Organic Geochemical Characteristics And Palaeoenvironment Reconstruction Of Inertinite-enriched Coal From Yan'an Formation In Eastern Dongsheng Coalfield

As the carrier of geological information,rich-inertinite coal can not only records the climatic conditions,coal-forming plants and marsh types during the coal accumulation period,indicates the natural geological events.This subject selects the No.2 coal of Jurassic Yan'an formation in Zhangjialiang mining area in the east of Dongsheng coalfield,guided by the theories of coal geology,coal petrology,organic geochemistry,stable isotope geochemistry.The collected coal samples of Jurassic Yan'an formation were tested by microscopic coal-rock composition analysis,organic elements determination,gas chromatography-mass spectrometry,stable isotope mass spectrum and so on.The coal-forming parent materials,sedimentary environment,evolution degree of organic matter,types of peat bogs and evolution characteristics of paleoenvironment in this study area are studied.Compared with the carbon content of all kinds of coals,except for the floor 2-13,the carbon content of organic elements in the other coal samples(64.6%?72.1%)is low,indicating that the metamorphic degree of organic matter is on the low side.Vitrinite reflectance(0.41%?0.49%)is low,which belongs to low-rank long-flame coal in bituminous coal.The macerals are mainly inertinite(57.3%),followed by vitrinite(42.1%)and liptinite(0.33%).The fusinite structure of inertinite is intact.The structures of homogeneous cell wall in silk charcoal indicate that forest fire occurred in the process of coal formation.The reflectivity of inertinite is less than 2%,indicating that the wildfire temperature in this area is on the low side.The carbon number distribution of n-alkanes in saturated hydrocarbons is incomplete(C₁₂?n-C₂₉).The main peak carbon number is mainly n-C₂₅,and most coal samples are post-unimodal pattern,only 2-7 and 2-10 are dominated by n-C₁₆,showing front-peak distribution,The ratio of Pristane to Phytane(2.11?10.6)in isoprenoid alkanes is higher.In regular steranes,C_29???20R>C_28???20R>C_27???20R are of"V"and"L"type.which further supports the contribution of mixed sources in this study area.The above parameters indicate that the parent material of Zhangjialiang No.2 coal is mainly from terrestrial higher plants and mixed with a small amount of lower bacteria and algae.Tricyclic terpenes are dominant in terpenes,and peak advantage of C₂₁TT is obvious.The index of gamma waxane(GI)is 0.00?0.20,and the GI index of floor 2-13 is on the high side,indicating that the coal-forming environment salinity of the coal samples is very low,and 2-13 is formed in brackish water environment.In the odd-even dominance,the odd carbon advantage is obvious,and the the number of long-chain carbon is weak.The value of Ts/(Ts+Tm)in hopane is 0.17?0.41,which indicates that the thermal evolution of organic matter is low.The aromatic series are mainly pyrene,phenanthrene and oxyfluorene series.9-MP>1-MP>2-MP>3-MP in methylphenanthrene,which proves that the thermal stability of organic matter is low and coal-forming plants are mixed with higher plants and aquatic organisms.The contents of oxyfluorene series(20.4?g/g)and fluorene series(19.5?g/g)are similar,which is much higher than that of thiofluorene series(1.47?g/g).It is confirmed that Zhangjialiang No.2 coal is mainly formed in lacustrine and delta facies under partial oxidation conditions.The abundance value of pyrene series is the highest,and the coal forming material is still the input by higher plants,which is closely related to the forest fire occurred in the period.A variety of high concentration pyrolysis of polycyclic aromatic hydrocarbons(PAHs)were detected in the study area,such as retene,cardarene,benzo[a]pyrene,benzo[k]fluoranthene,benzo[e]pyrene,etc.,which proved that forest fires occutted under low tempetrature combustion in this area.The atomic ratio of organic elements has the characterics of high C/N value(88.1%)and low H/C value(0.06%),indicating that the paleoenvironment of Jurassic Yan'an formation is warm and humid climate,and the coal forming plants are dominated by gymnosperms.Organic carbon isotopes(?¹³C_org)range from-24.3‰to-22.4‰,indicating that coal-forming plants in the whole study area are mainly C₃ plant types forms in warm and humid climate.