Gas Origin and Constraint of δ13C(CH4) Distribution in the Dafosi Mine Field in the Southern Margin of the Ordos Basin, China

Coalbed gas or coalbed methane (CBM) is a vital hydrocarbon gas which is mainly absorbed in the surface of the coal matrix and has multiple benefits as in being effective in development and utilization. To determine the gas origin, the distribution of the stable carbon isotopic composition for methane, and its constraint in the Middle Jurassic strata in the Dafosi mine area in the southern margin of the Ordos Basin, nine CBM samples were collected from pumping wells at the surface, arranged along three profiles located in different tectonic areas. The gas components and stable isotope compositions were measured, and the results show that the average methane and carbon dioxide concentrations in the coal seams measured are 77.44 and 1.04 mol % respectively. CBM has apparent dry gas characteristics, with more than 6835 distributed C-1/(C-2 + C-3) ratio values. The average values of the carbon and hydrogen isotopic compositions of CBM are -79.2 parts per thousand and -241.9 parts per thousand, respectively. The gas origin in the Dafosi mine field is mainly composed of biogenic gas from CO2 reduction and mixed thermogenic gas. The former content accumulated in the present coal seams ranges from 68.93% to 100.00%, as calculated by the binary mixing model. A web-based learning tool also indicates mostly CO2 reduction-biogenic gas. Fold types play an essential role in controlling the distribution of different genetic CBM with different stable carbon isotope ratios. Biogenic gas with low carbon isotopic composition lies in the southern parts of the Qjia anticline blocked by the footwall of the fault. The mixed CBM comprised of biogenic and thermogenic gases with high carbon isotopic composition is distributed in the areas of Anhua and Shijiadian synclines sealed with lithology and hydrocarbon concentration, respectively.