Coupling of diagenesis and sedimentary facies of Shanxi Formation in An'sai-Shenmu, Ordos Basin

The type of rocks, diagenesis, distribution of Permian Shan 2 Member in the central-eastern Ordos Basin were studied based on core observation, thin section analysis, cathodoluminescence and scanning electron microprobe. The reservoir mainly consists of quartz and feldspathic quartz sandstones with subordinate lithic sandstones. The sandstones experienced all kinds of diagenetic processes, such as mechanical compaction, pressure solution, quartz overgrowth, carbonate cementation and replacement, kaolinite filling, dissolution. The dissolution turns out to be the essential control for the formation of effective reservoirs. Laterally, diagenetic features were analyzed in detail, with a conclusion that the regional variations of diagenesis are mostly controlled by facies in the given area. Along the dispersal system of sediments, pressure solution-siliceous cementation and dissolution become prominent whereas compaction, carbonate cementation and replacement decrease in intensity. From the center of the distributary channels to both flanks, compaction, carbonate cementation, and matrix dissolution increase in intensity and siliceous cementation and dissolution wane accordingly. Pores are well developed in the sandstones in the distributary channel of deltaic plain, the subaqueous distributary channel and mouth bar of deltaic front.