Upper Carboniferous Volcanic Sedimentary Filling Sequence And Hydrocarbon Potential In Wulungu Area Of Junggar Basin

Junggar basin is a large petroliferous basin,located in northern Xinjiang Province,China.Wulungu depression is northernmost tectonic unit of the basin.Because of low exploration degree in Wulungu area,the drilling of Carboniferous system is less and Upper Carboniferous volcanic rocks are widely developed,which leads to the weak study of Upper Carboniferous source rocks and sedimentary environment.This study is based on the Zaheba and Takeerbasitawu area profiles on eastern margin of Wulungu,Wucaicheng and Baijiangou in southeast,Jimunai Formation and Qiaqihai Formation sections in western,the Wucan 1 well in basin,with a total of 6 field profiles and 1drilling data.Taking source rock test analysis,mudstone geochemistry analysis and rock-mineral identification as the means,then Upper Carboniferous sequence,sedimentary environment,and hydrocarbon generation potential of source rocks are studied.Primary understanding and conclusions have been drawn as followed:1.Upper Carboniferous stratigraphic sequence in Wulungu areaIn eastern and southeastern of the Wulungu depression,volcanic activities in the early Late Carboniferous are strong,lithology were dominated by intermediate-mafic volcanic lava rocks,pyroclastic rocks and pyroclastic lava rocks.In late Late Carboniferous,volcanic activity was intermittent,large sedimentary rocks were deposited during the intermittent periods,including fine sandstone,siltstone and silty mudstone,during which sedimentary rocks and volcanic rocks were interbedded in the southeastern margin.In early stage of Late Carboniferous,western margin,sedimentary rocks and volcanic rocks were mainly interbedded,and mudstone deposits appeared.In middle stage,volcanic activity is strong,and there is a large scale of volcanic lava rocks,pyroclastic rocks,and acid pyroclastic rocks.Volcanic activity ended and massive sedimentary rocks appeared in late.In early Late Carboniferous of the basin,it was characterized by thick carbonaceous mudstone and sandy mudstone,in late volcanic activity was intense,depositing large-scale intermediate-mafic volcanic pyroclastic rocks.2.Upper Carboniferous sedimentary environment in Wulungu areaThe eastern and southeast margin of Wulungu depression and the Upper Carboniferous were dominated by the coastal shallow lake and marsh environment.The water body of the Wucaicheng area and the Takeerbasitawu area was an anoxic fresh water environment,the Zhaheba area was an anoxic saline water environment,and the paleoclimate was a warm climate.The western margin Wulungu depression in Upper Carboniferous was dominated by fan delta front and shallow lake environment,water body was in the anoxic fresh water environment as a whole.The Qiaqihai Formation was short salted and the climate was warm.The Upper Carboniferous in the basin was dominated by half deep lake sediments,and the water body was an anoxic freshwater environment.The climate was in arid environment.3.Influence of sedimentary environment on source rocksThe Late Carboniferous deep water environment in the Wulungu area provided sedimentary sites for the accumulation of mudstones.Warm palaeoclimate was conducive to growth and reproduction of vegetation.It provides ample carbon source for organic matter accumulation,and shows that higher organic matter abundance.As Wucan1 was in arid climate,the organic matter abundance of source rocks was still moderate.The abundance of organic matter in the area affected by volcanic activity is higher,which shows that the organic matter abundance of source rocks in the east and southeast is higher than that in the western margin and basin.It is beneficial to the preservation of hydrocarbon source rocks that the organic matter deposited in the hypoxia environment is not easy to be decomposed by aerobic bacteria.4.Hydrocarbon generation potential evaluation of Upper Carboniferous source rocks in the Wulungu areaThe eastern and southeastern of the Wulungu depression are affected by volcanic eruption,which are characterized by high organic matter abundance and high maturity,low S₁+S₂ indexes influenced by weathering,and Type?kerogen.The abundance of organic matter in the source rocks of the western Wulungu depression is poor–medium,low maturity and maturity mature evolution stage,Type?kerogen and part of type?.In basin,source rocks are medium abundance of organic matter,low maturity mature evolution stage,and Type?kerogen.The development thickness of source rocks in Wulungu area is different from 13m to 1423m,and is currently in the gas generating stage.Thick source rocks can provide adequate gas source for Wulungu area.