Source rock heterogeneity of the Upper Jurassic Draupne Formation, North Viking Graben, and its relevance to petroleum generation studies

The Upper Jurassic Draupne Formation in the North Viking Graben, North Sea, has been studied to identify lateral and vertical variations in source rock characteristics and their effect on the timing of petroleum generation. Fourteen source rock samples from the western flank of the graben and five from the eastern flank were analysed using Rock-Eval analysis, microscopy, open system pyrolysis gas chromatography and kinetic experiments. Two major organic facies with different petroleum generation characteristics were identified: a stratigraphically higher, thermally less stable facies, corresponding to a type II kerogen with HI (Hydrogen index) values of >350 mg HC/g TOC; and a stratigraphically lower, more stable type II/III facies with HI values <350 mg HC/g TOC. Only the former is conventionally considered in basin modelling studies. Both organofacies are dominated by marine aliphatic-rich organic matter with variable contributions from transported terrestrial organic matter, resulting in the formation of paraffinic–naphthenic–aromatic petroleum with variable amounts of gas. Bulk kinetic experiments have shown that predicted petroleum formation temperatures are very different, with onset (TR 10%) temperatures of 126 °C (geological Tmax 144 °C) for the less stable facies and 153 °C (geological Tmax 170 °C) for the more stable. This corresponds to 400 m difference in burial depth or delayed onset of petroleum generation by 14 My. Based on a larger dataset from industry, the more stable facies may represent at least 40% of the Draupne Formation in our study areas. Our results imply that petroleum generation from the formation extends to greater depths than generally recognized and that the more stable Draupne organic facies also needs to be included in basin modelling studies to cover the full history of petroleum formation.