The Coal Quality And Its Controlling Factors In The Main Coalfields Of Junggar And Yili Basins,Northern Xinjiang Province

This thesis focus on investigation of the coal quality and its distribution,and analysis of its geological controlling factors,based on the analysis of data from the coalfield exploration,previously-published literatures,and studied samples of the eastern Junggar coalfield,souther Junggar coalfield,Heshentuoluogai coalfield,and Yining coalfield,northern Xinjiang Province,by using the methods of sequence stratigraphy,coal geology,coal petrology,coal chemistry,and coal geochemistry.The results as follows:1.The maceral of the Jurassic coals in Xinjiang Province consists mainly of vitrinite and inertinite with inertinite dominant in the coals.However,the maceral contents are variable among the coal-bearing basin,coalfields and coal-bearing sequences.The vitrinite contents of the Xishanyao Formation coals are higher in the Heshentuoluogai and southern Junggar coalfields than in the eastern Junggar and Yining coalfields.Comparison between the Badaowan Formation and Xishanyao Formation coals,the vitrinite contents are higher in the Badaowan Formation coals than in the Xishanyao Formation coals in the southern Junggar coalfield and Yining coalfield.The vitrinite contents of the Xishanyao Formation coals increase from west to east in the eastern Junggar coalfield and are higher in the thin coal seams than in the thick coal seams.The thickness of coal-bearing strata positively correlates with the vitrinite contents in the coals.In addition,the vitrinite contents in the coals of lacustrine expansionsystem tract are higher than the highstand system tract under the sequence framework.The difference of maceral in the coals is controlled mainly by the basin type,evolution stage of basin tectonic,structural unit,lake level and depositional system.Totally,the relatively rapid subsidence rate of basin base and rising lacustrine level contribute to the preservation of peat due to relatively high groundwater level.The maceral of the coals is primarily influenced by the water table and burial rate.The eastern Junggar coalfield is located on the uplifted zone of the Junggar basin and has relatively thin thickness of strata due to relatively low subsidence rate.The coal seams are formed in the highstand swamp peat under weak redox environment and its macerals consist mainly of inertite.The southern Junggar coalfield is suited in the southern thrust belt of the Junggar basin and the Heshentuoluogai coalfield is controlled by syn-depositional faults located to the northern and southern margin of basin.The two coalfields have relatively rapid subsidence rate and thus peat type is the wetland herbaceous swamp-wet forest swamp facies,and the maceral is primarily vitrinite.2.The mineral content is relatively low in the Jurassic coals in Xinjiang Province.The minerals consist mainly of kaolinite and quartz with varying proportions of carbonate minerals(e.g.,calcite,dolomite,ankerite,siderite)and trace amounts of sulfides(e.g.,pyrite),sulfates(e.g.,gypsum,barite),and aluminosilicate minerals(e.g.,microcline,illite,anorthite,chlorite,saponite,palygorskite,nepheline).The mineralogical composition varies greatly among the coalfields.The Xishanyao Formation coals are represented by kaolinite and quartzin the eastern Junggar coalfield and by quartz,kaolinite,and carbonate minerals in the southern Junggar coalfield.The minerals in the Badaowan Formation and Xishanyao Formation coals in the Yining coalfield are composed mainly of kaolinite and quartz,with higher proportion of siderite in the Xishanyao Formation coals and higher amount of calcite in the Badaowan Formation coals.The kaolinite and quartz are primarily of detrital origin;carbonate,sulfide,and sulfate minerals are the epigenetic authigenic origin.3.The Jurassic coals are characterized by low-medium ash.The ash yields of the Xishanyao Formation coals are the highest in the Heshentuoluogai coalfield followed by the southern Junggar and Yining coalfields,and are the lowest in the eastern Junggar coalfield.The ash yields of the Xishanyao Formation coals increase from west to east and from south to north in the eastern Junggar coalfield.The ash yields decrease from the southern and northern margin to basin center in the Heshentuoluogai and Yining coalfields.The ash yields of the Jurassic coals in Xinjiang Province are mainly controlled by subsidence rate of basin basement and distribution of depositional system.Basement subsidence rate controls the detrital material supply and then affects the distribution and scale of sedimentary system.For example,the southern Junggar coalfield is located in the southern thrust belt of the Junggar basin,and relatively fast subsidence rate leads to higher ash yields in the coals than those in eastern Junggar coalfield(located in the eastern uplifted zone,slower subsidence rate).The ash yields of the coals in the basin margin are generally higher than those in the basin interior and show low-zone and high-zone along the basin margin,which is mainly controlled by the direction of terrigenous detrital material supply and spatial distribution of depositional system.The ash yields in the coals of the Heshentuoluogai and Yining coalfields decrease from the northern and southern margin to the basin center due to detrital matter input from the southern and northern sources.The low-ash zone is located between deltaic lobe,and high-ash zone is located on the deltaic lobe due to the spatial distribution of deltaic lobe.4.The Jurassic coals in Xinjiang province are characterized by the medium-high to high volatile matter yield coals(candle coal or non-caking coal),with the low-volatile coals locally distributed in the southern Junggar coalfield.The volatile matter shows litter variability throughout coal seam.The volatile matter yields of the Jurassic Badaowan Formation coals are higher than those of the Xishanyao Formation coals,due mainly to the higher vitrinite contents in the Badaowan Formation coals than the Xishanyao Formation coals.The volatile matter is mainly controlled by the temperature,pressure,and time during coalification.The volatile yields decrease with increasing burial depth.The Junggar and Yili basins have relatively low geothermal gradient and thus have low coal rank.The volatile matter yield is influenced mainly by coal macerals in the low rank coal;the exinite has the highest volatile yield,followed by vitrinite and inertinite.Obviously,the Badaowan Formation coals have higher volatile matter yield than the Xishanyao Formation coals,mainly due to higher vitrinite contents in the Badaowan Formation.5.The coal-bearing strata were fromed in the freshwater environment,and thus the coals have low sulfur contents.The relatively high sulfur content are mainly distributed in the shallow part of the margin of the coalfield,which may be related to the burning of coal seams and the secondary enrichment of sulfur adjacent to the outcrop of coal seams..6.The source rocks of Jurassic coal-bearing strata in Heshentuoluogai,eastern Junggar,southern Junggar and Yining coalfields are mainly acidic rocks,followed by intermediate rocks.The Jurassic coals in Xinjiang Province have low major-and trace-elements contents,which are mainly related to the low amounts of minerals.Sodium is enriched in the coals of Yining,Heshentuoluogai and eastern Junggar coalfields.The enrichment of sodium in the eastern Junggar coalfield may be controlled by the sodium-rich groundwater infiltrating into the coal seam and occurring in the pores of the coal in the form of hydrate ions,and the sodium-rich fluids are from groundwater leaching roof and floor rocks or from late salinized surface water.The elevated contents of Sr in the coals of Heshentuoluogai coalfield occur mainly in carbonate and sulfate minerals(e.g.,barite)that are the epigenic origin of hydrothermal fluids.