Sourceand MigrationOrganic-rich marine mudstones in the upperJurassic Kimmeridge clay is the source material for most of the region’shydrocarbons (Brooks and Glennie 1987). Thereis possibility of hydrocarbon migration from the East through the flankingterrace, which is also most likely to have occurred. Hydrocarbon migration ismainly vertical, but with significant lateral migration restricted to the UpperJurassic and Paleogene successions. ReservoirThe Brent Group is the reservoir for this oil field.These marine/estuarine sandstones comprise an upward-fining medium to very-finegrained sequence between 12-18m thick that transgressed across the surface ofoutcropping Precambrian and Old Red Sandstone rocks. The sequence is uniform inthickness and forms a sheet sandstone of nearshore to offshore marine origin.
The reservoir lies at a depth of 1500-1800 m and is of good quality withporosities of 2530% and permeability in the range 100-1000 md. The sandstonesshale out eastwards into Heather mudstones. In West and Southwest Heather,oil-bearing Emerald sandstone directly overlies eroded basal Brent, which formsthe primary producing horizon in these fieldsSyn-rift reservoirs within producing fieldscomprise Upper Jurassic to Lower Cretaceous sandstones according to Brooks Brooks and Glennie (1987). Volgian-age sandstoones in Upper Jurassicchannels and wedges may also form reservoir in this area including Interbeddedsandstones and conglomerates and deposited submarine fans. Seal/TrapsThe field shows deferent types of trapping systemslike tilted fault blocks, domes, and stratigraphic closures. Thick, post-riftLower Cretaceous mudstones also provide a regional seal for many traps.
Sealare also provided by Heather or Kimmeridge Formation mudstones and ultimatelyLower Cretaceous Shetland Group Shales and marls. Faults intersection offsetsin the Eastern part of the boundary provide conduits for hydrocarbon migrationand entrapment.