A COMPARATIVE-STUDY OF THE STABLE CARBON ISOTOPIC COMPOSITION OF CRUDE-OIL ALKANES AND ASSOCIATED CRUDE-OIL ASPHALTENE PYROLYSATE ALKANES

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.