Geochemistry of Late Permian coals from the Moxinpo mine, Chongqing, Southwestern China: Emphasis on petrology, minerals, and organic matter

The indicative significance of organic matter in fossil fuels, particularly polycyclic aromatic compounds (PACs), is a current research hotspot. Petrology, minerals, and organic matter in Late Permian coals from the Moxinpo (MXP) mine located in Chongqing, southwestern China, were analyzed to evaluate the coal rank, coal-forming material source and thermal maturity of organic matter, and sedimentary environment of the peat swamp. Vitrinite and inertinite were dominated by collotelinite, collodetrinite, inertodetrinite, fusinite, and semifusinite. Owing to the high maturity of the coals, liptinite was not observed under the microscope. Clay (kaolinite) and pyrite were the primary minerals in the MXP coals, followed by calcite, quartz, trace amounts of anatase, gypsum, xenotime, and bastnaesite. The minerals in the MXP coals were authigenic primarily in the epigenetic stage. The extract yield of organic matter in coals was low (0.30%), and the saturated and aromatic fractions accounted for approximately 60% of the extractable organic matter. Low ratios of saturated to aromatic compounds reflected the terrestrial higher plants input and high maturity level of the coals. The extracted yield in the host rock samples was much lower than in the coals, while the proportion of saturated hydrocarbons was relatively high. The marine-continental transitional sedimentary environment, distribution of n-alkanes, ∑ C21−/∑ C22+, and pristane (Pr)/n-C17 vs. phytane (Ph)/n-C18 indicated that the organic matter originated from the input of terrestrial higher plants and was accompanied by the contribution of marine lower phytoplankton and aquatic organisms. The coal-forming environment of the coal seam was reductive according to the Pr/Ph and sulfur-containing compounds of dibenzothiophenes and benzonaphthothiophenes. The thermodynamic stability controls the presence of polycyclic aromatic hydrocarbons in extractable organic matter. Isomeric compounds with high stability had higher concentrations, such as 2-methylnaphthalene, 2,6 + 2,7-DMN, 4- methylbiphenyl, and 2- methylphenanthrene.