Analysis of the provenance direction and the depositional systems of Yanchang Formation of Upper Triassic in the southern Ordos Basin

Based on the study on the characteristics of the palaeocurrent and heavy mineral assemblages,the analysis of grain size and the characteristics of the sandstone framework mineralogy,the provenance and the characteristics of the mother rock during the deposition of the Yanchang Formation of Upper Triassic in the southern Ordos Basin are systemically studied.The results show that the studied area is affected mainly by four sediment sources.The delta deposition in the Yanchang Formation can be divided into four types of depositional systems according to the source direction,depositional environment and sandstone petrography,including meandering delta,braided delta,lacustrine and deep-water gravity flow.They are is Fuxian delta depositional system in the northeast,Miaowan-Yijun——Huangli delta depositional system in the southeast,Binxian-Changwu delta depositional system in the southwest and Qingyan——Heshui delta depositional system in the west in turn.The evolution process of different delta depositional systems and deep-water gravity flow and the relationship between the depositional systems and the hydrocarbon distribution are elaborated in detail.And it is shown that the hydrocarbon mainly distributes in the delta front subsea distributary channel,mouth bar and its superposition,turbidite sandstone body and sandy debris flow deposit,which are the most favorable sedimentary facies belt for the generation of Yanchang Formation reservoirs.The study results in this paper are useful to reasonable deployment of oil and gas exploration.