| The eastern thrust belt of the southern Junggar Basin has become an important oil and gas exploration area in the western China.The Lucaogou Formation shale is an important source rock and an unconventional reservoir of oil and gas with significant exploration potential of shale oil and gas.Based on a combination method of qualitatively theoretical analysis including structural geology,stratigraphy,sedimentology,petrography and the quantitative characterization of geochemistry,this study utilizes the field geological survey,drilling core observation,microscopic thin-section observation and sample analysis in association with the seismic data to carefully investigate the stratigraphic division,regional comparison,types of sedimentary system and spatial distribution of the southeastern Junggar Basin in middle-upper Permian series.The history of tectonic-sedimentary evolution of southeastern Junnger Basin in Permian is discussed,which specifically targets at lithofacies types,sedimentary processes,lithofacies assemblage sequence,origin evolution,and the enrichment mechanism of shale organic matter of the regional source rock and Lucaoagou Formation shale.This paper mainly acquired these conclusions:The stratigraphic correlation framework of the southeastern Junngar Basin in middle-upper Permian series is established according to the recognition of different formations of the lithologic interface and well phase interface of southeastern Junggar Basin in middle-upper Permian.Determine the types,distributions and evolutions of sedimentary system,distribution and evolution of the system on the southeastern Junggar region in Permian,in the meantime pointing out the depositional stage of Lu Cao Gou formation.The research area has two major depositional systems,which are the fan delta system in front of south Yilinheibiergen Mountain and the braided river delta system in the north concave of Wucai Bay,respectively.At the time,the Bogda is an submarine uplift,and what is between two delta depositonal system is a more extensive hemi-deep-deep lake deposits.In the depositional period of Hong Yan Chi formation,the lake level sank,taking fan delta system as the main body.The lower and upper part of the Quan Zi Jie formation belongs to alluvial fan debris flow deposition,while the middle part of the Quan Zi Jie formation belongs to braided channel deposition with flood plain.The Wu Tong Gou formation developed fan delta front-shallow lake deposits.In this period,the lake level relatively elevated,while the whole pattern of distribution of depositional system resembles the Quanzijie formation.On this basis,the depositional evolution sequence of fan delta-lake system of middle Permian Lucaogou formation and Hongyanji formation,alluvial fan system of upper Permian Quanzijie formation,and fan delta-lake system of Wutonggou formation in southeastern Junngar area is presented.According to the tectonic evolution stages in Junggar basin,two regional unconformity planes have been identified(equivalent to the first-level sequence interface)in the southeastern Junngar Basin.One is the middle-upper Permian series in southeastern Junggar basin and the whole developing unconformity plane between the middle and the lower Permian,while the other is the unconformity plane between the upper and the middle Permian series.After that,based on these two regional unconformity planes,the tectono-sedimentary evolution of southeastern Junngar region is divided into the period of sea-land-transition compressional basin in early-middle Permian and the period of the intracontinental compression basin in the late Permian-Paleogene.The period of sea-land-transition compressional basin in early-middle Permian includes the stage of early Permian Marine extrusion and the stage of middle Permian continental extrusion basin.The period of the intracontinental compressional basin in the late Permian – Paleogene includes the stage of continental compressional sunken in the late Permian – early Triassic.The tectonic evolutionary sequence of these two periods-three stages are also established.They are the period of sea-land-transition compressional basin in early-middle Permian and the period of the intracontinental compressional basin in the late Permian – Paleogene respectively.The angular unconformity of the upper Permian Quanzijie formation and the middle Permian Lucaogou formation or Hongyanji formation indicates that the Hercynian movement has a great influence on the tectonic movements of the adjacent areas in southeastern Junggar.This tectonic movement marked the continuous uplift of Bogda mountain out of the water,and the entire southeastern Junggar appears as the continental basin.In the late Permian,the compression from south to north was strong,Bogda mountain continued to rise due to the intense squeezing from south to north.The Chaiwobao depression was separated from the Changji depression of Junggar basin.The southeastern Junggar entered the phase of squeezing-depression starting from Guodikeng formation.The shale of Lucaogou formation is divided into 6 types of lithofacies such as lamellar shale facies,massive dolomitic mudstone facies,lamellar argillaceous dolomite facies,massive oolitic limestone facies,massive calcareous siltstone facies and 6 types of lithoface associations such as LA1-LA6.The lamellar shale facies and massive – lamellar calcareous mudstone mainly deposited in the environment of the hemi-deep – deep lake.The massive dolomitic mudstone facies deposited in environment of hemi-deep – shallow lake transition.The lamellar argillaceous dolomites deposited in the shallow lake environment which is influenced by the volcanic mantle source hydrothermal activities.The massive oolitic limestone facies formed in the shore-shallow lake environment.Lastly,the massive calcareous siltstone facies formed in the environment of front fan delta.The paleoclimate of research region reveals zoning and segmentation.The zoning of paleoclimate indicates that the northeastern Bogda had a wetter paleoclimate than the southwestern Bogda during the depositional period of Lucaogou formation.On the other hand,the segmentation of paleoclimate suggests that it is relatively humid during the depositional period of the lithofacies combination in upper and lower shale of Lucaogou formation,while it is dry in the period of lithofacies combination in middle shale of Lucaogou formation.The zonation of paleoclimate controlled the difference of TOC content,the enrichment mechanism of organic matter,the types and development patterns of ancient lakes in deposition period of the Lucaogou formation shale in the northeast and southwest Bogda of southeastern Junggar.The segmentation of paleoclimate controlled the types of lithofacies combination in vertical upward and different types of vertical evolutionary patterns of ancient lakes in the shale of Lucaogou formation in the northeast and southwest Bogda of southeastern Junggar.It is revealed that there is a developmental mechanism of zonal enrichment of organic matter in Lucaogou formation shale of southeastern Junggar region.The enrichment of organic matter in the Lucaogou formation shale of the southwestern Bogda near the southern provenance region is controlled by the paleoproductivity of the water surface,the redox condition of the water body and the amount of terrigenous detritus input during shale deposition.In the areas of southwest and northeast Bogda which is far away from the provenance region,the enrichment of organic matter in the Lucaogou formation shale is mainly controlled by the paleoproductivity of the surface water and a certain amount of terrigenous detritus input during shale deposition,rather than the redox condition of the water.Moreover,the enrichment of organic matter in the Lucaogou formation shale in the northeastern Bogda near the northern provenance region is mainly controlled by the paleoproductivity of the surface water,followed by the redox condition of the water,and finally the terrigenous detritus input at the time of shale deposition. |
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