The Ancient Lake Basin Characteristics And Its Depositional Response Of Chang 9 And Chang 10 Member In Longdong Area Of Ordos Basin

The Longdong area is located in the southwestern margin of the Ordos Basin.The Triassic Yanchang Formation is the main oil-producing layer.There are multiple sets of oil-bearing layers.Chang 9 and chang 10 are the initial depression period of the lake basin,which have certain exploration potential.There are few studies on the provenance systems,depositional systems,geneses of sandstone and spatial distribution,which cannot be effectively developed.Based on a large number of data of cores,drilling logs,seismic,and laboratory test,a procedural stratigraphic framework for the Yanchang Formation in the Longdong area was established.We studied provenance systems,depositional systems,geneses of sandstone and spatial distribution of Chang 9 and Chang 10 depositional stage.And analyzed the controlling factors and genesis models of sand bodies.What's more,we predicted favorable sand bodies.Based on heavy mineral analysis,rare earth element analysis,and zircon dating,it is believed that the Chang 9 and Chang 10 provenances in the study area are mainly from the northern Yinshan ancient land,the North Qinling and the southern margin of North China.The Longxi ancient land and Alashan ancient land are secondary source area.According to the analysis of trace elements,it is considered that the Chang 9 and Chang 10 deposition periods are generally in a warm and humid weak oxidation-weak reduction environment,and the temperature of Chang 10 and Chang9 is gradually increased,and the water body is a brackish water-brackish water environment;quantified by Co element Calculating the paleo-water depth,the average water depth of Chang 9 and Chang 10 is 20 m,and the water depth of Chang 10-Chang 9is increasing continuously.The paleo-geomorphic features of the study area Chang 9 and Chang 10 were restored through seismic and well data.Through the analysis of core,logging and seismic facies,five types of sand bodies such as underwater distributary channels,mouth bar,sand sheet,slope slide fans,and deep water turbidite fans were identified.Different types of sand bodies in the study area were analyzed.The superimposed relationship and distribution characteristics of the sedimentary facies clearly define the distribution of sedimentary facies in the study area.The distribution characteristics and genesis models of sand bodies are controlled by the characteristics of ancient lake basins,such as provenance,sedimentary environment,climate,water depth,and ancient landforms,which are the sedimentary responses of the ancient lake basin characteristics.Based on the characteristics of the ancient lake basin,this paper analyzes the sedimentary response of known sand bodies,summarizes the sand body distribution patterns,and predicts favorable areas.