Stable carbon isotope variation of n-alkanes in Central Graben oils

Abstract

While isotope analysis of bulk fractions is a well-tried and useful tool in oil correlation, use of compound specific isotope ratio mass spectrometry (CS-IRMS) of individual hydrocarbons is still in its infancy. This technique has been developed over the past few years as a useful research tool, but its potential in petroleum exploration has yet to be demonstrated. In this paper both bulk fraction and individual hydrocarbon isotopic data were acquired. Their use was evaluated and compared with traditional geochemical measurements such as gas chromatography and GC-MS. Data on 72 oils from the Norwegian sector of the Central Graben were acquired. Data include δ 13C isotope composition of whole oil and topped oil, as well as saturated and aromatic hydrocarbon, NSO and asphaltene fractions, and δ 13C isotopic composition of individual n-alkanes in the range nC 4 to nC 35. The same samples were also analysed by capillary GC and GC-MS. GC, GC-MS and bulk fraction isotope data suggest that many of the analysed Greater Ekofisk oils (e.g. Valhall, Edlfisk), i.e. within the Lindesnes Ridge Inversion zone have more than one source facies. Outside of this area, particularly towards the northwest, oils are probably mainly from a single source facies. The isotopic compositions of individual hydrocarbon compounds also reveal the same divisions. In addition, several further features were observed. The normal alkanes of many of the Greater Ekofisk oils are more depleted in 12C between nC 20– nC 25 than at higher or lower molecular weight (possibly representative of the original microbial precursors). Other oils have relatively uniform n-alkane isotope profiles, with n-alkanes which are enriched in 12C relative to the n-alkanes of oils with the other type of profile. The two different isotope profiles are interpreted as being separate source facies. The least mature Greater Ekofisk oils also become more enriched in 12C with decreasing molecular weight. Mature oils from this area tend to have very similar n-alkane isotope values throughout the measured n-alkane range. The higher maturity oils and condensates have n-alkanes < nC 8, which are more depleted in 12C than the n-alkanes > nC 8. The tentative interpretation of this is that the highly mature oils/condensates are sourced only from highly mature source rocks, i.e. of a condensate window maturity and are not products of thermal alteration in the reservoir.