Factors Affecting Hydrocarbon Generation, Migration, and Accumulation in Norwegian Central Graben and Northern Viking Graben
R. C. Leonard
The Norwegian Central graben and northern Viking graben are among the most prolific hydrocarbon provinces in Europe, with proved reserves exceeding 8 billion bbl of oil and 61 tcf of gas. Although are several successful plays are found in both grabens, this study concentrates on factors controlling accumulations in the most prolific trends--the Late Cretaceous to Danian chalk fields of the Central graben, and the Jurassic and Triassic sandstone fields of the northern Viking graben. Detailed modeling of hydrocarbon generation and migration in the two grabens demonstrates some similarities and major differences in the geochemical factors that control the accumulations.
In the basin model studies, we constructed a nine-layered, three-dimensional grid, summarizing burial history. Then we analyzed the source rock, calculated geothermal gradients, and estimated paleotemperatures. Following a maturation study, we calculated the amount of hydrocarbons generated and analyzed individual structures and catchment areas.
Both grabens are associated with Mesozoic rifting followed by Late Cretaceous-Tertiary subsidence. Accumulations are sourced predominantly by the Late Jurassic Kimmeridge Clay. Structural style ranges from salt tectonics in the Central graben to tilted fault blocks in the northern Viking graben. Geothermal gradients increase toward the center of the Central graben, whereas the reverse is seen in the northern Viking graben. High heat flow was probably present in both grabens during the Mesozoic; however, the northern Viking graben was also strongly affected by the early Tertiary opening of the Norwegian Sea. Hydrocarbon generation occurred predominantly from the middle Tertiary to Holocene in the Central graben, whereas generation was initiated from the Early Cretaceous to Holocene in he northern Viking graben. In both grabens, the maximum amount of hydrocarbons generated reaches up to 500,000 bbl/ac, with extensive areas exceeding 100,000 bbl/ac.
Major factors affecting hydrocarbon accumulations in the Central graben chalk fields are the maturation level of the Kimmeridge Clay directly under the crest of the structures and the presence of faults to act as conduits for vertical hydrocarbon migration. Lateral migration is severely restricted because of a lack of permeable carrier beds. The trapping efficiency (hydrocarbons in place/hydrocarbons generated in the catchment area) for tested closures averages 8%. In the northern Viking graben, the timing of hydrocarbon generation and the amount of hydrocarbons generated within the catchment areas are key factors in controlling the size of accumulations. The trapping efficiency averages 28% for tested closures, with the higher efficiency in the Central graben in part due to effective lateral migration.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.