Study on the sedimentary environment of the Middle Ordovician Yijianfang Formation in the south of Tarim River area,Xinjiang,China

The Middle Ordovician Yijianfang Formation mainly consists of carbonates in the south of Tarim River area of Xinjiang.The authors have observed 28 cores and identified 3200 slices.They think the rock types are bioclastic limestone,biolithite,algal pelleted limeston,oncolite limestone,oolitic limestone,intraclastic limestone,micrite,glauconite limestone.Algae,echinoderm,bivalve,and reef-building organisms including bryozoan,coral,stromatoporoids,and sponge are conserved in the carbonate of the Yijianfang Formation.This indicates a normal shallow marine environment with water cycle,normal salinity and sufficient oxygen.Based on the integrative analysis of seismic data,log data and petrologic character,three types of sedimentary facies can be recognized in the Yijianfang Formation.They are platform shoal,intershoal,and patch reef.Based on this,the paleogeographic maps about the early Darriwilian Stage and late Darriwilian Stage are mapped,respectively.At last,the authors describe the depositional facies distribution in space.This is very important for people to engage oil and gas exploration in study area.