Hydrocarbon generation and migration from Jurassic source rocks in the E Shetland Basin and Viking Graben of the northern North Sea

In the E Shetland Basin oil generation began 65 Ma ago; peak oil generation maturity occurs today at 3250 m (0.7% R₀) and was first reached 40–50 Ma ago; the oil generation threshold is at 2500 m. Highest oil saturations in the Kimmeridge Clay occur at 0.8% R₀; oil expulsion efficiencies are >20–30%. Oil phase migration has probably occurred through oil wet kerogen laminae, and through interconnected large pores aided by low oil/water interfacial tensions. Oil migrated along strong lateral fluid pressure gradients, from overpressured source rocks in half grabens to Jurassic reservoirs in tilted fault blocks.

In the Viking Graben the Kimmeridge Clay is at oil floor maturity below 4500 m; oil and peak oil generation began 7C–80 and 55–65 Ma ago respectively; 40 Ma ago the Kimmeridge Clay passed through peak generation, and gas generation by cracking of oil had begun. Peak dry gas generation from Brent coals occurs today below 5000 m, and began 40 Ma ago. The Frigg Field gas, probably generated from late Jurassic source rocks, migrated through microfractures in overpressured mudstones below 3500 m; above 3500 m methane probably migrated in aqueous solution and was exsolved in the early Tertiary aquifer.