Stratigraphic classification and correlation of Triassic in Caohu Sag of Shaya Uplift, Tarim Basin

Triassic was found well-developed and thick in the Caohu Sag of the Tarim Basin;however,stratigraphic classification was not precise,restricting petroleum exploration.According to the study of plentiful of Triassic sporopollen fossils in mudstones from several wells,4 sporopollen fossil assemblages were established,including Lundbladispora-Limatulasporites-Taeniaesporites,Punctatisporites-Aratrisporites-Calamospora,Apiculatisporis-Rugulatisporites-Colpectopollis,and Dictyophyllidites-Aratrisporites-Alisporites.Triassic was divided into three series,three formations and five members.From bottom to top,there are the Ketuer Formation(it was revised as the upper and lower segments)in the lower series,the Akekule Formation(the upper and lower segments)in the middle series and the Halahatang Formation in the upper series.Lithologic and logging characteristics are obvious in each member.The "pure mudstone segment" at the top of the lower member of the Ketuer Formation and the "big mud" at the top of the Halahatang Formation were regarded as the key beds in this area.There is lacuna between the lower series and the middle series.Triassic was divided into three second-order sequences and five third-order sequences.Based on biostratigraphy,combined with rock stratigraphy,sequence stratigraphy and logging strata,Triassic in the area was classified and correlated more precisely,providing data for the analyses of sedimentary facies,tectonic movement and petroleum accumulation.