The Permian Denison Trough, located along the western edge of the Bowen Basin, Queensland, is a graben complex containing a sedimentary sequence in which a range of terrestrial, nearshore, and marine deposystems is recognised. Distribution of organic matter within the deposystems is varied, most accumulations being associated with meandering stream, fluviodeltaic, and deltaic/back barrier systems.
Deposition of organic matter in the Denison Trough was controlled largely by plant types, palae odi mate, and palaeogeography. Plant matter was derived from glossopterids, sphenophytes, ferns and colonial algae, with lesser contributions from lycophytes, unicellular algae, conifers, fungi and bacteria. Differential distribution of organic matter and variations in plant tissue types and their associated palynofloras, as observed in the sediments, is attributed chiefly to pre-depositional variations in sites of growth along with variable proximity to sites of deposition and avenues of transport. Climatic changes affected the accumulation and preservation of organic matter through variations in water influx, while changes in external tectonic setting and internal depositional environment chiefly influenced clastic sediment input.
The petrological classification of coals and carbonaceous mudstones in the Denison Trough is based on such characteristics as plant origin, gelification, oxidation, particle reworking, particle size, and particle segregation. Five coal facies, the Detritai, Leaf, Wood, Herbaceous and Gel Coal Facies are defined. Various combinations of these facies can be distinguished in coal seams, banded coals and carbonaceous mudstones, leading to the recognition of seven coal facies assemblages. The classification of disseminated organic matter (DOM) relies on variations in quantity, physical degradation, and oxidation, and in general cannot be related to the more detailed criteria that distinguish coal facies. Six DOM suites, the Channel, Overbank, Swamp, Nearshore, Shoreline, and Offshore suites, are described.
Preservational characteristics of macerais vary throughout the range of coal facies and DOM suites. Reflectance/rank correlations for individual macerais show marked irregularities due to variations in plant and tissue type, and to degradation. Spectral fluorescence analyses of selected liptinites show wide variations with rank due to variations in preservation, with best preservation related to the major sites of growth and accumulation of the particular organisms.
Those macerais showing greatest potential for oil generation are low reflecting, highly fluorescing liptinites. They occur in trace proportions, as remnants of organic matter that was abundant prior to the generation stage, at which stage it was converted to oil.