an organic-rich source rock, the presence of a reservoir in which the hydrocarbons can accumulate, a suitable seal, and a series of tectonic events that allow the hydrocarbons to be generated after burial and then to migrate into the reservoir.

In the North Celtic Sea Basin, in Irish waters, a gasfield located in Albian–Aptian sandstones at Kinsale Head has been in production since 1979. In the South Celtic Sea and Bristol Channel basins, the equivalent sandstones are only thinly developed and the wells that have been drilled have all proved to be dry. The paucity of organic-rich source rocks in both basins is a major problem. Jurassic, organic rich claystones, which are major source rocks in the North Sea and Irish Sea basins, are not widely developed. Silesian strata with vitrinite organic debris have elsewhere proved a rich source for gas and some oil but, without a suitable cap rock, it is likely that the gas would have escaped during Variscan deformation. The low-grade pervasive metamorphism of the Devonian–Carboniferous strata was proved in a few offshore deep wells indicating that there is little possibility of a hydrocarbon source. Additionally, the tectonic history of both the South Celtic Sea and Bristol Channel basins makes successful hydrocarbon exploration unlikely.

North of the Variscan front, the 3000 m of Upper Palaeozoic strata and 6000 m of Mesozoic and Cainozoic sedimentary rocks in St George’s Channel Basin would seem to be a more likely target for hydrocarbon exploration. Drilling objectives have included sandstones in the Upper Lias (Bridport Sands equivalent) and Middle Jurassic, with Cretaceous green-sands, equivalent to those in the Kinsale Head Gasfield, as secondary objectives. Until now the only gas and oil shows have been poor. However, it is possible that further exploration in St George’s Channel Basin may prove the Sherwood Sandstone to be a prospective reservoir and the overlying Mercia Mudstone Formation halites to be an effective cap rock. The Dragon prospect, off Pembrokeshire, is a Jurassic reservoir, which has proved some oil shows. The potential of Upper Palaeozoic source rocks is likely to be better than in the basins to the south of the Variscan front as the rocks are not metamorphosed. In the East Irish Sea Basin, to the north of Wales, a widespread phase of oil and gas accumulated during late Mesozoic times. Oil-prone source rocks include Dinantian limestone, Namurian shale and Westphalian oil shale, cannel coal and marine bands. Gas-prone source rocks comprise Namurian shale and Westphalian coal and mudstone.

Bibliography[edit]

JACKSON, D I, JACKSON, A A, EVANS, D, WINGFIELD, R T R, BARNES, R P, and ARTHUR, M J. 1995. United Kingdom offshore regional report: the geology of the Irish Sea. (London: HMSO for the British Geological Survey.)

PENN, I E, and EVANS, C D R. 1976. The Middle Jurassic (mainly Bathonian) of Cardigan Bay and its palaeogeographical signifance. Report of the Institute of Geological Sciences, No. 76/6.

TAPPIN, D R, CHADWICK, R A, JACKSON, A A, WINGFIELD, R T R, and SMITH, N J P. 1994. United Kingdom offshore regional report: the geology of Cardigan Bay and the Bristol Channel. (London: HMSO for the British Geological Survey.)