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Understanding of Petroleum Systems from Palaeo-Petroleum Present in the Fingerdjupet Sub-Basin of the Bjørnøya Basin, Norwegian Barents Sea

Abstract

The Norwegian Barents Sea was for some time considered a gas-province due to significant uplift and the discoveries of Alke, Albatross and Snøhvit, but the recent advents of the Goliat and Johan Castberg oil discoveries followed by the Gohta oil discovery in 2013 has resulted in renewed optimism and work to understand better the “Petroleum Systems” of the region. Our angel of investigation to this is in part the residual oil in dry wells in the region. The present case study involves the study of residual oil and also petroleum inclusions in core samples from reservoir sandstones of exploratory wells 7321/7–1, 8–1 and 9–1 in the Fingerdjupet Sub-Basin in the Barents Sea. Gases from inclusions were studied on disintegrated samples and extracted bitumen samples were analyzed by Gas Chromatography - Flame Ionization Detector (GC-FID), Gas Chromatography - Mass Spectrometry (GC-MS) and Thin Layer Chromatography - Flame Ionization Detector (TLC-FID). The isoprenoid to n-alkane ratio from GC-FID and distribution of sterane isomers from GC-MS suggest the palaeo-oil to have been sourced from the Upper Jurassic Hekkingen Formation. The relation between isoprenoids ratio (Pr/Ph) and aromatic hydrocarbons (MDBT/MPHEN) also point to such a marine source rock facies. Vitrinite reflectance calculated from aromatic hydrocarbons and sterane isomers shows that expulsion of petroleum from source rock took place in a single event of migration at maturity =0.8%Rc, which is typical for the early to middle part of the oil window. Composition of gases from inclusions of the Late Triassic Fruholmen Formation and the Middle Jurassic Stø Formation reservoirs show with C2+ wetness between 13 to 22 % classifying the gas as condensate to oil associated and the butane ratios (i-C4/n-C4) fall in the range of 0.5 reflecting un-altered petroleum. We found a lateral variation in the Middle Triassic reservoir (Snadd Formation). In well 7321/7–1 this formation show inclusions with a drier gas composition (C2+ =5 %) and with higher butane ratio (=1.0) and also an abnormally high polar fraction (50 %) representing biodegradation. However, this same formation in the well 7321/9–1 contain un-altered petroleum (i-C4/n-C4=0.5) with higher wet petroleum components (14 %), illustrating the complexity of the Triassic formations. The level of maturation and the proven migration history of these wells suggest that the possibility is high for commercial oil discoveries up-dip in the general region.