Abstract:

Taiyuan Formation in eastern Ordos Basin is typical tight sandstone reservoir, with high content of clay minerals, so it has great influence on reservoir evaluation and natural gas exploration. Based on the analysis of petrography and geochemistry, the characteristics and origin of clay minerals of sandstone reservoir of Taiyuan Formation in eastern Ordos Basin were studied. The result shows that the reservoir sandstone of Taiyuan Formation is with high content of clay minerals, which is mainly of illite and kaolinite. The illite is characterized by high content of Al₂O₃ and K₂O and low content of FeO and MgO, and the Al₂O₃ content of kaolinite is more than that of typical kaolinite. Sr/Ba ratio of illite and kaolinite is less than 1. Meanwhile, illte and kaolinte are enriched in LREE, depleted in HREE, characterized by downwards sloping and some extent of δ Eu and δ Ce anomaly. Petrographical and geochemical characteristics show that illites and kaolinites are all related with hydrothermal fluid. Combined with tectonic-burial history and authigenic diagenetic minerals sequences, it is revealed that hydrothermal diagenesis began with the early Cretaceous thermal anomaly in Ordos Basin. Hydrocarbon fluid formed by thermal evolution of coal-bearing source rocks came into reservoir sandstone of Taiyuan Formation, which leads to the hydrothermal alteration to form large quantity of illtes, kaolinites and other authigenic minerals.

Key words:  lacustrine dolomite, primary biogenesis , penecontemporaneous metasomatism , hydrothermal metasomatism , primary origin related toexhalative hydrothermal activities , intergranular pore , geochemical analysis