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Abstract

The sandstones of the Lunde (Upper Triassic) and the Statfjord (Lower Jurassic) Formations from the Snorre Field of the Northern North Sea have good reservoir properties which are primarily controlled by sedimentary facies. Dissolution of feldspars and other unstable grains during early diagenesis has resulted in abundant well-formed large vermicular booklets of kaolinite, fine-grained kaolinite and blocky kaolinite. Textural evidence suggests that kaolinite and calcite predate quartz cementation. Kaolinite distribution in the Statfjord and Lunde Formations is related to the porosity and permeability characteristics of the sandstones, facies, homogeneity of sandstones, carbonate cement percentage in the sandstones and multiple stacking of channel units. The distribution of kaolinite is controlled by the flow of ground water with low K+(Na + )/Fl + ratios. The increased leaching observed in the channel sandstones could be due to focused flow during and after deposition through channel deposits as compared to the thinner and finer overbank sands. Greater leaching of feldspars and mica resulting in higher amounts of authigenic kaolinite in the Statfjord Formation can be attributed to more humid climatic conditions and higher meteoric flow rates during early Jurassic time.

Diagenetic illite occurs in relatively low concentrations in the Lunde and Statfjord Formation sandstones and may have formed mainly from smectite. The low illite content observed at the present burial depth suggests that illite requires higher temperatures than 80-95°C to form from feldspar and kaolinite. The present distribution of kaolinite in these reservoir rocks will control the distribution of illite at greater burial (>4 km) which would be important for the reservoir quality at those depths.

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