Structure-related geochemical (REE) and isotopic (K-Ar, Rb-Sr, δ18O) characteristics of clay minerals from Rotliegend sandstone reservoirs (Permian, northern Germany)

Euhedral illite cementing the gas-bearing sandstone reservoirs of the Rotliegend in the Niedersachsische rift system (northern Germany) was studied along a horst-to-graben cross-section to examine its chemical and isotopic characteristics. The data show that differentiated illite particles grew during a tectono-thermal event marked by distinct episodic hydrothermal activities along fault drains acid in the poral space of sandstones at 210 Ma and at 195 to 190 Ma in horst positions, at 185 to 175 Ma in the nearby graben, and at 170 to 165 Ma in both the horst and graben. Varied REE distribution patterns and initial Sr-87/Sr-86 ratios (from 0.7124 to 0.7142) relative to illite-particle size outline a high chemical variability of the fluids during illite growth. The delta(18)O values of illite range from +20.0 to +12.4 parts per thousand and those of chlorite from +12.0 to +6.9 parts per thousand. The changing delta(18)O values of the minerals most likely relate tovaried delta(18)O values of the fluids, or to different water/rock ratios during crystallization. Differences among the REE and Sr-87/Sr-86 tracers and the delta(18)O values of different generations of illite also suggest a changing fluid chemistry relative to time. Significant positive Eu anomalies and negative Ce anomalies in the REE distributionsof illite suggest feldspar alteration by the migrating fluids in oxidizing environments. (C) 1999 Elsevier Science Ltd.