Diagenesis and Hydrocarbon Accumulation, Brent Sandstone (Jurassic), Bergen High Area, North Sea

Diagenesis of Brent Group sandstones in the Bergen High area was studied and related to the history of hydrocarbon maturation and pore-fluid environment. Calcite and kaolinite are low-temperature diagenetic precipitates that formed as meteoric pore water entered the Brent at the end of Brent deposition and later, during late Cimmerian erosion. A major period of feldspar dissolution associated with quartz and illite cementation occurred at Huldra field. Precipitation of diagenetic illite in Brent sandstones was closely linked with hydrocarbon maturation in the adjacent Viking graben and accumulation of generated fluids in traps at structural highs. At Huldra field, illite K-Ar ages suggest onset of illite diagenesis in the latest Cretaceous, initial reservoiring of hydroca bons during the latest Paleocene to early Eocene (58 Ma), and filling of the reservoir by early Oligocene (38 Ma). Pore-water character during illite diagenesis, interpreted from oxygen isotope and quartz overgrowth fluid inclusion data, was warmer than predicted by burial history analysis and one and a half to two times seawater salinity. The incursion of saline, isotopically evolved fluids at Huldra suggests replacement of paleometeoric water by basin compaction fluid. Illite K-Ar ages from Veslefrikk field indicate a minimum age of hydrocarbon accumulation at about 30 Ma. Modern pore-water chemistry in the Brent at Veslefrikk is unlike the suggested water chemistry at Huldra and indicates that migrating compaction water did not completely displace older fresh connate water. This sugge ts that hydrocarbons migrated into Veslefrikk as a discrete phase, without great volumes of associated compaction water.