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Rundle oil shale containing 12.9 wt.% of organic carbon was retorted using a combustion-type retort. It was possible to quantify the reduction in oil yield (compared with Fischer assay) due to the combustion of oil vapour during retorting. The oil yield was only 7.2 wt.%, compared with a Fischer assay oil yield of 8.7 wt.%, when retorting was carried out with excess air but no additional source of carbon. Oil yields could be improved by mixing lignite, available from within the Rundle deposit, with oil shale before retorting. Oil yields, based on charge, from mixtures containing more than 10% lignite decreased with increasing lignite content, but were virtually identical with Fischer assay oil yields from the mixtures. The maximum oil yield was about 8.5 wt.% for 6 wt.% lignite in the mixtures. The resulting oils were slightly heavier and more aromatic than the oils obtained by Fischer assay, but showed no significant difference in oxygen content.

Oil shale and lignite from the Rundle/Stuart deposit and a range of mixtures of the two, have been pyrolysed in a Fischer retort. Oil yields decreased linearly from 8.67 to 5.19 wt% as the lignite content increased from 0 to 100%. Yields of gas, water and spent shale also varied linearly. H C atomic ratios of the resulting oils varied from 1.70 to 1.47 and, when plotted against oil shale percentage, a slightly concave curve resulted. Similarly, carbon aromaticity determined by n.m.r. varied from 0.33 to 0.48 and resulted in a slightly convex curve when graphed against oil shale content. It appears that oil produced by blending shale and lignite is more aromatic than expected for non-interacting components. Results show that oil from an intimate geological mixture is more aromatic than that from a comparable synthetic mixture.

Shale oil recovered from Fischer assays of fresh and artificially weathered oil shales have been compared using 1 H and 13 C nuclear magnetic resonance spectroscopy and elemental analyses. Rundle, Julia Creek and Green River shales were exposed to air at 100 °C for 60 weeks to simulate either geological weathering, or oxidation during drying or stockpiling prior to retorting. As a result of oxidation, the oils became more aromatic, with higher oxygen contents and shorter average aliphatic chain lengths. Variations in the distribution of phenols, acids and ketones, as well as heterocyclic compounds containing nitrogen and sulphur, have been detected.

An attempt has been made over 60 weeks to simulate experimentally the weathering of Rundle, Julia Creek and Green River oil shales. Finely-ground shales were subjected to three types of ‘weathering’: 1. (1) 22 °C in air without water or sunlight (‘internal’ conditions) 2. (2) 10–35 °C under water with sunlight (‘external’ conditions) 3. (3) 100 °C in air without water or sunlight (‘extreme’ conditions). All shales exhibited decreases in oil yields by Fischer assay, the highest loss being 90 wt % for Julia Creek shale under extreme conditions for 60 weeks. Solid-state 13C n.m.r. spectra of kerogens before and after weathering under extreme conditions for 60 weeks confirm increased aromaticity and greater concentrations of oxygenated functional groups. Elemental analyses show that, for these kerogens, H/C ratios decreased by ≈ 30% and O/C ratios increased by ≈ 300%. The results are important when considering natural geological weathering of shale seams, oxidation of stockpiled shale and prolonged drying of shale at elevated temperatures.

Differing degrees of oxidation, caused by weathering at the surface, are evident in the Julia Creek oil shale deposit. Weathering results in decreased oil yields and its extent is indicated by the oil/organic carbon ratio. In a series of carefully selected samples, this ratio (by weight) varies from 0.55 for ‘fresh’ shale to 0.38 for appreciably altered shale. Oxidation of kerogen is reflected in the elemental composition of kerogens obtained by demineralization of the samples. FT-i.r. spectra of kerogens show that CH concentrations decrease and COOH contents increase with in situ oxidation. Nuclear magnetic resonance spectroscopy gives clear evidence that, as well as oil quantity, oil quality progressively changes with shale weathering. Results are, in general, directly comparable with those for artificially weathered shales.

A fresh sample of Rundle shale oil, recovered during retorting, was subjected to a variety of storage conditions for 26 weeks. Changes in chemical composition were investigated by Chromatographic and n.m.r. techniques. Subsamples of the fresh shale oil were stored at 1. (a) −4 °C under nitrogen 2. (b) 22 °C in air, in the dark 3. c) 22 °C in air, exposed to artificial light 4. (d) 10–35 °C in air, exposed to sunlight. The most significant changes were observed in the ‘sunlight’ sample. Exposure to sunlight caused a marked decrease in the aliphatic and mono-aromatic fractions in the aged shale oil with a corresponding increase in polar constituents. The ratio of straight-chain 1-alkenes to alkanes was significantly lower in this sample, while the ratio of 1-alkenes to internal alkenes was greatly increased. The reactivity of cis and trans -2-alkenes appeared to be similar during ageing experiments.

The amount of oil that can be generated from a kerogen under natural conditions is an important parameter in source rock assessment. Similarly, the quantity of oil able to be formed from a coal or kerogen during much more rapid pyrolysis is a crucial factor in oil shale and coal conversion studies. An approximate relation based on atomic ratios is derived for slow geological heating but is also valuable for comparing samples under other conditions. The percentage of oil, on a weight basis, that can be generated is given by: Oil = 66.7 H C − 57.0 O C −33.3 . Typical results for twelve Australian coals and twelve world oil shales are given as examples.