Geochemical characterisation and genetic origin of oils and condensates in the South Viking Graben, Norway

Abstract

Geochemical analysis of oil and condensate samples from all major fields and discoveries in the Norwegian South Viking Graben, as well as source rock extracts of Upper and Middle Jurassic source horizons, was carried out to characterise reservoired hydrocarbons and establish genetic relationships. The South Viking Graben is in a mature phase of exploration, and this study aims at unravelling regional trends to improve predictions in exploration and will show the significance of the play models in the area. Seven hydrocarbon families have been identified, based on interpretation of molecular and isotopic characteristics and multivariate analysis of biomarker data. These families can be linked to three source horizons. Three families originate from different facies of the Draupne Formation and are characterised by Pr/Ph ratios below 2, dominance of C 27 regular steranes over the C 29 homologues, and carbon isotopic values for saturate and aromatic fraction lighter than −28 and −27‰, respectively. The Upper Jurassic Heather Formation and the Middle Jurassic Vestland Group each source one group of hydrocarbons. Typical for Heather sourced hydrocarbons are elevated Pr/Ph ratios between 2.35 and 3.08, low ratios of C 27/C 29 regular steranes and average isotopic values of −28.4‰ ( δ 13C Sat) and −26.8‰ ( δ 13C Aro). Hydrocarbons from a Middle Jurassic source are characterised by very heavy isotopic composition, highest Pr/Ph ratios of the data set and dominance of C 29 regular steranes over the C 27 homologues, among other factors. The two remaining families represent mixtures from Upper and Middle Jurassic sources, and are mainly encountered in the Greater Sleipner Area. The available data in this study allowed characterisation of 84% of the recoverable oil resources which proved that the main source rock in the area is the Upper Jurassic Draupne Formation. The play concept including a Draupne source and Tertiary reservoir proves to be most prolific, with 239×10 6 S m 3 of recoverable oil, equalling 88% of the total characterised oil resources. Oils in these Tertiary reservoirs are however likely to be affected by biodegradation. Based on the available data, the zone of biodegradation in the South Viking Graben appears to be limited to depths above 2140 m and temperatures below 70 °C.