Reservoir quality and burial diagenesis in the Statfjord Formation, North Sea

This paper presents models for reservoir quality prediction in the dominantly fluvial reservoir sandstones in the Lower Jurassic Statfjord Formation in the North Viking Graben. The models are based on theory and on empirical observations from wells from the Norwegian sector of the North Sea. At depths shallower than 3000 m, porosity and permeability in the Statfjord Formation sandstones show systematic trends versus burial depth and the net-togross ratio is mainly a function of the amount of sand deposited. Hence, confident reservoir quality prediction at shallow burial may be founded on regional or subregional models that address sandstone architecture and on regional or sub-regional porosity and permeability versus depth trends. Beneath 3000-3500 m, however, the reservoir quality is more variable due to variations in porosity and permeability reduction following compaction, quartz cementation and formation of fibrous illite. Between 3000 and 4000 m, formation of quartz cement and fibrous illite reduce the permeability and the net-to-gross ratio. Accordingly, permeability lower than 1 mD and net-to-gross ratios less than half the initial sandstone content are expected in many prospects below 4000 m. In some deeply buried reservoirs, porosity 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 hindered by limited potassium supply.