Division and Correlation of Middle-Lower Ordovician in Tazhong area,Tarim Basin

According to the palaeontological feature, petrophysical and electric properties from the new wells, with combination of data from thin-section and scanning electron microscope, the Middle-Lower Ordovician in Tazhong aera can be divided into Yijianfang formation (Middle Ordovician),Yingshan formation (Middle-Lower Ordovician) and Penglaiba formation (Lower Ordovician), Yingshan and Penglaiba formations can be subdivided into 4 or 5 members, which are of good comparable behavior in the whole area. There existed an unified carbonate platform in Early-Middle Ordovician among Cardac uplift, Tangutsbas depression and Guchengxu uplift. In Cardac uplift, Yijianfang formation was lack, and Yingshan and even Penglaiba formations were incoordinately eroded, being an angular unconformity with Upper Ordovician or Lower Carboniferous. But in Tangutsbas depression and Buchengxu uplift, Yijianfang formation was existent, being successive deposition with Yingshan formation.