--> Source Indicators to Unravel Mixtures of Oils in the South Viking Graben, North Sea: A Study Based on δ13C of Individual Hydrocarbons, PAHs and Sulfur Aromatic Hydrocarbons

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Source Indicators to Unravel Mixtures of Oils in the South Viking Graben, North Sea: A Study Based on δ13C of Individual Hydrocarbons, PAHs and Sulfur Aromatic Hydrocarbons

Abstract

The South Viking Graben (SVG) is located in the North Sea, one of the most important petroleum province in the world. In the SVG, oils and condensates represent mixtures from Jurassic source rocks dominated by marine algal material (Draupne Fm.) and terrestrial material (Heather Fm. and Vestland Group). In order to unravel mixtures of oils from Jurassic source rocks with slight maturity variations, this research is aimed to assess the influence of terrestrial source contribution on the δ13C of individual hydrocarbons and the relative abundance and distributions of specific methyl isomers of polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, phenanthrene and dibenzothiophene in unaltered oils from the SVG. Several genetic families of mixed oils were identified in the SVG. Mixed oils sourced mainly from type-II kerogen rich rocks were recognized in the Vølve, Varg, Sleipner Øst and Glitne fields, whereas mixed hydrocarbons with a high contribution from predominantly type-III kerogen-rich source rocks were identified in the Sleipner Vest and Øst fields, as well as to the north of the Greater Sleipner area. A 13C-enrichment of (C10-C14) n-alkanes, pristane, phytane, branched and some alkylcycloalkanes, as well as a higher abundance of methyl and dimethylnaphthalenes compared with trimethylnaphthalenes were found in mixed oils and condensates with a high terrestrial source contribution. The heaviest δ13C values were found in condensates with a high contribution from coals and coaly shales of the Vestland Group. In contrast, hydrocarbons derived mainly from marine algal material show higher alkylnaphthalene concentrations with increasing in methyl substitution. Contribution from calcareous source facies of the upper Draupne Fm. was recognized in the Vølve and Glitne fields by the lightest δ13C of isoprenoids and branched, cyclo and n-alkanes, the significantly more abundant methyl isomers of dibenzothiophenes and a distinctive “V-shaped pattern” of methyldibenzothiophene isomer distribution. In the SVG, the PAHs, sulfur-containing aromatic hydrocarbons together with δ13C ratios of (C10-C14) n-alkanes, branched and cyclic alkanes provided a means of discriminating mixed hydrocarbons from source rocks of distinct OM type and depositional environment conditions. These source indicators were suitable to unravel complex oil mixtures, especially in the Greater Sleipner area, where biomarker parameters were unable to distinguish compositional variabilities.