Chemical, molecular and isotopic differentiation of organic facies in the Tertiary lacustrine Duaringa oil shale deposit, Queensland, Australia

Abstract

Total organic matter, extractable organic matter (bitumen), and pyrolysates released on low-temperature sealed-tube pyrolysis of kerogen were analysed chemically and isotopically. Based on bulk bitumen and biomarker parameters, the sediments are immature to a depth of approx. 1300 m. Within the terrestrial and lacustrine depositional environments of the Tertiary Duaringa Basin, various organic facies have been classified according to inorganic input and maceral abundances of organic matter derived from land-plant and algal sources; the latter derived mainly from Pediastrum and with a varying Botryococcus content. The lacustrine upper lamosite is chemically and isotopically distinct from the lower lamosite due to a changing algal community, relative contributions from allochthonous land-plants and, for the lower lamosite, a slightly more saline and restricted lake environment. The lower lamosite is associated with the highest concentrations of C 28-C 30 4-methylsteranes and 13C depletion in TOC. In the upper lamosite, there is a strong positive correlation between Botryococcus content and 13C enrichment. The organically lean claystones have sources nearly identical to the lamosites, but the organic matter is diluted due to rapid influx of siliciclastics. The terrestrial facies, the cannel coal and carbonaceous shales, are mainly distinguished from the lacustrine facies by higher relative proportions of C 29 steranes, C 30 4-methylsteranes and δ 13C values for the pyrolysed hydrocarbons intermediate between the two lamosite facies. The carbonaceous oil shale has a composite geochemical signature intermediate between the two end-members, lamosite and cannel coal. Oleanane and ring-A-degraded triterpanes derived from lupane, arborane and oleanane skeletons are distributed throughout all organic facies, representing both autochthonous and allochthonous higher plant inputs. Higher concentrations of oleanane and des-A-lupane are associated with the terrestrial facies. Hopanes dominate over steranes in all organic facies and their concentration ranges are independent of facies. For the lamosites, the aliphatic biopolymer algaenan is implicated as the dominant source for n-alkanes from analysis of their yields, homologue abundances, and individual δ 13C values. In the terrestrial facies, resinites are paraffinic and are a major source for the n-alkanes released on pyrolysis. Compound Specific Analysis of extractable and pyrolysate hydrocarbons allowed deconvolution of different source inputs to free lipids and kerogen. For the lamosites, low-molecular-weight n-alkanes in the bitumen are derived mainly from a Pediastrum source while Botryococcus is associated with waxy n-alkanes. However, the situation is reversed in the kerogen; Pediastrum algeanan has a longer average alkyl chain-length than Botryococcus algaenan. The magnitude of the isotopic differences between single n-alkanes released at different temperatures is useful in determining multiplicity of sources.