Geothermal history and petroleum generation in the Norwegian South Viking Graben revealed by pseudo-3D basin modelling

Abstract

Hydrocarbon exploration in the Norwegian South Viking Graben (57°45′–60°15′N) began in the late 1960s and has now entered a mature phase. A map-based pseudo-three-dimensional (3D) basin modelling study is performed in order to unravel regional trends in hydrocarbon generation and expulsion and to evaluate the remaining potential of this mature petroleum province. Generation and expulsion from all major source rock horizons in the area, the Draupne, Heather, Hugin and Sleipner Formations, is simulated using a pseudo-3D model comprising 36 isochronous geological events. The map-based pseudo-3D model is built from 1D models, combined with subsurface and source rock quality maps. Hydrocarbon expulsion from the Middle and Upper Jurassic source rocks in the area occurred in two major phases. The first phase lasted from Palaeocene to Middle Miocene with peak oil and gas expulsion during the Early Miocene. The second, Quaternary, phase of expulsion, which supplied 11% and 13% of all oil and gas, respectively, is related to increased subsidence rates during this period. A total of 74×10 9 Sm 3 oil and 8.2×10 12 Sm 3 gas has been expelled in the area. The Frigg area in the north is with 2.9×10 12 Sm 3 gas expelled the most gas-prone area, and the Greater Balder area seems to be the most oil-prone area with 17×10 9 Sm 3 oil expelled. The lower, syn-rift section of the Upper Jurassic Draupne Formation dominated oil expulsion (54% of all expelled oil), whilst the Heather Formation dominated gas expulsion with 37% of all gas expelled. Model predictions were successfully applied to explain the fill and secondary alteration history of the Greater Balder Area, Norway. The modelling supports the Draupne Formation origin of the oils in the Greater Balder Area, as well as the presence of oils deriving from multiple charge phases found in correlation studies. Based on the modelled cumulative volumes expelled and estimates for in-place volumes in the area, generation–accumulation efficiencies (GAEs) of 1.01–1.05% for oil and 9.67–12.89% for gas have been estimated for the South Viking Graben. In light of comparisons with other petroleum systems and critical evaluation of the elements of the petroleum systems, the estimated GAEs should be regarded as encouraging for further exploration of the area.