Dr. A. Parker writes: Dr Hancock claims that in the Brent Sand there is an inverse correlation between authigenic illite and authigenic kaolinite, and considers that the illite is forming by progressive alteration of the kaolinite with increasing depth. He does not believe there to be a significant amount of detrital clay present.

Except in the case of the Upper Brent sand, this correlation is far from obvious by simple visual inspection of the data, and some quantification would appear necessary. Furthermore, Dr Hancock has not determined the mineralogy of the shales that are interbedded with the sandstones. In a similar investigation, I have found that the clay mineralogies of adjacent shales and sandstones are often identical, both qualitatively and quantitatively. Although there may be some clay authigenesis in these Jurassic shales, it is reasonable to assume that most of their clay mineral component is detrital in origin, the Scandinavian platform having contributed fair amounts of kaolinite. I have found that it is very easy to overestimate the proportion of authigenic clay in such studies; when using the SEM in particular, it is far easier to identify well-crystallized material than poorly crystalline or amorphous material, and one tends to be influenced by this in making visual estimations of relative proportions. In my experience, partial recrystallization and re-distribution of detrital clay-sized material, particularly in pore throats, are usually more significant in permeability reduction than authigenic kaolinite, which generally forms in actual pores.

A Parker, Sedimentology Research Laboratory, The University, Whiteknights, Reading,

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