Depositional Environment and Origin of Inertinite-rich Coal in the Ordos Basin

Inertinite-rich coal is widely distributed in the Ordos Basin, represented by the No. 2 coal seam of the Middle Jurassic Yan'an Formation. This paper combined coal petrology and geochemistry to analyze the origin of inertinite, changes in the coal-forming environment and control characteristics of wildfire. Research has shown that there are two forms of inertinite sources in the study area. Alongside typical fusinization, wildfire events also play a substantial role in inertinite formation. There are significant fluctuations in the coal-forming environment of samples at different depths. Coal samples were formed in dry forest swamp with low water levels and strong oxidation, which have a high inertinite content, and the samples formed in wet forest swamp and limnic showed low inertinite content. Conversely, the inertinite content of different origins does not fully correspond to the depositional environment characterized by dryness and oxidation. Nonpyrogenic inertinites were significantly influenced by climatic conditions, while pyrofusinite was not entirely controlled by climatic conditions but rather directly impacted by wildfire events. The high oxygen level was the main factor causing widespread wildfire events. Overall, the combination of wildfire activity and oxidation generates a high content of inertinite in the Middle Jurassic coal of the Ordos Basin.