Nature and origin of fractures in Permian coals from the Bowen Basin, Queensland, Australia

A field investigation into the development of cleat and other fractures within Permian coal measures was undertaken in the Bowen Basin of eastern Queensland, Australia to provide predictive information on coal permeability for use in coalbed methane exploration. Data were collected from five open-cut and three underground mines in the southern and central Bowen Basin, covering the Upper Permian German Creek Formation and Rangal Coal Measures. Fractures noted in the coals fall readily into the four categories of faults and shear zones, extension and compression-related joint sets, mining-induced fractures and coal cleats. Four geometric varieties of coal cleats were identified: class A orthogonal sets, class B sinusoidal sets, class C polygonal sets and class D chaotic sets. Superimposed or overprinted type A sets were also observed locally. On a regional scale the cleats typically parallel dip (face cleats) and strike (butt cleats), but local departures from this pattern occur in proximity to faults, where face and butt cleat directions were often reversed, rotated or multimodal. Field data suggest that coal cleats of classes A B and C were formed by brittle fracturing of the coal during burial, wheras class D and superimposed cleats were formed during later compressional events. Constraints on the timing of deformation events affecting the Bowen Basin, and geochemical data provided by other workers indicate that the cleats were formed within 5 Ma of formation of precursor peats, but were in many instances modified by later structural events