Effect of the depositional environment on the formation of tar-rich coal: A case study in the northeastern Ordos Basin, China

Abstract

Tar yield, as a basic evaluation index of tar-rich coal, is unclear to be affected by the depositional environment. In this work, the vertical variations in tar yield, macerals, and major/trace elements were investigated for highly volatile bituminous coal in the Ordos Basin, China, to analyse the effect of the depositional environment on the tar-rich coal distribution. The results show that the tar yield more than 7% (tar-rich coal) accounts for 70% in this formation, which formed in a depositional environment with a stable supply of terrestrial sediments, a warm and humid climate, and weakly reducing to reducing conditions. For the coal facies, all samples were distributed into two groups of coal facies, with the first group containing limnic, dry forest swamp, wet forest swamp and another group contains bog, forest bog. Moreover, limnic and bog with less woody tissue and a higher water table contribute to the higher tar yield of coal. Correspondingly, the vitrinite content and the gelation index (GI) were positively correlated with tar yield, and the tissue preservation index (TPI) and vegetation index (VI) were negatively correlated with tar yield. The inorganic composition, the ash yield, which is mainly composed of SiO2 and Al2O3, reduced the tar yield. CaO and MgO in small proportions promote the tar yield of coal due to catalytic effects. The relationship between tar yield and the parameters Sr/Ba, Sr/Cu, Cu/Zn, (Cu + Mo)/Zn, U/Th, Ni/Co, δU, Ce/La, δCe, and δEu are also discussed here. It was concluded that the paleoclimate had little effect on the tar yield, while the paleoreduction conditions had an obvious influence on the tar yield. The depositional environment with higher reduction and lower salinity conditions was conducive to the increase in tar yield of coal. This work aims to accurately identify and predict tar-rich coal based on its depositional environment, which provides guidance for its exploration and development.