Stable Isotope Geochemistry of Coalbed Gas with Special Focus on CO2

Coalbed gas is generated during coalification process and remain in the coal as an adsorbed form. This paper discusses the mechanism of generation of coalbed gas and identification of its origin. Two samples has been retrieved from exploratory borehole of Jharia Coalfield. Location of the study area was shown in figure 1. Gas content of these coal samples has been found and some gas is collected in glass bottle from desorption canister. After collection of this adsorbed gas, it was analyzed through gas chromatography and isotopic ratio mass spectrometer. Analyzed coalbed gas contains primarily CH4, ranges from 72.25% to 83.21% and CO2, ranges from 2.63% to 3.05%. Stable isotope geochemistry shows that isotopic fraction is between CH4, C2H6 and CO2. Stable isotopic fractionation and coalbed gas composition is shown in table 1. Isotope of δ13CO2 varies from -21.3% to -17.9%. CDMI and hydrocarbon index (CHC) was also calculated. CDMI index ranges from 3.51 to 3.54 which that coalbed gas might be of thermogenic origin. CHC index ranges from 68.77 to 95.07 % which shows generated CO2 is of organic in nature. Calculated dryness index ranges from 0.986 to 0.990 which shows that coalbed gas is dry to very dry in nature. Coal samples was characterized on the basis of petrochemical and petrographic for the coalbed methane recovery from Jharia Coalfield. Analyzed coalbed gas contains mainly thermogenic methane with substantial amount of biogenic methane. δ13CO2 for biogenic was assumed to be -20% to -15% for the samples. Biogenic proportion for the samples was observed 24.65% and 13.77% respectively while for thermogenic proportion, it was found around 75.35% and 86.23%.