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

Abstract

Recent significant oil discoveries have focused exploration attention on the Atlantic Margin areas north and west of Britain, in particular the West of Shetlands area. These discoveries, the Foinaven and Schiehallion fields, each have reserves of 250 to 500 million barrels (39.8 to 79.5 × 10 6 m 3) of 24–27° API oil while an earlier heavy oil discovery, the Clair Field, is the largest undeveloped oil discovery on the U.K. Continental Shelf with resources estimated in the range 3–5 billion barrels (477 to 795 × 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 Jurassic Kimmeridge Clay Formation equivalent, the Middle and Lower Jurassic and the Middle Devonian. The results of geochemical studies of the oils, reservoir core extracts and source rocks from released wells from these West of Shetlands discoveries show them to be genetically related to North Sea oils sourced from the Kimmeridge Clay Formation equivalent. Detailed geochemical analysis (biomarkers, source rock reaction kinetics, compound specific isotope 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 place, with an intervening biodegradation event. Oil charging from Middle Jurassic lacustrine source rocks is also indicated in the “Foinaven Complex” oils. 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 directly linked with the tectonic evolution of the West of Shetlands with major oil generation from very oil-prone Kimmeridge Clay and Middle Jurassic lacustrine facies into shallow-buried reservoirs. Biodegradation of the oils 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/III kerogens similar to those seen in the Kimmeridge Clay Formation or Heather Formation in wells on the basin margins. Kinetics studies show the Middle 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 maturity.