Depositional environment and hydrocarbon potential of Early Permian coals deposit in the Huanghua Depression, Bohai Bay Basin

The Permian coaly source rocks of coal and carbonaceous mudstone contained more hydrogen-rich vitrinite and liptinite, and formed in the weak oxidation–reduction conditions. Coaly shale was characterized by a most hydrogen-poor vitrinite and deposited in the weak to the strong reduction environment. A comprehensively accumulation model of Permian coaly source rock was established.