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Norwegian North Sea, South Viking Graben; petroleum mixtures; compound-specific stable carbon and hydrogen isotope compositions; n-alkanes; acyclic isoprenoid alkanes; oil-source correlation; unraveling oil mixtures; thermal maturation; hydrogen exchange
Abstract:
In order to resolve oil mixtures and for oil-source correlation purposes, both hydrogen (δ2H) and carbon isotope compositions (δ13C) of n-alkanes and acyclic isoprenoid alkanes have been determined in petroleum mixtures and extracts from Jurassic source rocks of distinct organic matter type, depositional setting and lithology in the South Viking Graben, Norway. δ2H values vary from -90 to -140‰ for n-alkanes and from -130 to -165‰ for pristane (Pr) and phytane (Ph) in oil and condensate samples. At the same maturity level, in the main oil generative window, individual n-alkanes from most oils and condensates show a 2H-enrichment with n-alkane chain length by 15 to 50‰. With the increase in the estimated input of hydrocarbons from terrigenous source material to petroleum mixtures, the n-alkanes (>C20) exhibit a higher 2H-enrichment while n-alkanes (C10-C14) show a higher 13C-enrichment. δ2H values of n-alkanes (>C20) are useful to discriminate mixed oils that were not well differentiated by δ13C data. δ2H values of individual n-alkanes from oils suggest a possible exchange of C-bound H with other hydrogen during their generation from precursor molecules. We find that δ2H values of n-alkanes from Jurassic source rock extracts, at the onset of oil generation, were mainly controlled by the isotopic composition of biosynthetic precursors and varied from -90 to -150‰. Combination of carbon and hydrogen isotope compositions of n-alkanes proved to be a powerful tool to discriminate Jurassic petroleum mixtures from distinct sources in the South Viking Graben.
