Origin and geological implications of super high sulfur-containing polycyclic aromatic compounds in high-sulfur coal

Polycyclic aromatic compounds (PACs) are ubiquitous in fossil fuels. As the main component of PACs, sulfur-containing polycyclic aromatic compounds (SPACs) have been widely studied because of their biological markers and maturity indicator functions. SPAC content is especially high in high-sulfur coal. However, the origin and formation of SPACs remain debatable. Herein, we discuss the origin and geological implications of abnormally enriched SPACs in high-sulfur coal of the Heshan Coalfield, Southern China. Fifteen coal and 6 parting samples were collected from the nos. 41 and 42 coal seams of the Heshan Coalfield, Guangxi Zhuang Autonomous Region, Southern China. Heshan coal is a superhigh organic sulfur coal (SHOSC). These collected samples were investigated using a microscope and organic geochemical methods. The microscopic results indicated that the random vitrinite reflectance (Ro) was 1.88% and 1.75% in the 41 and 42 coal, respectively. The dominant fractions of the extract were aromatic fractions with an average ratio of 79.62% in the 41 and 86.22% in the 42 coal samples, respectively, and 72.70% in the parting samples. Detailed quantification of the PACs was performed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC–MS). Seventy SPAC species were identified. The highest ratio of total SPAC/total PAC reached 89.22 wt% in the coal and 83.07 wt% in the parting samples. Such high SPACs are attributed to high sulfur, depositional environment, and organic matter thermal evolution. SPAC species and contents were initially controlled by the sulfur content, but gradually modified by the maturity of the organic matter. In addition, possible comprehensive formation pathways for major SPAC species have been proposed.