VARIATION IN THE ISOTOPIC COMPOSITION OF SINGLE COMPONENTS IN THE C4-C20 FRACTION OF OILS AND CONDENSATES

Determination of the isotopic composition of individual compounds in the C4-C-15 range has been difficult due to the lack of suitable instrumentation. It has also been suggested that isotopic fractionation during storage might occur. With the introduction of online GC-IRMS, an instrument is now available to test whether or not isotopic fractionation occurs during storage. To test this, two condensates were analysed using the GC-IRMS method. Samples were taken for analysis at regular intervals, during storage at elevated temperatures. Only small changes in isotopic composition were observed during evaporation, with a slight enrichment in C-13 in some compounds. However, the changes are so small that even prolonged storage times do not significantly affect the carbon isotopic composition of the compounds as long as there is enough material left for analysis. Two separate suites of samples, one from the Central Graben, the other from the Viking Graben, were used to demonstrate the use of the technique in correlating oils and condensates. This study shows that the oil/condensates in the Central Graben are not sourced from a single source rock, differing only in maturity, as reported earlier, but are sourced from various source facies, where especially Edda differs significantly from the others, i.e., the compounds in Edda are 1-2 parts per thousand heavier than the rest of the analysed samples in the Central Graben. The analysed Viking Graben oils and condensates were divided into three groups based on this study.