The case for exploration of deep plays in the Variscan fold belt and its foreland

Abstract

The hydrocarbon prospectivity of Paleozoic rocks in the Northern Variscide Chain and its foreland has been examined. A model of the Variscides involving an opening and closing Rheic Ocean is preferred to existing models. Seismic reflection profiles are interpreted as evidence for early Paleozoic extension, which led to the formation of the ocean. In addition to prospects in late Paleozoic rocks, the early Paleozoic rocks, near the Variscan Front and on the foreland, should be targets for exploration. Along strike within the external zones, differences in Westphalian coal maturity indicate higher geothermal gradients, rather than burial effects. Carboniferous rocks reside in both oil and gas windows, north of the Variscan Front. A low maturity area coincides with Variscan uplift of the Worcester and Usk highs. Potential reservoir, source and sealing rocks have been removed from a large part of this structure. Adjacent basins, therefore, offer the best prospects. Untested Silurian shales, beneath thick Devonian sediments, may lie in the oil window. Prospects and leads are indicated on several British Geological Survey and commercial seismic profiles. The foreland area of St George’s Channel–Cardigan Bay basins includes definite Carboniferous rocks. Coals are probably present, combined with Triassic reservoirs, salt seal and basin margin structures. A depth-maturity profile is required before prospectivity can be compared to other productive deep basins. The internide zones of the Variscides are generally overmature, and reached optimum maturity earlier than the externides. Nevertheless, overlying limnic coal basins are prospective, and can be expected within the Saxo-Thuringian zone (offshore UK only). Thin-skinned tectonics and sub-thrust plays are probably restricted to SW Wales and east of Kent. Hydrocarbons are produced, from early Paleozoic rocks in deep basins, on all the other plates (forelands) surrounding the former Paleozoic oceans.