Thrust-related permeability in the South Wales Coalfield

Abstract

The South Wales Coalfield is a Variscan foreland basin extensively deformed by both linked and isolated thrusts in response to regional NW-SE compressive stress. Thrusts normally strike NE-SW, but transpression has caused a variable dextral rotation of the thrusts to strike E-W and even NW-SE. The range in orientation has allowed thrust-related fractures locally to be opened within the neotectonic stress field in which 0'1 is oriented NW-SE. It is argued that similar thrust-related permeability should be developed in other coal-bearing foreland basins both associated with the Late Carboniferous Variscan/ Appalachian orogeny and with younger compressional tectonic systems. The dominant meso- to major-scale structures formed during the compression of coalbearing sequences are thrusts and folds. Strains at leading and trailing tip-lines of isolated thrusts and in the immediate hanging wall and footwall generate tension cracks which may act as methane conduits. Unsealed, these allow permeability parallel to thrust strike. Decollements form as bed-parallel detachments within coals, developing as pervasive shear zones, characterized by cleavage duplexes and C-S fabrics in which a penetrative new hinterland-dipping fabric is formed. This fabric, under changing regional stress conditions, may be opened to form a highly effective gas migration pathway. Coal-bearing strata develop chevron folds with flexural slip in fold limbs and tension gashes in competent strata, generating porosity and permeability parallel to the strike of fold limbs. Incompetent coal seams are strongly sheared, producing cleavage duplexes with contrasting vergence in opposing limbs.