Abstract
Euhedral illite cementing the gas-bearing sandstone reservoirs of the Rotliegend in the Niedersächsische 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 and 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 87Sr/ 86Sr ratios (from 0.7124 to 0.7142) relative to illite-particle size outline a high chemical variability of the fluids during illite growth. The δ 18O values of illite range from +20.0 to +12.4‰ and those of chlorite from +12.0 to +6.9‰. The changing δ 18O values of the minerals most likely relate to varied δ 18O values of the fluids, or to different water/rock ratios during crystallization. Differences among the REE and 87Sr/ 86Sr tracers and the δ 18O 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 distributions of illite suggest feldspar alteration by the migrating fluids in oxidizing environments.
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