The effects of varying tissue preservation on the aliphatic hydrocarbons within a high-volatile bituminous coal

Detailed petrographic analyses of the Barrett Coal, an Upper Permian high-volatile bituminous coal in the Hunter Coalfield, Australia, reveal cycles of peat formation defined by the Tissue Preservation Index. These cycles correspond to coal plies that are separated by dirtbands. The petrographic cyclicity is also manifested by a corresponding stratigraphic cyclicity in the biomarker composition of the extracted aliphatic hydrocarbons. Higher amounts of hopanes and lower Carbon Preference Index values (n-C22 to n-C30) show greater bacterial contribution in samples characterised by low values of Tissue Preservation Index. The inherited biodegradation signal from the diagenetic stage has now been partially diluted by the generation of new hydrocarbons from the macromolecule. The C31 αβ (22S/22S + 22R) hopane and C29 ααα (20S/20S + 20R) sterane ratios show least variation through the seam. The homohopane epimer ratios have attained equilibrium, whilst the hopane/moretane ratios are typical of coals with unsupressed vitrinite reflectance of > 0.65%. However, other hopane and sterane maturity ratios appear slightly affected by the degree of preservation of organic matter. The C29 (αββ/αββ + ααα) and C29 ααα (20S/20S + 20R) sterane ratios were not found to be useful for maturity comparisons with other coals because of the low values (average 0.18 and 0.43, respectively) due to heating-rate effects. These and other biomarker distributions are syndepositional features of the coal precursor and therefore are controlled by the environmental conditions during the stage of peat accumulation and its subsequent coalification. All these changes are evident in a coal member only 2.2 m in thickness. The petrographic and geochemical evidence suggest that the environmental affinity of the Barrett Coal is that of a delta-plain depositional setting.