Print this pageOrigin of High Porosity in Central North Sea HPHT Clastic Reservoirs: Influence of Overpressure on Cementation and Compaction
OSBORNE, MARK J.
High pressure high temperature (HPHT) clastic reservoirs of the Upper Jurassic Fulmar Formation, North Sea, have anomalously high porosities (up to 30%) for their depth of burial (4.0-6.0km). Porosity preservation results from the dynamic interaction of pressure, temperature, and compaction, plus water and rock chemistry. These high porosity, deeply buried reservoirs are highly overpressured, but other factors control localized porosity distribution within the reservoir itself, including cementation by ankerite, quartz, illite and albite. However, the highest porosities are found in well sorted sandstones which contain abundant secondary porosity (after feldspar), with grain rimming microquartz cements inhibiting later quartz overgrowths. These HPHT reservoirs have less quartz cement than slightly overpressured reservoirs buried to similar depth, and lack compactional features such as interpenetrating grain contacts and stylolites. Variation in quartz cement abundance with pressure state (effective stress) indicates that overpressure has inhibited pressure solution and associated quartz cementation. While feldspar grain dissolution has contributed to macroporosity, it is difficult to calculate whether ions released during dissolution have been exported from the reservoir or not. This is because the Fulmar Formation contains abundant detrital clay (up to 40%). Mass balance calculations show only minor export, and K and Al ions could easily have been absorbed by these smectitic clays as they became altered to diagenetic illite. Porosity preservation is primarily due to inhibition of chemical compaction.