A review of the biogeochemical controls on the occurrence and distribution of polycyclic aromatic compounds (PACs) in coals

The organic matter in coal contains polycyclic aromatic compounds (PACs) of varying quantities in diverse soluble and insoluble forms. PACs in coal are of special interest for organic geochemical studies because they have been successfully used as biological marker compounds (biomarkers) and indicators of thermal maturity. In addition, they could provide critical information to understand the structural and compositional evolution of the coal organic matters during coalification, and to predict PAC releasing characteristics during coal utilization. The main purpose of this paper is to provide a critical review on the current state of knowledge on the potential use of PAC composition to understand the underlying biogeochemical factors that control PAC occurrence and distribution in coal. The published PAC data of world coal are first compiled according to their ranks (natural coke, anthracite, semi-anthracite, bituminous coal, low volatile bituminous coal, medium volatile bituminous coal, high volatile bituminous coal, sub-bituminous coal, lignite), countries (Brazil, Canada, China, France, Germany, Japan, USA), formation periods (Carboniferous, Permian, Jurassic, Cretaceous, Early-Middle Cenozoic). Then, the biogeochemical factors affecting the occurrence and distribution of PACs in coal, including biogenic precursor materials, coal rank, maceral composition, depositional environment, and geological settings, are comprehensively reviewed. The results show that the geochemical parameters of PACs in coal are robust tools to trace the coalification processes from the molecular perspectives, and to understand the corresponding biogeochemical process (e.g. depositional environment, geological settings, igneous intrusion) occurring during coal formation.