Organic geochemistry of semianthracite from the Gequan Mine, Xingtai Coalfield, China

Abstract

Eighteen semianthracite samples of the No. 9 coal seam in the Gequan Mine, Xingtai Coalfield, China were investigated, primarily by microscope and organic geochemical methods. The petrological results showed that the vitrinite random reflectance (Ro) is 2.36% and the maceral group was dominated by vitrinite. The gas chromatograms (GC) of the saturates display a monomodal or bimodal outline of n-alkane peaks, generally from n-C10 to n-C31, resulting in two ranges of ∑(C1–C21)/∑(C22–C34) values. Most of the coal samples exhibit consistently similar parameters, such as pristane/n-C17 and phytane/n-C18 ratios, whereas samples with different Pr/Ph values may have been the result of varied redox depositional conditions. The distributions of the polycyclic aromatic compounds (PACs) identified in the aromatics were generally similar due to the similar high rank of the coal samples. Under the high thermal maturity, the PACs with greater thermodynamic stabilities (including 2-methylnaphthalene, 2,6- and 2,7-dimethylnaphthalenes, 2- and 3-methylphenanthrenes, and 2,6- and 2,7-dimethylphenanthrenes) were present at higher concentrations than those of less thermodynamically stable PACs. A portion of these PACs most likely originated from a secondary generation. The generally higher contents of dibenzothiophenes and benzonaphthothiophenes were most likely generated under the control of hydrothermal fluids. However, the coal-forming environment may also have played a role for higher contents of dibenzothiophenes and benzonaphthothiophenes.