Geochemical characteristics of lignite from Shengli Formation andPaleo-environmental geological significance in Shengli Coalfield

The Early Cretaceous Shengli Formation is the main coal-bearing strata in the Shengli Coalfield in eastern Inner Mongolia. In order to find out the elemental characteristics of coal and sedimentary environment of coal seams from Shengli Formation and provide a geological basis for coal mining, coal and rock samples from Shengli Formation were systematically collected. Tests and analyses of petrography, physical chemistry, geochemistry, etc. were carried out. Based on theoretical methods such as element geochemistry, the sedimentary facies, Paleo-salinity, Paleo-climatic characteristics and Paleoproductivity of the Shengli Formation were analyzed. The results show that the sample from the Shengli Formation is brown-black lignite, mainly containing vitrain and durain. The maximum reflectance (Ro) of vitrinite is 0.2% to 0.5%. The moisture content of each coal seam is medium to high (Mad=12.12%), ash yield of each coal seam is low to medium (Ad, 18.34%), volatile matter yield of each coal seam is high (Vdaf=45.55%), fixed carbon yield of each coal seam is extra-low (FCad=24%), the total sulfur content of each coal seam is low to medium (St.d=1.39%), and calorific value of each coal seam is medium to high (Qnet=22.19 MJ/kg).The major elements of the samples are mainly Si, Fe, and Al, a small amount of Ca, Mg, etc., and the ash composition index is low (mean value is 3.29), which indicates that the coal formation period is a low-level peat swamp environment, with relatively low reduction. The ratio of MgO/Al2O3 is medium (18.78), and the ratio of CaO/(MgO×Al2O3) is high (0.15), indicating that the Paleo-climate is relatively dry, the evaporation is large, and the water body has high salinity, which is basically a salt water environment. The ratios of trace elements V/(V＋Ni), V/Cr, Ni/Co, U/Th, V/Sc are 0.83, 2.14, 2.15, 0.87, 10.06, respectively. Combined with U abundance of 0.98 μg/g, it is judged that the bottom water environment of the study area is dominated by long-term hypoxic reduction conditions, accompanied by many short-term periods of oxygen enrichment. The ratio of Sr/Ba and Th/u judges that the sedimentary water body of Shengli Formation is brackish-salt water environment. from the vertical direction view, with the gradual shrinkage of the lake basin and the increase of evaporation, the upper peat swamps have become shallower, and the salinity of the water body has increased. There is no evident enrichment of Mo, Ba, Cu, and Ni; it is speculated that ancient productivity was moderate at that time. It appears as a middle peat swamp, a favorable deposit environment for coal formation and coal accumulation.