ABSTRACT: Variation in the Isotopic Composition of Single Components in the C3-C20 Fraction of Oils and Condensates

BJOROY, MALVIN, PETER B. HALL, and RITA P. MOE, no affiliations

Variation in the isotopic composition of components in the C15+ fraction of oils and extracts has been reported lately by a number of authors. Large variations have been found for terpenoids in immature samples, while there is much less difference between terpenoids in mature samples. For n-alkanes and isoprenoids the variations are minor.

Very little work has been reported on the isotopic composition of the lighter components. Earlier reports have shown a strong variation in the distillation cuts for this range of hydrocarbons, depending on maturity and type of kerogen generating these hydrocarbons. Some of the arguments for the observed variation have been isotopic fractionation during storage/distillation. To test this, five oils and five condensates were analyzed using the GC-IRMS method, where samples were tested at regular intervals during storage in open containers and at elevated temperatures. The variations observed in the different components will be discussed in detail. The analyzed oils and condensates show variation in isotopic composition between n-alkanes, branched alkanes, cyclic alkanes, and aromatic co pounds of 4 to 6 percent for compounds with the same carbon numbers. The cause of this variation and the relationship between the compounds and their precursors will be shown and discussed. Oils and condensates generated from marine source rocks in the North Sea and from nonmarine source rocks in Africa and Australia are used in the study.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)