Rb-Sr and K-Ar isotopic determinations were combined with scanning electron microscopy (SEM) observations and x-ray diffractometry (XRD) controls to date the diagenetic formation of clay minerals from oil-bearing sandstones of the Middle Jurassic Brent Sandstone in the North Sea.

The separated clay fractions often contain detrital components, especially K-feldspars, which are reduced to sizes smaller than 0.4 µm either by intense in-situ alteration or by destructive sample preparation. Direct Rb-Sr and K-Ar isotopic dating of diagenetic illite therefore was not possible. Isotopic data obtained on mixtures of detrital and newly formed components showed, however, that illite formed about 40 to 45 Ma. This value also was obtained directly on clay fractions separated by a gentle method of disaggregation.

Illitization is related to emplacement of hydrocarbons and associated waters into the formation. Aqueous and hydrocarbon fluids simultaneously trapped in quartz-overgrowth inclusions indicate late silicification contemporaneous with this fluid migration. Minimum trapping temperatures of at least 110°C for these fluid inclusions are too high for the present burial depth. Hot fluids are assumed to have originated in deep source rocks beneath the Brent reservoirs and migrated upward during the middle Eocene.