Study on kinetics of hydrocarbon generation from coals in the Qinshui Basin

A new method for the quantitative assessment of hydrocarbon generation potential from coals by means of the chemical reaction kinetics has been developed gradually over the recent years. In this paper, the kinetic parameters of hydrocarbon gas generation are determined by high temperature and pressure, and closed- system thermal simulation for Late Paleozoic coals in the Qinshui Basin and the kinetic characteristics and the histories of hydrocarbon gas generation were studied using the parameters obtained. Results show that during the longer period from the Triassic (T) to the Middle Jurassic (J2), the coal-derived methane yield increased more slowly under lower palaeogeotemperature in the Qinshui Basin; however, the shorter period from Late Jurassic to Early Cretaceous, the coal-derived methane yield increased more rapidly under higher palaeogeotemperature. The correlation between the thermal simulation and the factual data shows that C1/ (C1+C2−4) coefficients computed by the histories of methane and C2-C4 hydrocarbon generation can provide evidence for the identification of the genesis of coal bed gas in the different areas of the Qinshui Basin. The kinetic simulating experiment of hydrocarbon generation for the peat considered as the original matter of coal formation was performed for the first time and the simulated results were compared with the characteristics of hydrocarbon generation from coals undergoing various palaeogeotemperature in the Qinshui Basin. The result indicates that the peat has a higher potential of hydrocarbon generation than that of coals. Therefore, the hydrocarbon generation results obtained from kinetic simulation for coal with higher maturation rank could not stand for their original hydrocarbon generation potential and thus would lead to an underestimation for coal-bed gas resource. The generative amount of coal-derived gas in the Qinshui Basin was predicted using kinetic simulation results for the peat and their maximum was estimated. The calculation shows that the generative amount of coal-derived gas from Taiyuan Formation and Shanxi Formation coal-beds in the Qinshui Basin is the highest in Yangcheng area, higher in Qinyuan area and the lowest in Huoshan area. Therefore, the conditions of coal-bed gas generation are the best in Yangcheng area, the second in Qinyuan area and not very good in Huoshan area.