Study On Sedimentary Microfacies And Depositional Environments Of Upper Ordovician In BachuYijianfang Area

The outcrop of Upper Ordovician limestone is distributing in the Bachu area of central uplift of Tarim Basin. The reef-bank complex outcrop of Lianglitage Formation are well exposed. Base on the principle of stratigraphy, integrating outcrop and laboratory analysis, using the knowledge of petrography, carbonate microfacies, sedimentology and geochemistry, four mearsured outcrop sections are correlated, and their macrofacies types and depositional environments are researched.By the means of microfacies analysis, fossil assemblage, particle composition, matrix type and its structure has been studied of the Upper Ordovician limestone in Bachu area, the microfacies of Tumuxiuke Formation is packstone composed of thin-shell Ostracodas and Trilobites. The macrofacies of Lianglitage Formation are packstone with bioclasts, pelletoidal limestone, intraspararenite, oospararenite, algal limestone, pelletoid-algal bondstone.According to the analysis of microfacies types and their combination types, we argue that Tumuxiuke Formation is deposited in the slope facies, forming condensed member in three order sequence, while Lianglitage Formation deposited in the regression progress of sea water, and the depositional environments are mainly open platform, restricted platform with reef-bank developed. Reef building organisms are mainly calcareous green algae. Overall, it shows the depositinal patterns of northward deeper and southward shallower in studied area, with reef developing in north, oolitic bank developing in south.The research of Carbon and Oxygen stable isotopes shows that limestone of Upper Ordovician in Bachu area is mainly deposited in the normal marine environment,? Eu and ?Ce appear obviously negative anomaly, which reflect that the diagenetic environment is anoxic condition by analysis of trace elements.Analysis of diagenesis and reservoir physical properties indicates that the diagenesis of Lianglitage Formation are micritization, cementation, silicification, dissolution, recrystallization,compaction and pressure solution. Reservoir spaces of the Lianglitage Formation are mainly the pore spaces in the reef and mound and dissolution fractures and pores in oolitic limestone.