Subject : Research On Sedimentary Sequence And Coal Accumulation Of Xishanyao Formation In Eastern Junggar Basin,xinjiang

Junggar basin is located in the northwest of China and in the center of Eurasia.Surrounded by Tianshan Mountains,Altay Mountains,kelameishan mountains and Zaire mountains,the basin is a nearly triangular internal drainage basin,which is the key battlefield for CNPC to "stabilize the East and develop the west".The study area is located in the east of Junggar basin,which contains abundant coal resources.The research area is Xishanyao Formation of Middle Jurassic in eastern Junggar basin.The research content is mainly based on sequence stratigraphy theory,combined with the basic theory and method of geological multidisciplinary.The sequence stratigraphic framework of Xishanyao Formation of Middle Jurassic in eastern Junggar basin is established to reveal the filling of Xishanyao Formation sequence in time and space In order to analyze and summarize the accumulation characteristics and occurrence rules of coal resources in Xishanyao Formation,and provide important guidance for the evaluation of coal mineral potential in Junggar basin.The main achievements of this study are as follows:In this paper,the stratigraphic framework of the Xishanyao Formation of Middle Jurassic in the eastern Junggar basin is established.It is considered that there are one super long cycle(structural sequence)and two long-term cycles(third-order sequence)in the study area.Based on the sequence stratigraphic framework,the sedimentary system of Xishanyao Formation of Middle Jurassic in the eastern part of the gar basin is proposed.The main sedimentary facies types are braided river facies,meandering river facies,delta facies and lacustrine facies.Based on the analysis of paleoclimate and palaeogeography,it is considered that the Middle Jurassic Xishanyao Formation in the eastern part of the gar basin was dominated by subtropical humid climate,which was favorable for coalaccumulation.The thickness of the strata in the northeast is thin and the variation gradient is small.The provenance mainly comes from kelamei mountain in the north and Bogda mountain in the south.Finally,based on the analysis of the sedimentary characteristics,sequence stratigraphy,paleoclimate and palaeogeography of Xishanyao Formation in the study area,the sedimentary characteristics and sequence stratigraphy of Xishanyao Formation in the study area are analyzed.It is considered that thin coal seams are developed in lowstand system tracts and have poor stability.During the period of lacustrine transgressive system tracts and highstand system tracts,there are many thick coal seams superimposed by parasequence facies to extremely thick coal seams.The thickness of coal seams gradually thinned in the center of basin and the direction of provenance until they pinch out.The coal seams in the study area are mainly controlled by the subsidence rate of the basin basement and the change of lake level.The subsidence of the basin base can provide a place for coal accumulation,and the expansion of lake level submerges the peat swamp,which may lead to the end of coal forming conditions.The synsedimentary depression is the most favorable area for coal seam development when the basement subsidence rate and peat growth rate balance each other during coal accumulation period.