Unusual distribution of biological markers in an Australian crude oil

Abstract

The stereochemical configuration of hopane-type triterpanes in source rock extracts or crude oils is normally determined by the maturity of the sample1. Naturally occurring precursors of the hopane hydrocarbons are dominated by the 17βH, 21βH stereochemistry with only one diastereomer at the C22 position in the C31 and higher homologues2. Note that the 17βH, 21βH-C28 homologue has never been reported in any sample. Mature crude oils in general contain hopane hydrocarbons dominated by the thermally more stable configuration of 17αH, 21βH stereochemistry and a mixture of the two diastereomers at the C22 position for C31 and higher homologues in the ratio of ∼3/2. We present here evidence for the presence of an unusual distribution of hopanes in an Australian crude oil. A study by computerized gas chromatography–mass spectrometry (C–GC–MS) designed to determine whether different oils from the same basin have similar sources has revealed the presence of the thermally unstable 17βH, 21βH isomers of hopane-type triter-panes in conjunction with C22 diastereoisomers in the unusual ratio of 1:9. We propose that there are two possible explanations for this unusual distribution of the triterpanes. (1) The oil is immature or (2) these compounds have accumulated in the oil as it migrated through coals in the region of the reservoir which are also known to contain the unusual hopane distribution.