Unraveling petroleum mixtures from the South Viking Graben, North Sea: A study based on δ13C of individual hydrocarbons and molecular data

Abstract

Mixtures of compositionally different petroleum occur in the Norwegian North Sea, one of the world’s major oil-producing regions. The objective of the present contribution was to appraise the source, maturity, in-reservoir mixing and alteration processes in a set of condensate and oil samples covering the main producing areas in the South Viking Graben (SVG). Furthermore, this study focused on unraveling complex mixtures of petroleum and quantifying mixing ratios of hydrocarbons generated from Jurassic source rocks. The present research was based on a multiparameter approach that comprised the molecular composition of light hydrocarbons and heterocyclic and polycyclic aromatic compounds, as well as the δ13C of individual hydrocarbons in oils, condensates and Upper and Middle Jurassic source rock extracts from the SVG. According to the relative contribution of oils from Type-III kerogen-rich source rocks, seven populations (A-G) of mixed petroleum were identified by combining source-related ratios of heterocyclic and non-heterocyclic aromatic hydrocarbons and δ13C values of n-alkanes, pristane (Pr) and phytane (Ph). The heterocyclic and polycyclic aromatic hydrocarbons provided a useful means to discriminate mixed hydrocarbons from source rocks of distinct organic matter type, depositional environment and lithology, in the SVG. The 13C-enrichments of (C10-C14) n-alkanes and Pr and Ph were good indicators of the scale of the terrigenous source contribution and allowed quantitative determination of the proportions of Jurassic source contributions enriched in Type-II and Type-III kerogens in complex petroleum mixtures from the southern part of the SVG.