Origin and geochemical characteristics of coalbed methane in abandoned coal mines, Panzhuang block, southern Qinshui basin

In order to analyze the carbon and oxygen composition and origin of the coal seam gas(CSG) in abandoned mines, Panzhuang block, south of Qinshui basin, in this paper, the coalbed methane and produced water samples were collected. The results show that the CSG of abandoned mines is dominated by CH₄, the average is 91.99%, with minor amounts of N₂(6.73%) and CO₂(1.26%). The δ¹³C values of CH₄ range from -31.36‰ to -33.53‰, with an average of -32.25‰, the hydrogen isotope(δD) value is between -182.76‰ and -193.20‰, and the average value is -187.538‰. The main ions in produced water of abandoned mine are mainly composed of Mg²⁺, K⁺, HCO₃^-, Cl^-, Na⁺, SO₄^2- and NO₃^-, and the type is Mg-(HCO₃)₂. The CH₄ was mainly generated from thermal cracking during coalification, with some microbial genetic methane. Compared with the unexploited coal seams, the abandoned coal mine is more suitable for methanogen to produce the CH₄.