Provenance Analysis And Sedimentary System Of Upper Paleozoic Shanxi Formation And The 8^thmember Of Shihezi Formation In The Southeast Part Of Ordos Basin

In recent years,with the southward progress of the Upper Paleozoic exploration and development in the Ordos Basin,it is an important step to strengthen the exploration of the southeastern part of the basin in order to make new breakthrough in Ordos Basin.However,there are few studies and relatively low degree of research in this area.For example,there are still some deficiencies in the aspects of provenance,sedimentary characteristics,sand body distribution and reservoir characterization,which could not determine the favorable exploration area,so as to directly affect the deployment of exploration wells,resulting that follow-up work can not be effectively carried out.In this paper,based on the previous studies,focusing on Shanxi Formation and the 8^th number of Shihezi Formation,using drilling core and logging data,provenance analysis and depositional system research are carried out and finally determine the evolution of sedimentary facies and distribution of sandbody to provide the evidence for“sweet point”prediction.Based on a series of sedimentary methods of provenance analysis,such as detrital composition of sandstone,lithic fragments composition,typomorphic characteristic of quartz,quartz cathodoluminescence,feldspar content and type,heavy mineral assemblage and a geochemical method-rare earth elements analysis.It is concluded that there are three provenance areas in the depositional period of Shanxi Formation in the study area.They are northwestern source,northeastern source and southern source.During the 8^th member of Shihezi depositional period,compared to Shanxi Formation,the southern provenance move forward to north considerably,having a larger influence on sediments,and the northwestern source almost no longer supply sediments to the study area.At the same time,sediments supply from northeastern provenance narrowed,while the southern provenance had a larger impact on sediments of study area.The study area is controlled by two provenance systems as a whole:the northern part is the far-source,where parent rock is from ancient Yin Mountain;the southern part is the near-source,where parent rock is from North Qinling Mountain.The lithologic characters,sedimentary structure,palaeontological facies markers and environmental facies markers of the strata in the study area were observed by the drilling core.Combined with the logging data,single well interpretation,well tie sedimentary facies correlation and analysis of plane facies,it is concluded that delta sedimentary system is mainly developed in the Shanxi Formation and the 8^th member of Shihezi Formation,including meandering river delta system and braided river delta system.From the Shanxi Formation to the 8^thmember of Shihezi Formation,the sedimentary environment in the northern region has been transformed into a braided delta from meandering river delta due to the advancement of southern provenance north to almost the entire study area,while the southern part has been braided river delta all the time.There are altogether four types of sedimentary microfacies of meandering river delta,which are underwater distributary channel,underwater natural dike,inter-distributary bay and distributary channel.Among them,the underwater distributary channels are the most developed microfacies,and the channels mainly develop in the middle and lower section of the first member of Shanxi Formation.The braided delta has identified six sedimentary microfacies:braided,floodplain,underwater braided,underwater braided,and estuarine.Among them,the underwater distributary braided channels are the most developed microfacies.