Geochemistry of Inertinite-Rich Coals from the Zhundong Coalfield, Xinjiang: Organic Compounds and Paleoenvironment Reconstruction

Organic compounds in fossil fuels (coal, petroleum), particularly polycyclic aromatic hydrocarbons (PAHs), serve as significant molecular fossils to reveal paleoenvironments, paleoclimates, and major geological events and have become the focus of multidisciplinary research. Inertinite-rich coals are widely distributed in the Jurassic coal of Xinjiang, yet the organic geochemical significance and their paleoenvironment implications remain underexplored. This study elucidates the indicative significance of organic compounds in inertinite-rich coals from the Yihua mine of the Zhundong Coalfield, Xinjiang, China, aiming to reveal the coal-forming mechanisms and paleoenvironmental implications of Jurassic low-rank coals in the Zhundong Coalfield. A comprehensive analysis was conducted on 36 coal-seam samples, including maceral identification, vitrinite reflectance measurement, Soxhlet extraction, and GC-MS analysis of saturated and aromatic hydrocarbons. The results show the coal is dominated by inertinite (69.6%), with vitrinite (23.7%) and trace liptinite (0.7%), and classified as low-rank bituminous coal (Ro = 0.42%). Extractable organic matter yield is low (0.20%), with aromatic hydrocarbons (35.19%) far exceeding saturated hydrocarbons (10.57%). PAHs are dominated by high-ring (4–6 ring) compounds, such as benzofluoranthene and retene. The molecular geochemical parameters suggest these PAHs are primarily derived from biomass combustion, recording significant wildfire activity during the Middle Jurassic. Terpenoid biomarkers (cadalene and dehydroabietane) confirm terrestrial higher plant inputs, while isoprenoid parameters suggest an oxidizing coal-forming environment. This research demonstrates that organic compounds, especially PAHs, serve as reliable tracers for paleowildfires and inertinite genesis, providing critical geochemical evidence for understanding coal-forming processes in arid-semiarid regions.