Ketones in a Proterozoic dolerite sill

Abstract

A dolerite sample from the Middle Proterozoic McArthur Basin in northern Australia has solid bitumen on fracture surfaces and liquid hydrocarbon fluid inclusions in secondary calcite mineralization. The extract of this sample is rich in polar compounds and contains an unusual and novel series of saturated ketone isomers, including homologous series of n-alkan-2-ones and n-alkan-3-ones, together with a complicated series of branched mid-chain ketones. These ketone isomers were identified by the monitoring of diagnostic mass chromatograms, including those due to rearrangement ions. n-Alkanes are only three to four times more abundant than the ketones. The n-alkan-2-ones have a similar distribution to the n-alkanes and may have been formed by the pyrolysis and β-oxidation of carboxylic acids, followed by decarboxylation of the resultant β-keto acids. The n-alkan-3-ones and the mid-chain isomers have a distribution which is biased towards the low molecular weight isomers, probably due to the pyrolysis of a different kerogen moiety, perhaps alkyl chains cross-linked by ether groups. Ketones are known to occur when oil shales are pyrolysed and in hydrothermal oil, so by analogy the natural pyrolysis of kerogen in the fine-grained sediments by the dolerite sill resulted in the generation of liquid products (predominantly n-alkanes and ketones) which were preserved as hydrocarbon-bearing fluid inclusions and as solid bitumen in a fracture.