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Basin Analysis |
Brabant Petroleum Ltd, Suffolk House, 154 High Street, Sevenoaks, Kent TN13 1XE, UK
Permo-Triassic basins, both onshore and offshore UK, typically have good reservoir rocks and frequently good seals, but their continental depositional environment requires that source beds of a different age must be invoked if they are to be prospective. In the Dorset-English Channel Basin, the overlying Liassic shales provide an oil prone source, while in the Southern Gas Basin of the North Sea and the East Irish Sea, the underlying Carboniferous has generated large volumes of gas and some oil. Other Permo-Triassic basins, onshore, are generally devoid of hydrocarbons and this can in large part be explained by an absence of an adequate source rock.
The Cheshire Basin lies adjacent to the East Irish Sea Basin and contains a similar, thick Permo-Triassic sequence. The presence of outcropping Coal Measures on three sides of the Basin gives encouragement as to sourcing potential. However, the six exploration wells drilled in the Basin to date (four during the last ten years) have proved dry.
Review of recent seismic data, particularly mid-late 1980s vibroseis surveys, suggests that the Basin developed as a Permo-Triassic half-graben over a Hercynian inversion. Therefore, as one moves from the western edge towards the basin centre progressive erosion of Westphalian strata below the base Permian event may be observed, culminating in a likely Lower Palaeozoic subcrop in the southeastern part of the Basin. A large, faulted high in the centre of the Basin, in many ways similar to the Morecambe Field structure, was proven dry by the Burford-1 well, apparently due to an absence of an underlying source. Nevertheless, seismic data in the northern part of the Basin, supported by the Knutsford-1 penetration, suggest the presence of underlying Coal Measures. However, regional projections suggest that the thick, Dinantian-Namurian bituminous shales which appear to have sourced the East Irish Sea fields, are absent from much of the Basin.
A further difference between the Cheshire and East Irish Sea Basins is the presence, in the former, of the Tarporley ‘Siltstone’ Formation above the main reservoir objective of the Helsby Sandstone Formation. The former may be regarded as a likely non-sealing waste zone over much of the central and eastern parts of the Basin, but in the SW where the coarser Malpas Sandstone facies is developed, and in the north, it becomes a viable reservoir target in its own right, sealed by the Bollin Mudstone Formation and, ultimately, by the Northwich Halite.
It is therefore proposed that the successful exploration for hydrocarbons in the Basin is likely to depend on finding a combination of sub-cropping Coal Measures and reservoir-grade Tarporley Siltstone, sufficiently deeply buried to retain an effective Mercia Mudstone and preferably halite seal. These conditions appear to prevail in the NW part of the Basin, where the search for suitably large, closed structures is currently in progress.
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