Hydrogen and Carbon Isotope Composition of Hydrocarbon Gases Generated during Pyrolysis of Peats from Different Environments

For the purpose of understanding whether the formation environment of peat precursor has a meaningful effect on the isotope ratios of thermogenic coalbed gas, pyrolysis experiments were carried out on herbaceous and reed marsh peats derived from high-latitude areas with cold dry climate and a herbaceous peat of low-latitude area with tropical moist climate. Our results show that the hydrocarbon gases generated from the herbaceous swamp peat of the high-latitude area had lighter hydrogen (−56 to −46‰) and heavier carbon (3.3–8.2‰) isotopic compositions than those generated from the low-latitude area peat. As the peat samples were pyrolytically matured to vitrinite reflectance levels of 2.5, 3.5, and 5.5%, respectively, the hydrogen and carbon isotope ratios of the generated hydrocarbon gases increased, and the average hydrogen isotopic differences between them decreased from −32 to −10‰, whereas the carbon isotopic differences increased from 0.9 to 3.6‰, respectively. We suggest that the cause of these differences may originate mainly from the environmental influence on hydrogen and carbon isotopic compositions of the peats. Our results also showed that the influence of the formation environment of the peat precursor on the isotopic compositions of the pyrolysis hydrocarbon gases is less than that of types of peat. A good positive correlation of the δD or δ13C value with maturity as well as the δD value with the δ13C value of the generated hydrocarbon gases exists between the two kinds of samples, respectively, and their mathematical expressions were also established. These results showed that the environmental influence on isotopic compositions of peat should be considered when evaluating the genesis of thermogenic coalbed gas using hydrogen and carbon isotopes.