Thermal Simulation Experiment and Application of Staged Evolution of Coalbed Methane Carbon Isotope

Abstract: Coal bed methane (CBM) accumulation models include continuous gas accumulation and staged gas accumulation. In this paper, the influence factors of carbon isotope of CBM formed by staged gas accumulation and carbon isotopic composition of CBM formed in different maturity intervals were researched by thermal simulation experiment. The results showed that the carbon isotopic composition of CBM formed by staged gas accumulation is related to initial Ro and final Ro values in the evolutionary stage. With the increase of initial Ro and final Ro values, carbon isotope of CBM became heavier. In some maturity intervals, the carbon isotopic composition of CBM formed at higher heating rate was lighter than that at lower heating rate. At the same time, nature of coal also affects carbon isotope of CBM. The carbon isotope of methane generated by peat was the lightest of all three samples. The carbon isotopic composition of CBM formed by coal parent material with Ro  values of 1.2%, 1.6%, 2.0%, 2.4% and 2.8% rising to 3.0 and 4.0%, respectively, was determinated. This provides scientific evidence for studying the genesis of CBM generated in different maturity intervals. These results were used to study CBM from the southern Qinshui basin, and it is found that CBM from the southern Qinshui basin was accumulated after terminal Middle Jurassic (Ro >1.6%) and characterized by the staged gas accumulation.