Geochemical characteristics of produced gas by CBM wells in western Guizhou

Based on the gas components,methane carbon and hydrogen isotopic tests and productivity data of produced gas samples from 17 coalbed methane (CBM) wells in Western Guizhou,the hydrodynamic conditions and geological characteristics of the study area are combined,the geochemical characteristics of CBM,the geological control factors and their relationship with CBM productivity are studied in the study area. Research shows that methane is the main component of CBM in the study area,which belongs to dry gas ~ extremely dry gas. The volume fraction of methane is between 91. 504% and 99. 508% ,followed by N2 and CO2 . The heavy hydrocarbon content of produced gas by different CBM wells varies greatly. The δ13 C1 -CH4 values are between -44. 1‰ and -27. 8‰,and the δD-CH4 values are be- tween -196. 5‰ and -120. 8‰,indicating that methane is the thermogenic gas. The GP well group and Z-1 well have the characteristics of primary CBM,and ZJ well group and other wells in the eastern part of the study area are obviously affected by the secondary action of migration and diffusion. The δ13 C1 -CH4 and δD-CH4 values of produced gas by CBM wells in western Guizhou tend to be high in the northeast and low in the southwest. The metamorphic degree of coal plays a major role in controlling the geochemical characteristics of the produced gas. The distribution of δD-CH4 values is also controlled by the change of sedi-mentary environment. In the same coalbed methane well group,the δ13 C1 -CH4 and δD-CH4 values are greatly affected by the change of hydrodynamic conditions in the well group and the buried depth of coal seam. Meanwhile,the δ13 C1 -CH4 and δD-CH4 values are negatively correlated with the daily gas production,the essence of which is that the methane carbon and hydrogen isotopic are controlled by the diversity of buried depth and hydrody- namic conditions dominated by depth. With the change of production time,the composition of coalbed methane and the δ13 C1 -CH4 and δD-CH4 values show a certain regular trend,while the δ13 C1 -CH4 and δD-CH4 values fluctuate and be- come heavier,which is related to component fractionation and isotope fractionation.