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Geological Society, London, Special Publications; 1996; v. 109; p. 103-120;
DOI: 10.1144/GSL.SP.1996.109.01.08
© 1996 Geological Society of London

Coal as a Reservoir

Coal clasts in the upper Westphalian sequence of the South Wales coal basin: implications for the timing of maturation and fracture permeability

R. A. Gayer1, J. Pesek2, I. Sykorová3 & P. Valterová4

1 Department of Earth Sciences, University of Wales Cardiff, PO Box 914, Cardiff CF1 3YE, UK
2 Faculty of Science, Charles University, Albertov 6, 12843 Prague Czech Republic
3 Institute of Rock Structure and Mechanics, Academy of Sciences, V Holesovickách 41, 18209 Prague 8, Czech Republic
4 Geofond, Kostelní 26, Prague 7, Czech Republic

Coal clasts, common within channel lag deposits within the upper Westphalian C and Westphalian D Upper (Pennant) Coal Measures of the South Wales coal basin, represent reworked previously deposited coal-forming material. Analysis of clast shape, coal petrology and palynology at 16 localities within the eastern part of the South Wales coalfield suggests that three types of clast are present: (1) large elongate rafts of coal, with axial ratios up to 90:1, with irregular, often ‘fish-tail’ terminations and showing post-depositional compaction relative to the enclosing sandstone; (2) rafts of similar dimensions to (1), but showing no evidence for differential compaction and terminated by cleat fractures; (3) small near-equidimensional pebbles that either show no evidence for differential compaction or in which the surrounding sandstone shows greater compaction than the coal pebble. Both types 1 and 2 rafts have miospore assemblage ages that are indistinguishable from coal seams in adjacent sediments and show a similar range of vitrinite-dominated, maceral group composition. The pebbles have miospore assemblages suggesting derivation from coal seams ranging from a similar age to those of the rafts to Westphalian A (Lower Coal Measures) and with a wider range of maceral group composition than the rafts. Type 1 rafts were derived by erosion of partially consolidated and lithified peat deposits from the contemporary alluival plain, and cleat fractures were developed in situ during compaction. They formed as extensional cracks parallel to the regional NNW-SSE Variscan compressive stress. In type 2 rafts and pebbles cleat partly controlled the shape of the clasts and was developed before erosion of the clasts from their source coal seams. Vitrinite reflectance (Rm%) indicates that maturation of the rafts to bituminous coal rank developed in situ within the Pennant Measures, but in the pebbles maturation to bituminous rank may have been developed in the source coal seam before erosion. It is suggested that this maturation, and the concomitant compaction and cleat development, occurred within {approx} 1 Ma of deposition after rapid burial to {approx} 1 km in a high heat flow regime, associated with hot thrust-guided fluids in a foreland basin setting. Rapid maturation of type III kerogen into the oil and gas generation windows, together with the development of cleat, forming effective migration pathways, has important implications for coalbed methane exploration.