Geochemistry of Oils in the Northern Viking Graben
The compositions of crude oils in the northern Viking graben area reflect the influence of both the types of organic matter and the thermal maturity of Jurassic source rocks in the major basin depocenters, referred to as the Oseberg and Troll kitchens. Forty-four oil samples were analyzed and classified primarily on the basis of chemical composition, but with recognition of constraints on origins imposed by geographic and stratigraphic locations. Five groups are recognized: (1) Snorre oils, (2) Statfjord-Gullfaks oils, (3) Brent-Oseberg Beta-Veslefrikk oils, (4) Oseberg Alpha-Troll oils, and (5) Agat and Blocks 35/8 and 35/11 oils. The compositions of oils in the first three groups reflect the large relative proportions of marine organic matter and the lower temperatures of expulsion characteristic of oils derived from the Draupne Formation. The compositions of oils in the last two groups reflect source rocks having more terrestrial organic matter and higher temperatures of expulsion.
The initial phase of hydrocarbon expulsion and migration from the Draupne Formation in the northern Viking graben began about 56 m.y.a. in the Troll kitchen and about 45 m.y.a. in the Oseberg kitchen. With continued subsidence and heating of Jurassic source rocks, gases and condensates were expelled, beginning about 40 m.y.a. in the Troll kitchen and about 30 m.y.a. in the Oseberg kitchen. This secondary phase of hydrocarbon generation has mixed with and displaced some of the earlier accumulations.