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Geological Society, London, Special Publications; 1986; v. 23; p. 55-72;
DOI: 10.1144/GSL.SP.1986.023.01.04
© 1986 Geological Society of London

Regional Studies

Palaeozoic Prospects on the Northern Flanks of the London-Brabant Massif

S. R. Tubb, A. Soulsby & S. R. Lawrence

Exploration Consultants Limited, Henley-on-Thames, Oxon, RG9 4PS

Seismic data across the northern flank of the London-Brabant Massif, between the United Kingdom and the Netherlands, have highlighted some interesting exploration leads in the sub-Permian section. Carboniferous sediments have been affected by low-amplitude Hercynian folding, Mesozoic rift faulting and Cretaceous inversion-related compression and fault re-activation. These effects have created a variety of trap situations in potential Palaeozoic reservoir units. In addition to early Permian Rotliegende sandstones, potential reservoirs are formed by late Carboniferous Barren Measures and Millstone Grit sandstones, Carboniferous Limestone carbonates, and basal or pre-Carboniferous clastics. Also Carboniferous Limestone ‘reef’ developments have been tentatively identified along a paleo hinge-line. A significant stratigraphic break is identified near the base of the Barren Measures in the late Carboniferous which has been designated the ‘Symon Unconformity’. The unconformity and depositional patterns within the Barren Measures are explained by the effect of lateral heat flow on normal basin development in the late Carboniferous. An analysis of well log data has demonstrated that Barren Measures’ sandstones constitute a good reservoir objective. Reservoir quality and sand thickness/frequency increase towards the flanks of the London-Brabant Massif.

Westphalian A and B coals subcrop the Symon Unconformity, and together with the Millstone Grit, onlap the Carboniferous Limestone surface on the flanks of the London-Brabant Massif. The coals constitute a proven commercial gas source which is in direct contact with the recognised potential reservoirs. Maturation studies, based on geochemical analysis, demonstrate that gas-generative maturity is largely dependent on Jurassic burial. The source/migration/entrapment model suggests that conditions have been favourable for gas accumulation in Palaeozoic reservoirs contained within rift-related fault-blocks and more subtle stratigraphic traps.