Geochemical characteristics and its origins of CBM in deep-seated coal seam around Panji mining area of Huainan

The distribution and the origins of carbon and hydrogen isotopes of methane in coalbed methane ( CBM) from deep-seated coal seams around Panji mining area is significant in revealing the formation and the distribution of CBM reservoir,and the source evaluation of CBM. For these reasons,25 coal samples were mainly sampled from the deep-seated coal seams around Panji mining area,the desorbed CBM in Lab were conducted to test CBM component,carbon and hydrogen isotopes of methane. The distribution characteristics of CBM component,stable isotope of methane were analyzed,the variation characteristics between stable isotope of methane and the burial depth of coal seam were discussed. The geological effect of the formation and the evolution of CBM,the metamorphic degree of coal were ana- lyzed,the origins of CBM were revealed by Whiticar chart,the ratios of CBM with different origins were estimated based on the associated empirical model. The results show that the content of CH4 in CBM from the deep-seated coal seams is between 16. 2% -96. 68% with an average of 71. 60% ,the content of heave hydrocarbon in CBM is between 0. 35% -32. 13% with an average of 9. 86% ,the content of N2 is between 0. 13% - 74. 72% with an average of 21． 20% ,and the content of CO2 is between 1. 62% -27. 26% with an average of 7. 30% . Carbon isotope of methane varies between -45. 46‰--31. 17‰ in CBM from the shallow coal seam to the deep coal seam,with an average of -40． 92‰. Hydrogen isotope of methane varies between -199. 99‰--133. 87‰ with an average of -178. 04‰. CBM are characterized by rich heavy carbon and hydrogen isotopes. Tectonic-thermal history of Panji mining area shows that CBM is the thermogenic gas accompanied by the biogas. The average Ro,max of coal seams in the studied area is be- tween 0. 702 2% -0. 998 3% ,the types of coal in the studied area are gas fat coal,the generated CBM is mainly wet gas,the input of oil type gas causes the rich of δ13 C1. The analytical Whiticar chart suggests CBM in the studied area is the thermogenic gas reconstructed by secondary role. The estimated results show that the ratio of the thermogenic gas in the generated CBM from coal Nos. 13-1,11-2,8,7,6,5,4,3 and 1 is above 80% ,and the biogas is generated by the reduction of CO2 .