Petroleum source, maturity, alteration and mixing in the southwestern Barents Sea: New insights from geochemical and isotope data

• Mixed oils from distinct sources, maturity levels and ages were identified.
• δ13C of long chain n-alkanes differentiates Triassic from Jurassic contributions.
• δ13C of n-alkanes was suitable for quantifying oil source contributions.
• Diamondoids suggest a late gas charge in the entire Hammerfest Basin.
• Secondary oil migration seems not to have significant effects on δD of n-alkanes.

The Hammerfest Basin (HB) is an important area for petroleum exploration in the southwestern Norwegian Barents Sea. This contribution is aimed to characterize oils and condensates hosted in Triassic and Jurassic reservoirs, establish genetic relationships and to assess maturity, mixing and petroleum alteration processes. The research was based on geochemical as well as bulk and compound-specific stable carbon and hydrogen isotope analyses of oil and condensate samples from all major fields and discoveries in the HB, including extracts from Jurassic and Triassic source rocks. Interpretations of light hydrocarbon composition and isotope data clearly identify four main petroleum groups affected by microbial degradation, water washing, evaporative fractionation and high thermal maturity level. Based on polycyclic aromatic hydrocarbons (PAHs), marine and higher-plant-derived biomarkers and carbon isotopic composition (δ13C) of individual petroleum hydrocarbons, four petroleum families (I–IV) are recognized in the HB and a distinction can be made among oils and condensates from different stratigraphic levels and sectors of the Basin. Most oils and condensates from the HB consist of mixtures with small maturity variations; however, oils from Middle Triassic reservoirs in the Goliat field showed larger maturity variations according to biomarker, PAH and light hydrocarbon–maturity ratios. In the HB, geochemical and isotopic data allowed unraveling mixtures of hydrocarbons derived mainly from Jurassic, Triassic and Paleozoic source rocks.