Abstract: The experimental data from 120 samples for XRD and SEM analysis show that in Xiaermen Oil Field, clay evolut from shallow to deep with apparent characters. The clay minerals groups in the shallow-upper, Middle-lower, and deep layers are smectite-kaolinite, chlorite - kaolinite -interlayered I/S, and chlorite-illite respectively. Seven types of clay minerals microstructures have been described. The study on these clay minerals microstructures displays that the stability of formation is determined by the dominated types of microstructure in the reservoirs. The order of microstructure stability (from strong to poor) is as following lamina, cabbage, domain, book house, pseudo sphere, honeycomb and cardhouse. Predictions based on comprehensive analysis of sensitive minerals, pore structures, formation fluids and core flow tests indicate that the most serious factors of formation damage are alkaline, fresh water and salted water sensitivity, as well as sand production.

Key words: Clay minerals, clay microstructure, Formation damage, Core analysis, Xiaermen oilfield