A study of carbon isotope stratigraphy of the middle Ordovician Yijianfang Formation on the north slope of Tazhong area

161 carbon isotope samples of the Ordovician carbonate from well GL1 and SN1 were analyzed. According to the standard of the carbon isotope established for the Central Uplift Zone, the authors divided the Ordovician carbonate layer system according to the features of carbon isotope, lithology and conodont. Based on the summary of lithology and electric features, the authors divided well GL3 and SN2 through correlation of lithology and electric features. On such a basis, the authors further studied the thickness variation of Yijianfang Formation. The results show that the thickness of Yijianfang Formation in the east of the Shuntuogule Uplift is thicker than that of Yijianfang Formation in the Guchengxu Uplift, possibly because of the difference of the sedimentary environment. According to the evolution characteristics of conodont from the interface between the Yijianfang Formation and the Qrebake Formation in well GL1 and the seismic reflection termination of the T₇⁴ interface of well SN1, the authors have reached the conclusion that the T₇⁴ interface is a unconformity interface near the Tazhong Uplift, because the upper part of the Yijianfang Formation was subjected to obvious erosion. Nevertheless, the T₇⁴ interface should be a conformity interface in other areas. In other words, the strata between the Yijianfang Formation and the Qrebake Formation are continuous in other areas.