Illitic clay, ranging from pure illite to highly illitic illite/smectite (I/S), is the most abundant diagenetic phase in the eolian and sabkha facies of the Rotliegende Formation of the southern North Sea and northeastern Netherlands. Most K/Ar ages of diagenetic illite and I/S in the eolian sandstones are between 100 and 175 Ma. Illite formation is related to two major phases of tectonic activity, the Jurassic Kimmerian orogenic movements and Late Cretaceous-early Tertiary inversion movements.

The burial depth of a sample at the time of diagenetic illite formation can be derived from the K/Ar age and the appropriate burial-history curve. Illite formed in different localities at different times and at different depths. Generally, diagenetic illite is more abundant in samples more deeply buried at the time of illite formation. The correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties. Specifically, we would expect reservoir permeability of the Rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the Late Jurassic through the Late Cretaceous.

Calculated ^dgr¹⁸O values of most illite-forming fluids lie between 0 and 4^pmil (SMOW), indicative of meteoric or marine waters modified by interaction with rock. However, ¹⁸O-depleted fluids (as low as -5^pmil), indicative of a major meteoric component, are inferred to have been involved in the formation of illite in the Groningen gas field at a time when nearby sections of the Rotliegende were eroded and exposed at the surface.