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 t he C4-C-15 range has been difficult due to the lack of suitable instru mentation. It has also been suggested that isotopic fractionation duri ng storage might occur. With the introduction of online GC-IRMS, an in strument is now available to test whether or not isotopic fractionatio n occurs during storage. To test this, two condensates were analysed u sing the GC-IRMS method. Samples were taken for analysis at regular in tervals, during storage at elevated temperatures. Only small changes i n isotopic composition were observed during evaporation, with a slight enrichment in C-13 in some compounds. However, the changes are so sma ll that even prolonged storage times do not significantly affect the c arbon 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 dem onstrate the use of the technique in correlating oils and condensates. This study shows that the oil/condensates in the Central Graben are n ot sourced from a single source rock, differing only in maturity, as r eported earlier, but are sourced from various source facies, where esp ecially Edda differs significantly from the others, i.e., the compound s in Edda are 1-2 parts per thousand heavier than the rest of the anal ysed samples in the Central Graben. The analysed Viking Graben oils an d condensates were divided into three groups based on this study.