The Stratigraphic Classification and Correlation of Jurassic in Caohu Depression,Tarim Basin

The Jurassic of Caohu depression in the Tarim basin developed fully,with large thickness,but the precision of stratigraphic division and correlation is lower,which is one of the important reason that restrict the exploration of oil and gas in this aera.According to the fossil material collected from drilling hole,established three palynological assemblages,there are Cyathidites-Osmundacidites-Piceites,Cyathidites–Neoraistrickia-Disacciatrileti and CyathiditesClassopollis-Piceaepollenites,Aneuletes cucuma and A.stellerides-A.lufengensis,A.maxima-P.kucheensis.The Jurassic was divided into three series,five formations,From bottom to top are Ahe Formation and Yangxia Formation in lower series、Kizilnur Formation and Qiakmak Formation in middle series and Qigu Formation in upper series(Kelazha Formation is lacuna).Lithological and logging characterastics are obvious in each member,the coal stratificationat the lower and black mudstone sagment at the top of the Yangxia Formation,the coal stratification in the kizilnur Formation and the red mudstone sagment were regarded as the key bed in this research area.There is sedimentic absence in the Ahe Formation and the lower parts in the Yangxia Formation,denuded absence from Qiakmak Formation to Kelazha Formation in most aera.Jurassic was divided into three second-order and five third-order sequences,relied on the seismic sequence stratigraphy and borehole sequence stratigraphy,the detailded classification and correlation of Jurassic in the aera has been achieved,which based on biostratum and combined litology,sequence and logging strata.Those discoveries made it possible to analyze the tectonic evolution,sedimentary facies and hydrocarbon accumulation in Caohu sag and neighbouring regions.