Hydrocarbon generation, migration, alteration, entrapment and mixing in the Central and Northern North Sea

Abstract There is little doubt that sediments of Upper Jurassic to lowermost Lower Cretaceous age, particularly those of the Kimmeridge Clay Formation and its equivalents, are the source of the vast bulk of the oil in the Central and Northern North Sea. This is true both of oil in conventional Mesozoic plays and in Tertiary clastic reservoirs. The timing of oil generation and migration ranges from the Late Cretaceous through to the present. The oil productivity of the Kimmeridge Clay Formation in the Central Graben, Moray Firth and South Viking, Graben areas of the North Sea is estimated to be in excess of 250 billion barrels, of which about 25% is accounted for in accumulations discovered to date. There is strong areal differentiation between oil trapped in Mesozoic reservoirs as opposed to Upper Cretaceous Chalk and Tertiary reservoirs. This paper is particularly concerned with hydrocarbons in Tertiary reservoirs, which have a wide range of compositions and appear to have complex accumulation and degradation histories. Migration to Chalk and younger Tertiary reservoirs occurs often through thick shale sequences. The evidence for the timing and mechanism of this process is considered and a preferred model for the vertical migration and subsequent lateral migration, mixing and degradation is proposed.