CARBON ISOTOPE VARIATIONS IN NORMAL-ALKANES AND ISOPRENOIDS OF WHOLE OILS

Carbon isotope analysis of oils and source rock bitumens has been performed mainly on distillate cuts or whole fractions (i.e. saturated and aromatic hydrocarbons and NSO compounds). Until recently, previous studies on the carbon isotope composition of individual hydrocarbons has been restricted by the problem of balancing adequate resolution of individual compounds and obtaining enough material for good analysis. Previous studies on the carbon isotopic variations of individual hydrocarbons have used packed column gas chromatography to collect enough of each individual compound for analysis, or more recently combined with an interface to a mass spectrometer for immediate analysis after separation. In both techniques, resolution is insufficient for accurate measurement and, while the former technique, is also time-consuming. As a result of the development of a new isotope ratio-gas chromatograph combination in which capillary colums are used for improved resolution, both analytical problems have been removed. Experimental results obtained with this instrument (VG Isogas Isochrom II) show that there are clear differences in the carbon isotope composition of n-alkanes and acyclic isoprenoids in oils. In addition, differences in the isotope composition of these compound types appear to be related to source and maturity variations. Examples from a selection of oils are presented and discussed.