CH4/N2 Ratio as a Potential Alternative Geochemical Tool for Prediction of Thermal Maturity of Natural Gas in Tarim Basin

In this context, the bulk ratio of CH4/N2 is examined as a potential alternative geochemical parameter for the assessment of thermal maturity of natural gas and compared to other previously published data. Open-system non-isothermal pyrolysis of low-mature coal from the Manjiaer sag, Tarim basin, yielded generation curves for methane and nitrogen. Analysis of the change of vitrinite reflectance indicates a two-stage process of thermal maturation with increasing temperatures. The relationship between Ro and pyrolysis temperature could be expressed by the following equations: Stage I: Ro = 0.0014T + 0.109, r = 0.9931(Ro < 0.6%); Stage II: Ro = 0.0067T −1.5855, r = 0.9996 (Ro > 0.6%). A kinetic interpretation of the characteristics of nitrogen and methane generation in humic coal during laboratory pyrolysis indicates that the bulk ratios of methane and nitrogen as thermal maturity parameters may be applied to assess the maturity of gas-sources. Thus, the maturity of the source rocks of natural gases in the Tarim basin was expressed in terms of CH4/N2 ratios and compared with other geochemical natural gas parameters, such as stable isotopes. The predicted results were found to be consistent with the published data.