Geochemistry Characteristics and Coal-Forming Environments of Carboniferous–Permian Coal: An Example from the Zhaokai Mine, Ningwu Coalfield, Northern China

In order to study the geochemical characteristics of coal in the Ningwu Coalfield of Shanxi Province and the coal-forming environments reflected by it, a detailed geochemical study was carried out on the No. 5 coal of the Zhaokai Mine. The results show that the content of major-element oxides SiO2 and Al2O3 is high. The trace elements Ni, Nb, Mo, Cd, Sn, Hf, Ta, W, Th, and U are slightly enriched, while the elements Li and Zr are enriched, indicating an overall LREY enrichment type in the samples. Elemental parameters suggest that the sedimentary environment in the study area is continental sedimentary, and the whole environment is reductive. The macerals in the coal samples are mainly vitrinite, with an average vitrinite reflectance (Ro) of 0.744%. The distribution range of n-alkanes in the coal samples is from n-C14~n-C32, with the main peak carbons being n-C24 and n-C25, showing the post-single-peak type distribution pattern. The average odd–even predominance index (OEP) is 0.40, the average of the light and heavy hydrocarbons ratio (∑C21−/∑C22+) is 0.42, and the average of Pr/n-C17 and Ph/n-C18 are 1.08 and 0.23, respectively. The coal samples also contain various aromatic hydrocarbons, mainly from the naphthalene- and phenanthrene-series compounds. Biomarker parameters indicate that the parent material of the coal samples in the study area is mainly continental higher plants. The maturity is low, and the coal-forming environment is a reduction environment. This study of the No. 5 coal’s geochemical characteristics has laid a foundation for the efficient, green, and comprehensive exploitation of coal resources in this region, and has also provided an important basis for the sustainable development of coal resources.