Geological Significances of CoalBed Methane Carbon Isotope in Coal Bed Methane Exploration

Key words: 
• coalbed methane /
• carbon isotope

Abstract: As an unconventional natural gas, the characteristics of carbon isotope in coal bed methane (CBM) is similar to conventional gas. This paper summarizes the character of China coal bed methane carbon isotope according to field desorption,thermo-modeling and carbon isotope testing. Taking Qinshui CBM field as a example, it points out the potential area and profitable target zone for CBM exploration and development from carbon isotope fractionation effect and desorption,diffusion and movement effect of CBM,combined with hydrology and structural condition. Geologic target area in China has complex tectonic and high maturation, lots of CBM was lost during evolution stage. The variation of carbon isotope value can directly reflect the CBM accumulation degree. Due to desorption, diffusion and movement effect of CBM, the thermo-modeling value of carbon isotope was much higher than carbon isotope value. Meanwhile,the carbon isotope,which also reflects the preservation condition of CBM trap, is important to evaluate the origin of CBM reservoir and exploration potential. Four different types are divided in terms of carbon isotope value distribution. According to this theory, Qinshui CBM basin in Jincheng area, Shanxi province was found. Rope coring technique was used and carbon isotope testing of field desorption gas was firstly conducted in Qinshui CBM basin. The desorption gas sample for carbon isotope analysis was collected at 4,24 and 96 hours by water displacement. Because of ¹²C having less electrical property than ¹³C, CBM molecule with ¹²C was prior to molecule with ¹³C in desorption procedure and lead to carbon isotope fractionation, the early adsorption gas had lighter carbon isotope than late adsorption gas. Qinshui CBM field is a large gas field and can be divided into 3 zones in terms of carbon isotope distribution feature. Carbon isotope in coal seam 15 is lighter than coal seam 3 because of being interfered by carbonate water. The best location lies in primary-secondary zone with carbon isotope at - 30‰～ - 40‰.