RESERVOIR QUALITY AND BURIAL DIAGENESIS IN THE STATFJORD FORMATION, NORTH-SEA

This paper presents models for reservoir quality prediction in the dom inantly fluvial reservoir sandstones in the Lower Jurassic Statfjord F ormation in the North Viking Graben. The models are based on theory an d on empirical observations from wells from the Norwegian sector of th e North Sea. At depths shallower than 3000 m, porosity and permeabilit y in the Statfjord Formation sandstones show systematic trends versus burial depth and the net-to-gross ratio is mainly a function of the am ount of sand deposited. Hence, confident reservoir quality prediction at shallow burial may be founded on regional or sub-regional models th at address sandstone architecture and on regional or sub-regional poro sity and permeability versus depth trends. Beneath 3000-3500 m, howeve r, the reservoir quality is more variable due to variations in porosit y and permeability reduction following compaction quartz cementation a nd formation of fibrous illite. Between 3000 and 4000 m, formation of quartz cement and fibrous illite reduce the permeability and the net-t o-gross ratio. Accordingly, permeability lower than 1 mD and net-to-gr oss ratios less than half the initial sandstone content are expected i n many prospects below 4000 m. In some deeply buried reservoirs, poros ity loss by quartz cementation and compaction is retarded by chlorite coatings or by high pore pressure. Good permeability may be preserved to depths greater than 4000 m if the porosity is preserved (i.e. by a high overpressure or chlorite coatings) and if illitization is hindere d by limited potassium supply.