Authigenic clay minerals, in an almost complete sequence of oil-saturated Brent Sand from an oilfield in the Viking Graben in the northern North Sea, show a transition from kaolinite in the upper part into illite in the basal part of the reservoir; replacement textures indicate that the illite has replaced kaolinite.

After early kaolinite formation, it is suggested that illite diagenesis took place synchronously with oil migration, and that the progressive extension of oil saturation down through the reservoir terminated diagenetic processes. Early diagenetic stages are thus 'frozen' and preserved in the upper part of the reservoir.

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