In the eastern edge of the Ordos Basin,the coalbed methane(CBM)development has not made substantial progress in the past20 years,and the origin of gas can be used to guide the CBM block-selecting and development.Based on the 37 sets of carbon isotope data,the origin of the gas was determined and the origin mechanism was studied in this work.The 13CPDB of methane ranges from 70.5‰to 36.19‰in the eastern edge in the Ordos Basin and the value becomes heavier from the north to the south.The secondary biogenic gas and the thermogenic gas are mixed in the shallow area and the thermogenic gas occurs in the medium and deep levels.The phenomenon is controlled mainly by the distribution of coal rank and hydrodynamics.Firstly,based on the relationship between China coal rank and methane 13CPDB,the medium rank coal is dominant in the eastern edge of the Ordos Basin,and the mixture of the secondary biogenic gas and the thermogenic gas is formed in the coal of vitrinite reflectant ratio(Rmax)between 0.5%and 2.0%if there is appropriate hydrodynamics;at the same time,because of the shallow burial depth,and the well-developed coal outcrop,meteoric water and other surface water carrying bacteria recharge the coal reservoir,metabolize the organic compounds at a relatively low temperature,and generate methane and carbon dioxide.Wherever the trapping mechanisms occur in the coal,such as Liulin and Hancheng,modern gas content should be high.
