Regional petroleum alteration trends in Barents Sea oils and condensates as a clue to migration regimes and processes

Abstract

To date, most condensates and gases found in the Hammerfest Basin exist in distal, central basin settings, in traps with tight cap rocks of class 1 traps, whereas low-gas–oil-ratio (GOR) oils occur systematically in proximal basin settings, where cap rocks of class 3 traps prevail. Multiple fill-spill events resulted in the redistribution of oils toward structurally higher basin margins. In a systematic evaluation of light hydrocarbon parameters from condensates and oils, it was found that oils in general exhibit more traceable alteration effects than do condensates. Whereas 75% of condensate and 13.3% of oil samples are fractionated, 6.25% and 10%, respectively, show signs of biodegradation. Long-distance migration is indicated for 12.5% of condensate and 50% of oil samples. In addition, clear evidence is shown for the mixing of recently migrated high-GOR petroleum phases with older, low-GOR paleo oils. In general, variation in source-specific parameters is surprisingly less pronounced. Decreasing thermal maturity of entrapped petroleum from the eastern part of the Tromso Basin toward the Masoy-Nysleppen Fault Complex is observed, whereas high maturities are shown for the Nordkapp Basin and the Finnmark Platform in the eastern part of the study area. Low-to-medium maturities are recorded for oils from the basin margins of the Hammerfest Basin. Alterations in the composition of the petroleums by physiochemical processes and distribution patterns of the petroleums are closely associated with uplift and erosion.