The Jurassic petroleum system north and west of Britain: a geochemical oil-source correlation study

Recent significant oil discoveries have focused exploration attention on th e Atlantic Margin areas north and west of Britain, in particular the West o f Shetlands area. These discoveries, the Foinaven and Schiehallion fields, each have reserves of 250 to 500 million barrels (39.8 to 79.5 x 10(6) m(3) ) of 24-27 degrees API oil while an earlier heavy oil discovery, the Clair Field, is the largest undeveloped oil discovery on the U.K. Continental She lf with resources estimated in the range 3-5 billion barrels (477 to 795 x 10(6) m(3)). Other discoveries include Suilven, the very heavy oil in well 204/28-1 and the Quadrant 205 Strathmore and Solan fields. Potential source rocks in areas adjacent to the West of Shetlands include the Upper Jurassi c Kimmeridge Clay Formation equivalent, the Middle-and Lower Jurassic and t he Middle Devonian. The results of geochemical studies of the oils, reservo ir core extracts and source rocks from released wells from these West of Sh etlands discoveries show them to be genetically related to North Sea oils s ourced from the Kimmeridge Clay Formation equivalent: Detailed geochemical analysis (biomarkers, source rock reaction kinetics, compound specific isot ope analysis and fluid inclusion "crush-leach" analysis) of the Clair Field and "Foinaven Complex" oils showed that at least two episodes of charging from different source facies within the Kimmeridge Clay Formation took plac e, with an intervening biodegradation event. Oil charging from Middle Juras sic lacustrine source rocks is also indicated in the "Foinaven Complex" oil s. The source rocks are shown to exhibit variations in organic facies, from very oil-prone to mixed oil and gas, with a corresponding range of kinetic parameters. This complex generation, migration and trapping scenario is di rectly linked with the tec tonic evolution of the West of Shetlands with ma jor oil generation from very oil-prone Kimmeridge Clay and Middle Jurassic lacustrine facies into shallow-buried reservoirs. Biodegradation of the oil s appears to have occurred shortly after migration into the traps, followed later by a second charge of unaltered oil. This appears to have been from a different source facies, with more terrestrial, slower reacting Type II/I II kerogens similar to those seen in the Kimmeridge Clay Formation or Heath er Formation in wells on the basin margins. Kinetics studies show the Middl e and Upper Jurassic kerogens to be fast reacting and very oil-prone, with little potential for later gas generation. This finding appears crucial in the understanding of the West of Shetlands petroleum system as it explains the lack of large gas accumulations in a basin of such high thermal maturit y. (C) 1998 Elsevier Science Ltd. All rights reserved.