Sedimentary Characteristics Of The Upper Cretaceous Donggou Formation In The Southern Margin Of Junggar Basin

Donggou Formation has long been considered to be widely distributed and thick in the southern margin of Junggar Basin.However,more and more paleontological data,outcrop data and drilling data show that the Donggou Formation is pinched out at the location of the Anjihai anticline and thin on the Horgos anticline.At present,the stratigraphic distribution and sedimentary characteristics of Donggou Formation in the southern margin of the basin are unclear,which restricts the oil and gas exploration of the formation.Based on the outcropped stratigraphic profiles in the field,the stratigraphic distribution and sedimentary characteristics of the Upper Cretaceous Donggou Formation in the southern margin of the basin have been known to some extent by using the principles and methods of sedimentology through the field measurements and sample analysis,drilling data and seismic data.The boundary symbols between the Donggou Formation in the outcrop area and the overlying Ziniquanzi formation,and the lower Tugulu member are obvious,and the top boundary of the Donggou Formation is usually large trough-shaped interbedded calcareous cemented conglomerates of the Ziniquanzi Formation overlying the Donggou Formation..The boundary between sandstone and gravel-bearing sandstone with large cross bedding at the bottom of Donggou Formation and mudstone of Lianmuqin Formation shore-shallow lacustrine facies is obvious.The outcrop strata of the Donggou Formation show a certain degree of segmentation,which can be roughly divided into four sections from bottom to top.The main distribution characteristics of Donggou Formation are as follows: Donggou Formation is only 2m thick in Nananjihai River section.The Donggou Formation is missing in the west of Well Huo 10,and the Donggou Formation pinches out in Well An7 and Well An4.The western boundary of Donggou Formation in the study area is determined.The Donggou Formation in the Dongdagou section of the Taxi River is about 30 m thick,and the Dongdagou Formation is missing from the Eastern Bank of the Taxi River at about 2.5km northwest of the Dongdagou section of the Taxi River.The southern part of Aketun anticline is the southern boundary of Donggou Formation.From West to east,the thickness of Donggou Formation has a trend of thinning first and then getting thicker.To the north,the drilling area is still thicker in the east than in the west.From south to north,the thickness of the study area increases first and then decreases,with a small reduction.Through the observation of the field profiles,we can know fifteen types of lithofacies are developed in Donggou Formation in the southern margin of Junggar Basin.They can be combined into 11 types of lithofacies assemblages.Based on the analysis of outcrop sedimentary environment and drilling facies,it is found that braided river,alluvial fan and braided river delta deposits are mainly developed in the Donggou Formation in the southern margin of the basin.The alluvial fan and braided river delta environments are mainly developed in the western part of the southern margin of the basin,while the sandy braided river deposits and braided river delta environments are mainly developed in the eastern part of the basin.From north to south,the overall grain size of the Donggou Formation in the south of the southern margin is coarser,and alluvial fan facies and braided river facies are developed,while from north to north,braided river delta facies is mainly developed.The outcrop area mainly develops alluvial fan environment and braided river environment,while the drilling area mainly develops Braided River Delta environment.The study area develops a set of alluvial fan-braided river-braided river delta sedimentary system with sufficient source.During the Late Cretaceous,there were obvious uplift and denudation processes and intense tectonic activities in the southern margin of Junggar Basin.The source of the sandstone clastic components in the Donggou Formation of the Upper Cretaceous mainly comes from the transitional island arc and the recycling orogenic belt,and the source mainly comes from the Yilinheibiergen Mountains in the south.