1. Research Institute of Exploration ＆ Production，SINOPEC，Beijing 10083，China; 2. Exploration and Development Research Institute，Northwest Company，SINOPEC，Urumqi 830011，China

Based on the theory of corresponding relation between carbon isotope value fluctuation with sedi- mentary environment，sea level eustacy，formation interface mutation，etc，the authors study the stratigraphy by carbon isotopic method，and establish the classification standards for each unit，which solved the difficult problems on determining of the strata without index fossils in carbonate rocks． The Ordovician carbon isotopic curves present a process of stable- rising- positive drift in descending order． The δ 13 C values vary from － 4. 2 × 10 ^－3 to － 0. 2 × 10 ^－3 in Lower Ordovician Penglaiba Formation and Yingshan Formation，characterized by the values below zero; from －0. 4 ×10^－3 to 0. 6 ×10^－3 in Middle Ordovician Yijianfang Formation，characterized by distributed near the zero value，and vary from 0. 7 ×10 ^－3 to 3. 1 ×10 ^－3 in Upper Ordovician Qiaerbake Formation and Lianglitage For- mation，featured by the values all above zero． On this basis，with the carbon isotopic standards，the correlation of the Lower Ordovician strata of the typical outcrop profiles in Bachu area with the wells of Margaiti slope was stud- ied． It is confirmed that the Upper Ordovician developed in middle Margaiti slope and the eastern fault depression zone． Except the eastern fault zone，all the areas commonly remain the Micrite Member at the top of the Lower Or- dovician．