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.

Geochemical characterisation and genetic origin of oils and condensates in the South Viking Graben, Norway

“…6). A similar depth relationship was found by Barnard and Bastow (1991) in the South Viking Graben, including data from neighbouring English blocks. The hydrocarbons in PreTertiary reservoirs form two groups.…”

Ankerite Cementation in Deeply Buried Jurassic Sandstone Reservoirs of the Central North Sea

“…In particular, late-diagenetic feldspar dissolution caused net porosity gain in reservoir facies owing to export of reaction products (Wilkinson and Haszeldine 1996;Wilkinson et al 1997). Peak hydrocarbon generation in the Kimmeridge Clay of the SCG was at about 40 Ma (Barnard and Bastow 1991). The Franklin Field was filled during the Miocene, but the Elgin Field was filled only within the last 5 Ma.…”

Geology and assessment of undiscovered oil and gas resources of the Jan Mayen Microcontinent Province, 2008

“…In the North Sea, the Kimmeridge Clay yields total organic carbon (TOC) values that average about 6 percent and locally are in excess of 10 percent and have hydrogen indicies ranging typically from 100-400 mg/g TOC (Kubala and others, 2003). In Norway, the correlative Spekk Formation is oil-prone, with average TOC values between 6 and 8 percent and hydrogen indices (HI) commonly greater than 350 mg/g TOC (Barnard and Bastow, 1991). In East Greenland, the correlative Hareelv and Bernbjerg Formations have TOC values of 2 to 13 percent with HI of 100 to 250 mg/g TOC, although HI values from the Bernbjerg Formation are mostly below 100 mg/g TOC, indicating a gas-prone source (Christiansen and others, 1992).…”