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.

The Genetic Mechanism of Inertinite in the Middle Jurassic Inertinite- Rich Coal Seams of the Southern Ordos Basin

also an important geological information carrier of coal forming environments.In the southern section of the Ordos Basin,the No.4 inertinite-rich coal seam of the Middle Jurassic Yan’an Formation in the Binchang Coal field was selected as an example to study the genetic mechanism of the inertinite.In this study,the results obtained from experimental tests of coal rock,including principal and trace elements,stable carbon isotopes,scanning electron microscopy,inertinite reflectance,sporopollen and free radical retorting methods,were analyzed.Then,the findings were combined with the previous understanding of the oxygen content in the atmosphere and ground fire characteristics,in order to discuss the genesis mechanism of inertinite in the No.4 coal seam.The obtained research results were as follows:(1)During the coal forming period of the No.4 coal seam,the overall climate had been relatively dry.There were four relatively dry-wet climate cycles in the No.4 coal seam,which were controlled by the eccentricity astronomical period.The inertinite content were relatively high during the dry periods;(2)The temperature range suitable for microorganism activities during the oxidation processes was between 0 and 80℃.The simulation results of the free radical concentrations showed that the maximum temperature of fusain in the No.4 coal seam during the process of coalification had not exceeded 300℃,which was significantly higher than the temperature range of microorganism activities.Therefore,these were not conducive to the activities of microorganism and formation of inertinite during the coal-forming period;(3)The genesis temperature of the inertinite in the No.4 coal seam was calculated according to the reflectance of the inertinite,which was lower than 400℃.This result supported the cause of wildfire of the inertinite and reflected that the type of wildfire was mainly ground fire,along with partially surface fire.Moreover,the paleogeographic location,climatic conditions,atmospheric oxygen concentration,etc.of the study area showed that the conditions for wildfire events were in fact available;(4)There were dense and scattered fusinite observed in the No.4 coal seam,and the thickness of cell walls were found to differ.It was speculated that this was related to the type of wildfire,combustion temperatures,combustion timeframes,and different initial conditions of the burned objects during the coal forming periods.