STABLE CARBON-ISOTOPE VARIATION OF N-ALKANES IN CENTRAL GRABEN OILS

While isotope analysis of bulk fractions is a well-tried and useful to ol in oil correlation, use of compound specific isotope ratio mass spe ctrometry (CS-IRMS) of individual hydrocarbons is still in its infancy . This technique has been developed over the past few years as a usefu l research tool, but its potential in petroleum exploration has yet to be demonstrated. In this paper both bulk fraction and individual hydr ocarbon isotopic data were acquired. Their use was evaluated and compa red with traditional geochemical measurements such as gas chromatograp hy and GC-MS. Data on 72 oils from the Norwegian sector of the Central Graben were acquired. Data include delta(13)C isotope composition of whole oil and topped oil, as well as saturated and aromatic hydrocarbo n, NSO and asphaltene fractions, and delta(13)C isotopic composition o f 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 fract ion isotope data suggest that many of the analysed Greater Ekofisk oil s (e.g. Valhall, Edlfisk), i.e. within the Lindesnes Ridge Inversion z one have more than one source facies. Outside of this area, particular ly towards the northwest, oils are probably mainly from a single sourc e facies. The isotopic compositions of individual hydrocarbon compound s also reveal the same divisions. In addition, several further feature s were observed. The normal alkanes of many of the Greater Ekofisk oil s are more depleted in C-12 between nC(20)-nC(25) than at higher or lo wer molecular weight (possibly representative of the original microbia l precursors). Other oils have relatively uniform n-alkane isotope pro files, with n-alkanes which are enriched in C-12 relative to the n -al kanes of oils with the other type of profile. The two different isotop e profiles are interpreted as being separate source facies. The least mature Greater Ekofisk oils also become more enriched in C-12 with dec reasing molecular weight. Mature oils from this area tend to have very similar n-alkane isotope values throughout the measured n-alkane rang e. The higher maturity oils and condensates have n -alkanes < nC(8), w hich are more depleted in C-12 than the n-alkanes > nC(8). The tentati ve interpretation of this is that the highly mature oils/condensates a re sourced only from highly mature source rocks, i.e. of a condensate window maturity and are not products of thermal alteration in the rese rvoir.