Cambrian lithofacies paleogeographic characteristics and evolution of southern margin in Ordos basin

Through analysis of outcrop profile,drilling and core observation,based on stratigraphic division and correlation,the Cambrian lithofacies palaeogeographic characteristics of the southern margin of the ordos basin in different periods has been systematically analyzed.The study results show,influenced by ancient land and sea-level change,the Cambrian in the study area has undergone three evolutionary stages.In Xinji-Zhushatong formation,it is shallow sea sedimentary facies,gradually transited to carbonate gentle slope sedimentary facies.In Mantou formation,it is carbonate gentle slope sedimentary facies,and from north to south,it is developed inner gentle slope,outer gentle slope and deep water basin facies.In Zhangxia formation,the transgression reached the maximum degree,carbonate platform formed and platform margin oolitic beach was abnormally developed.In Sanshanzi formation,it is composed by platform-margin beach,gentle slope and deep water basin sedimentary facies.Affected by later hypergene dissolution,high-quality reservoirs have been formed in platform-margin beach of Zhangxia formation and regional parts of Sanshanzi formation,which is the most favorable reservoir facies belt in Cambrine system.