Study on evolution law of ancient peat swamp from main coal seam in Qinglongsi Minefield

Coal facies and related geochemical characteristics can reveal the water medium conditions and evolution sequence of ancient peat swampwhen the main coal seam was formed, and provide a geological basis for the study of clean coal utilization and the occurrence characteristics of oil-rich coal. Based on the test and analysis results of the micro-composition characteristics, coal chemical properties and element geochemistry of the main coal seam 5-2 in Qinglongsi mine field in ShenFu mining area, this paper divided the macro coal and rock types, summarized the coal chemical characteristics, and focused on detailed research on the coal facies and trace element distribution characteristics of the coal seam. The results show that: from the floor to the roof, the vertical microscopic composition of No. 5-2 coal seam constitutes 6 evolutionary cycles, corresponding to the six stages of coal facies evolution. From stage Ⅰ to stage Ⅲ, coal formation starts from the lake wetland swamp facies and ends with the lower swamp facies in the lower delta plain or the dry forest swamp facies in the alluvial plain of the mountain piedmont. From stage IV to stage VI, the main body of coal formation experiences the lake, wetland and swamp facies. In the time series, the water medium conditions of the marsh water body of the No.5-2 coal seam shows a fluctuating change. The hydrodynamic conditions of the first, second and fourth stages experienced an evolution process from strong to weak, and the hydrodynamic conditions in the third stage have undergone an evolutionary process from strong to weak and then strengthened. From stage Ⅴ to stage Ⅵ, the evolution trend of hydrodynamic conditions is from weak to strong with varying degrees of fluctuations within the stages. A single cycle is mostly characterized by low bottom salinity and a high top salinity, and the trend of salinity change in each evolution stage is basically consistent with the change trend of hydrodynamic conditions indicated by coal and rock parameters.