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

Abstract

Inertinite is an important type of organic maceral found in coal deposits, and also an important geological information carrier of coal forming environments. In the present study the results obtained from experimental tests of coal rock elements and carbon isotopes using scanning electron microscopy, inertinite reflectance, sporopollen and free radical retorting methods and so on, 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 of the Yan'an Formation. The No. 4 inertinite-rich coal seam of the Middle Jurassic Yan'an Formation in the Binchang Coal field, located in the southern section of the Ordos Basin, was selected as an example. The obtained results were as follows: 1) During the coal forming period of the No. 4 coal seam of the Yan'an Formation in the Binchang Coal Field, the overall climate had been relatively dry. There were four cycles of relatively dry-wet climate conditions, which had been controlled by an eccentricity cycle. It was determined that the of inertinite content had been relatively high during the dry periods; 2) The temperature range suitable for microbial activities during the oxidation processes was between 0 and 80℃. This study’s simulation results of the free-radical concentrations showed that the maximum temperature of fusain in the No. 4 coal seam of the Yan'an Formation during the process of coalification had not exceeded 300℃, which is significantly higher than the temperature range of microbial activities. Therefore, these were unfavorable conditions for both microbial activities and inertinite formation; 3) The genesis temperature of the inertinite in the No. 4 coal seam of the Yan'an Formation was calculated according to the reflectance of the inertinite, which was lower than 400℃. It was determined 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. However, it was considered that the inertinite of oxidation origin could not be excluded, but it may not have been the main genesis type; 4) There were dense and scattered fusinite observed in the No. 4 coal seam, and the thicknesses of cell walls were found to differ. Therefore, 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.

Characteristics and evolution of inertinite abundance and atmospheric pO2 during China’s coal-forming periods

“…In China, the three repetitions comprised the Late Triassic to Early Jurassic; Middle Jurassic to Late Cretaceous; and Paleogene to Neogene. The difference lies in the fact that the abundance of inertinite during the Middle to Late Jurassic in China was much higher than that in other parts of the world (Wang et al 2019). Diessel (2010) conducted a systematic study regarding the stratigraphic characteristics of global inertinite.…”

Coalbed Methane Enrichment Regularity and Model in the Xishanyao Formation in the Santanghu Basin, NW China

“…The Jurassic strata in the Santanghu Basin belong to a weakly water-rich aquifer, with insufficient surface water replenishment, resulting in the formation of "dry coal seams" [58]. The salinity of coal seam water is high, and the corresponding mineralization is also high (Table 4) [70]. The low mineralization of the coal seam water is suitable for the survival of methanogen and is conducive to the formation of biogenic gas [71].…”

Widespread wildfires linked to early Albian Ocean Anoxic Event 1b: Evidence from the Fuxin lacustrine basin, NE China

“…Wildfires have played an important role in Earth system changes since the Silurian period when land plants first appeared on Earth and profoundly influenced global ecosystem patterns and processes (Glasspool et al, 2004;Glasspool et al, 2015;Lu et al, 2021). Charcoal, as the byproduct of wildfires, is recorded from the late Silurian (~420 Ma) to the present (Scott and Glasspool, 2006) and provides evidence for wildfire events in the rock record (Scott and Glasspool, 2007;Scott, 2010;Shao et al, 2012;Wang et al, 2019a;Wang et al, 2021a).…”