Petrographic evaluation of the Middle Jurassic sandstones of the Brent Group, North Sea

Abstract

Quartz cementation and precipitation of clays are two of the most porosity and permeability reducing processes in the Brent Group sediments. Dissolved feldspar and authigenic kaolinite occur at shallow depths as a function of pore-water flow and pore-water chemistry. No significant increase of secondary porosity with depth is shown to occur. Petrographic analyses of the Ness, Etive and Rannoch formations show a reduction of K-feldspar with depth. This can be explained by dissolution of K-feldspars during illitization of kaolinite at temperatures exceeding 130°C (>3.5 km) and albitization of K-feldspars at depths between 2.5 and 3.5 km. Petrographic analyses have also shown that the amount of feldspar and kaolinite and their ratio are controlled by depositional environments. In deeply buried sandstones (>3.5 km), quartz cementation and illitization of kaolinite are the two most porosity and permeability damaging processes. Small amounts of illite are also observed in sandstones at temperatures around 70°C. This can partly be explained by infiltration of some smectite into channel sandstones due to flooding of the delta plain. Carbonate cementation occur at shallow depths and can be very damaging with respect to porosity. Extensive carbonate cementation is in literature mainly linked to marine facies. However, thin layers of extensive carbonate cement are also observed in the more fluvial delta plain environment of the Ness Formation. This suggests that fluid barriers formed by carbonate cementation should not be disregarded in non-marine facies.