N.J.P. Smith
British Geological Survey
Keyworth, U.K.
The hydrocarbon prospectivity of Paleozoic rocks in the northern Variscide chain and its foreland has been examined.
A model of the Variscides (Fig. 1) involving an opening and closing Rheic Ocean is preferred to existing models.
Seismic reflection profiles (on which Fig. 2 is based) are interpreted as evidence for the extension that occurred, leading to 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 seating rocks have been removed from a large part of this structure.
Adjacent basins (Oxfordshire basin, Berkshire basin, and Southeast Wales in Fig. 1) therefore offer the best prospects. Untested Silurian shales beneath thick Devonian sediments may lie in the oil window.
OIL, GAS POTENTIAL
Prospects and leads are indicated on several British Geological Survey and commercial seismic profiles.
Drilling depths of about 2 km should be adequate to test the prospects.
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 that of 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 (off the U.K. only).
Thin skinned tectonics and sub-thrust plays are probably restricted to Southwest Wales and east of Kent.
Hydrocarbons are produced from early Paleozoic rocks in deep basins on all the plates (forelands) surrounding the former Paleozoic oceans. Examples include Kaliningrad-Lithuania (Cambrian), Hassi Messaoud, Algeria (Cambrian-Ordovician), Clinton, Ohio (Silurian), and Arbuckle, Oklahoma (Cambrian).
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
