Geochemical characteristics of Jurassic coal and its paleoenvironmental implication in the eastern Junggar Basin, China

摘要

This study is the first to provide geochemical and paleoenvironmental information of Jurassic Xishanyao Formation coal in the eastern Junggar Basin, China. Twenty-seven coal core samples were collected from the zk0413 boreholes in Dajing area to determine the geochemical variability. Proximate analyses show that the Xishanyao Formation coal has the medium moisture content (average 13%), special low ash yield (average 6%) and high volatile matter (average 34%), which slightly fluctuate from bottom to top of coal seam. Organic matter in the coal are type III kerogen, with coalification degrees determined as subbituminous A or high volatile bituminous C. All coal samples have poor hydrocarbon generating potential with the yield between 31.93 and 72.36 (average of 43.57) mg HC/g.rock. The primary input of organic matter for coal-formation was higher terrestrial plants, with greater input from algae and micro-organisms and greater role of bacteria in plant degradation higher in the seam. The process of peatification in the lower section of the seam occurred under more acidic and oxic conditions. Coal accumulated in a lacustrine freshwater environment with the rise of the (ground) water table from bottom to top. There is evidence of higher salinity of water within the lower seam and decreased salinity (brackish to fresh water) within the middle of the seam, then an increase to moderate salinity (fresh water to brackish). The increasing stratified water column upward with reduced oxidation in the coal seams may result from the increased temperature in periods of a relatively hot and dry climate. The environment changes demonstrated in this study illustrate the importance of studying combined elemental and organic geochemistry to reconstruct paleoenvironment.