Organic geochemistry of Middle Jurassic coal seam in the Sandaoling Coalfield, Northwest China: Insights into the depositional environment

The study of coal seams provides valuable insights into the environmental changes that have occurred throughout Earth's geological history. Coal formation was notably widespread during the Jurassic period in mid-latitude East Asia. To reconstruct the paleoenvironmental conditions prevailing during the formation of peat in the Turpan-Hami Basin, China, we conducted a comprehensive study of 44 Middle Jurassic coal samples. Our investigative approach encompassed coal petrographic analysis, and gas chromatography-mass spectrometry to compare our findings with existing literature. Biomarkers from steranes and diterpenes suggest OM in coals of No. 4 seam predominantly originated from terrestrial higher plants, with conifers being one of the important sources of OM. Vitrinite reflectance, hopane, and sterane maturity biomarkers indicate that most of the samples exhibit low thermal maturity. Previous studies suggested that the No.4 coal seam formed in a saline water environment based on trace element indicators. However, this study, based on biomarker analysis (low gammacerane index, high Pr/Ph ratio, and low sulfur content), suggests that peat accumulation occurred in freshwater environments. The anomaly of element indicators in previous studies may be attributed to the introduction of trace elements from overlying strata into the coal seams via surface (groundwater) water or hydrothermal fluids. Since the vegetation and depositional environments indicated by biomarkers are more consistent with the Jurassic geological setting, biomarkers offer a more reliable indication of depositional environment compared to trace element indicators.