Organic geochemical characteristics of bark coal in Changguang area: evidence from aromatic hydrocarbons

Seven coal samples were collected from Changguang area, a typical bark coal field in Zhejiang Province, China. The organic geochemical characteristics and depositional environment of bark coal were studied by Rock–Eval pyrolysis, gas chromatography (GC), and gas chromatography–mass spectrometry (GC–MS) analysis. The results show that all samples are type II kerogen. The values of total organic carbon (TOC), potential yield (S 1 + S 2 ), and hydrogen index of these samples ranged from 65.80 wt%–87.00 wt%, 170.43–232.89 mg HC/g, and 234–295 mg HC/g TOC, respectively. In aromatic hydrocarbons compounds of bark coal samples, the phenanthrene series occupy the largest proportion (43.48%–53.74%), followed by the dibenzothiophens (7.33%–16.53%). The distributions of naphthalenes, diphenyls, and chrysene of the samples indicate the source inputs are mainly derived from terrestrial higher plants. Furthermore, the ratios of naphthalene and phenanthrene series compounds reveal the samples are in the mature stage. Some biomarkers (Pr/Ph and C 35 /C 31–35 homohopane), aromatic hydrocarbons (fluorene, dibenzofuran and dibenzothiophene), and high inertinite contents (14.7%–41.8%) suggest that bark coal was formed under relatively oxic conditions and was influenced by marine or brackish lakes environment.