Study On The Characteristics Of Organic Matter In Highly Metamorphosed Coal In Magmatic Intrusion Contact Zone

The occurrence environment of raw coal will be changed by magmatic intrusion,and the chemical characteristics of coal will be changed obviously by thermal contact and thermal metasomatism.The influence of magmatic intrusion on coal chemical property is a new problem in coal geology.This thesis is based on organic petrology,Infrared spectroscopy,Organic geochemistry and isotopic geochemistry,a combination of industrial analysis,optical microscopy,infrared spectroscopy and stable isotope mass spectroscopy,the coal quality,coal petrology and organic matter composition of No.8and No.9 coals in Yunjialing coal mine are studied,and the influence of magmatic intrusion on carbon and nitrogen isotopes in coals is discussed in detail.The results show that the No.8 and No.9 Coals in the magmatic contact zone of Yunjialing coal mine are anthracite with low ash content,extremely low moisture content,high sulfur content and extremely low volatile content,and the random reflectivity of vitrinite is very high(R_o,ran=5.06),it shows that the degree of metamorphism of coal is high.In addition to the original macerals,a series of new macerals with special occurrence,different morphology and strong optical anisotropy can be observed in coal under the influence of magmatic intrusion,it includes irregular granular mosaic structure,natural coke matrix and graphite microcrystal,etc.The newly formed macerals are mainly mosaic structures with strong optical anisotropy,and the inert group is less affected by heat because of its highly condensed aromatic structure.The results of FTIR analysis show that the aromatic system of dense nucleus increases,the volatile matter decreases and the macromolecular structure of coal becomes unitary and compacted.The organic extraction rate of the samples was low（average 0.04%）.The carbon number distribution of n-alkanes ranged from C₁₃ to C₂₇.The main carbon peaks of most samples were located near C₁₆ and C₂₄,showing a bimodal distribution with a prepeak distribution,the averageΣC₂₁^-/ΣC₂₂⁺ is 1.45,which indicates that the alkanes with high carbon number were cracked during the process of magmatic intrusion.The mean values of Pr/Ph,Pr/n C₁₇ and Ph/n C₁₈ were 0.84,0.38 and 0.46 respectively.With the exception of n-alkanes,most of the identifiable compounds in the samples were branched-chain alkanes and hydrocarbon derivatives.The branched-chain alkanes in the samples are mainly methyl and polymethyl branched-chain alkanes.The total amount of branched-chain alkanes in the samples near magmatic intrusion is relatively enriched,which indicates that the formation of branched-chain alkanes is related to temperature the detected hydrocarbon derivatives included amides,esters,alcohols,phenols,carboxylic acids and ethers.The hydrocarbon derivatives in the samples were mainly composed of esters and amides,it may be related to the secondary reaction of hydroxyl groups in carboxylic acids to form carboxylic acid derivatives such as amides and esters.The content of ethers is lower than that of alcohols and phenols,which may be related to the formation of alcohols and phenols by hydrolysis of ethers.There are some differences between the contents of hydrocarbon derivatives in gangue and coal,which may be related to the protective effect of clay minerals in gangue on some hydrocarbon derivatives.Theδ¹³C value of the samples（including coal gangue）ranged from-24.8‰to-23.3‰,with an average of-24.1‰.The variation range ofδ¹³C value in No.8 and No.9coals of Yunjialing is small due to magmatic intrusion,which is similar to that of high metamorphic anthracite in other areas,the reason may be related to the thermal cracking of organic matter in coal and the pyrolysis volatilization of lipid group caused by magmatic intrusion.δ¹⁵N value ranged from-2.6‰to 4.8‰,with an average of 1.3‰.Theδ¹⁵N value of gangue in No.8 coal seam are generally heavier than those of coal samples.Theδ¹⁵N value of the samples in No.9 coal seam show a negative trend as a whole.The variation range ofδ¹⁵N value in all samples was larger（>5‰）,which may be related to the difference of thermal stability of different organic nitrogen（pyrrole nitrogen,pyridine gas,quaternary nitrogen）in coal.Theδ¹⁵N value may be influenced by the nitrogen-bearing fluid carried by magma and the migration of nitrogen-bearing organic matter in coal.