塔里木盆地塔东地区古城8井上寒武-中下奥陶统碳酸盐岩碳氧同位素特征及意义OA

Carbonate reservoirs play a pivotal role in petroleum exploration endeavors within the Tarim Basin.How-ever,due to limited core sampling,severe dolomitization,and challenges associated with conventional well logging interpretation,the classification and correlation of Cambrian-Ordovician carbonate strata have long remained conten-tious.Stable carbon isotopes offer a promising avenue for facilitating global or basin-scale stratigraphic correlation,thereby presenting a novel approach to addressing this issue.This study systematically investigates the carbon and oxygen isotopes of 116 carbonate samples sourced from the Upper Cambrian to Lower-Middle Ordovician in Well GC8,situated in Gucheng area,Tarim Basin,NW China.The results show that the distribution of δ13C values ran-ges from-2.32‰ to+0.53‰,with an average of-0.95‰.The overall δ13C exhibits a negative trend from bottom to top with a small change in amplitude,typified by two cycles of gradual negative excursions succeeded by abrupt positive excursion.The δ18O values portray a predominantly negative trend with limited variability,characterizing by two weak positive excursions followed by strong positive excursions.Based on the conodont biostratigraphy,com-parative analysis with other representative sections elucidates the presence of TOCE(top of Stage 10 isotopic carbon excursion),COTICE(Cambrian-Ordovician transition isotopic carbon excursion)and MDICE(mid-Darriwilian iso-topic carbon excursion)through the Upper Cambrian to Lower-Middle Ordovician in Well GC8.Notably,the docu-mentation of TOCE and COTICE excursions within the Tarim Basin represents a pioneering discovery,with potential implications for positioning the Cambrian-Ordovician boundary at the inflection point of δ13C positive excursion after the TOCE.Furthermore,regional disparities in the MDICE development across the Tarim Basin hint at potential sedimentary hiatus of the Yijianfang Formation in Well GC8,with likely exhibiting the absence of the Pygodus anserinus conodont zone and a substantial portion of the Pygodus serra conodont zone in the Yijianfang Formation in Gucheng area.