Distribution and significance of Middle Ordovician Yijianfang Formation in Shuntuoguole lower uplift, Tarim Basin

The Middle Ordovician Darriwilian stage conodonts have been recovered from key wells in the Shuntuoguole lower uplift, Tarim Basin. They are Eoplacognathus suecicus, Pygodus anitae and Pygodus serrus in ascending order. These fossil provide the basis for the division and correlation of the Yijianfang Formation which is one of the main exploration targets. The characteristics of the Yijianfang Formation were analyzed based on isotopic strati-graphy, lithologic and petrophysical features and seismic data at the same time. On this basis, the distribution of the Yijianfang Formation in this district was analyzed through the comparison of the stratigraphic division, precise tracking and interpretation of seismic horizons. This formation is widely distributed in the Shuntuoguole lower uplift, with a thickness of 140-220 m. It has the largest thickness (> 200 m) in the SN1 to ST1 well area. There is no erosion unconformity between the Yijianfang Formation and the Qiaerbake Formation, which did not allow the development of paleo karst in the Middle Caledonian.