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A comparative study of the stable carbon isotopic composition of crude oil alkanes and associated crude oil asphaltene pyrolysate alkanes

Lookup NU author(s): Professor Stephen Larter

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Abstract

Normal-alkanes and isoprenoid alkanes in crude oils and micro-sealed vessel pyrolysates of related asphaltenes, sourced from different source rock types, have been analysed by gas chromatography isotope ratio mass spectrometry (GC-IRMS). Seven sample sets, each consisting of the oil and the related asphaltene, from marine non-clastic-sources, marine clastic-source, a lacustrine clastic source and two from sources from land plant-dominated deltaic facies source rocks (angiosperm- and gymnosperm-dominated plant inputs, respectively) were studied. The utility and precision of the GC-IRMS technique are assessed by analysing (a) a standard n-alkane-mixture of known isotopic values, and a blend of this standard mixture with (b) a biodegraded oil and (c) a branched/cyclic saturated hydrocarbon fraction. The precision of the technique is better than ca. 0.25 parts per thousand. In four sample sets, the individual n-alkanes of the asphaltene pyrolysates are enriched in C-13 (up to 4 parts per thousand) compared to the n-alkanes in the corresponding oils, and follow broadly the same trend with increasing carbon number. In the remaining three sample sets (marine-clastic-, lacustrine- and deltaic gymnosperm derived samples) the opposite trend is observed, i.e. here the n-alkanes from the asphaltene pyrolysate are isotopically lighter, thus perhaps indicating different sources for the n-alkanes in the oil and n-alkyl-moieties in the asphaltenes.

Publication metadata

Author(s): Wilhelms A, Larter SR, Hall K

Publication type: Article

Publication status: Published

Journal: Organic Geochemistry

Year: 1994

Volume: 21

Issue: 6-7

Pages: 751-760

Print publication date: 01/06/1994

ISSN (print): 0146-6380

ISSN (electronic): 1873-5290

Publisher: Pergamon

URL: http://dx.doi.org/10.1016/0146-6380(94)90017-5

DOI: 10.1016/0146-6380(94)90017-5

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