Geochemical characteristics and genesis of coalbed methane in Baode area on the eastern margin of Ordos Basin

CNOOC EnerTech-Drilling & Production Co., Tianjin 300452, China

^* Corresponding author: huwq5@cnooc.com.cn

Abstract

The degree of coalbed methane exploration and development now in Baode area is relatively low. The lack of systematic understanding of the formation of coalbed methane in this area restricts the further exploration and development of coalbed methane. Based on the comprehensive study on the geochemical characteristics of coalbed methane components, hydrocarbon isotopes, water quality detection and hydrogen oxygen isotopes in coal seam in Baode area, the origin of coalbed methane in this area is discussed. According to the research, the hydrocarbon gas in the composition of coalbed methane in Baode area is mainly CH4 and a small amount of ethane. Both of their drying coefficients are more than 0.99, so they belong to the extremely dry coal bed methane. The value of δ¹³C(CH4) coalbed methane is on the low side and the value of δ¹³C(CO2) is on the high side, the mean value of δD(CH4) is -247.5‰, which shows the characteristics of terrestrial biogas. The water produced by coal seam is weak alkaline and belongs to the NaHCO3 type of water. which is similar to the surface water ion composition, salinity , δD(H2O) and δ¹⁸O(H2O) values, indicating that the hydrodynamic conditions of the coal seam in this area are more active. There is a recharge of external water, which is benificial to the mass reproduction of CH4 producing bacteria and the formation of biogas. In this area, the coalbed methane is a mixture of thermogenic and biological genesis, mainly composed of thermogenic gases and supplemented by biogenic gases generated through carbon dioxide reduction.