The Yingshan Formation is part of the upper Qiulitag Group in the northeast Tarim Basin. Be- cause the Yingshan Formation is widely distribution, and it is an important carbonate reservoir, its strati- graphic subdivision and correlation is very important. According to international standard, the Lower-Mid- dle Ordovician boundary is defined at the first appearance of the conodont Bartoniodus triangularis. How- ever, B. triangularis has not yet been found in Tarim carbonate platform facies. Thus, we suggest to use the first appearance of Pteracontiodus eacilis to define the Lower-Middle Ordovician boundary within the Yingshan Formation. Accordingly, the Yingshan Formation is broken up into two formations in order to improve the precision of Ordovician stratigraphic correlation for oil and gas exploration in the Tarim Basin.
