Abstract

Middle Jurassic sandstones of the Brent Group from the Huldra Field, North Sea show extensive framework grain dissolution. Sandstones of the Etive Formation, which represent a laterally extensive sandstone facies, show more leaching than equally permeable sandstones of the Ness Formation, which are likely to be less continuous laterally. This variation in amount of secondary porosity is consistent with leaching and mass transfer associated with advection, and this may give information about internal flow properties within the reservoir. Kaolinite and relicts of feldspar are found in secondary pores, suggesting the formation of secondary porosity caused by feldspar dissolution associated with kaolinite precipitation. The least kaolinite is found in the sandstone facies of highest secondary porosity, suggesting that the mobility of aluminum is related to the rate of the pore-water flow. This is most likely explained by meteoric pore-water circulation due to the isotopic signature of the kaolinite, indicating formation at a temperature < 60 degrees C (Glasmann et al. 1989b), and also by the fact that illite is formed at elevated temperatures. Secondary porosity is also formed at greater burial depths at temperatures higher than 60 degrees C, but to a lesser extent. This leaching, which is selective for K-feldspar, is possibly enforced by the illitization of smectite in finer-grained associated sandstones and siltstones and by minor illitization of kaolinite. There is no net increase of porosity due to this process, and mass transfer is by diffusion.

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