Key controls of the gas and oil preferences of Chinaʹs major coal-bearing sequences

The key controls on the gas and oil preferences of coal-bearing strata are discussed from the relationship of material foundation between coal formation and hydrocarbon generation, the variation of the initial gas to oil ratio (GOR) of organic matter in coal-bearing strata, and the feature of coal-bearing sequence related to the formation and evolution of source rocks. The patterns of density of maceral compositions, the effect of GOR redistribution of coal measures in the processes of coalification, and the change of sequence base levels are the key factors for the coal measures being oil-prone or gas-prone. The maceral compositions of major coal-bearing sequences (coal basins) in China can be classified into three patterns, vitrinite liptinite/sapropel assemblage, vitrinite inertinite assemblage and transitional assemblage (intergradation), which represent oil-prone, gas-prone and transitional coal measures, respectively. Due to the control of sequence base level change, the latter two patterns are common. It is natural that gas prone is characteristic of most of the major coal-bearing sequences in China.