The devolatilization of coal seams in South Wales

Abstract

In South Wales there is a pronounced regional devolatilization of the coals towards a major thrust—the Main Thrust—which is observable immediately beyond the north-western margin of the coalfield. There is also a subsidiary regional devolatilization towards the Gower zone of compression, but this is apparent only on the southern rim of the coalfield. In South Wales, as well as devolatilization in a horizontal plane, there is marked devolatilization of coal seams with depth. This devolatilization in a vertical plane is towards a plane of shear or of stress (inferred to be the underground extension of the Main Thrust) and is demonstrated by the trend and by the values of the coal-seam isovols. From the trend of the isovols it is deduced that at depth the plane dips gently, approximately towards the south-south-east. The proposition is propounded that the volatile content of a South Wales seam (dry ash-free basis) expressed as a percentage of the original volatile content of a true coal seam, varies with the square of the distance of the seam from the plane of shear or stress, and the following formula is deduced :— where V 1 represents the volatile content (expressed as a percentage as indicated above) of the upper seam; V 2 the volatile content of the lower seam; y the vertical distance in feet between the seams; θ the angle of inclination of an underlying plane of shear or stress. The formula is tested in 49 cases where the volatile contents of upper and lower seams in vertical sequence have been determined by chemical analyses. These cases cover the full range of coal types with vertical distances between the seams up to 2457 feet. The close degree of correspondence between the actual and calculated values constitutes a measure of the validity of the formula. A corollary to the formula is the recognition of a coal-type band for South Wales. With the aid of the formula, the South Wales anthracite field is delimited, and the net reserves are estimated at 4000 million tons.