Several types of authigenic illite differing in their distribution and morphology are described in Rotliegend (Permian) gas-bearing sandstones of northern Germany. The microtextural environment seems to control the K-Ar age variations of illite-fractions of different sizes. Thin, uncompacted, and radial coatings are 200 Ma old, whereas uncompacted fibrous laths filling enlarged secondary porosity are 180 Ma old, and compacted or recrystallized tangential particles yield intermediate or even younger ages. Such Liassic ages relate to a thermal event, the effects of which on the clay material have not been erased subsequently, despite present-day burial depth of 4500 m and temperatures of 150°C.

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