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

Abstract

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.

Carbon Isotope Fractionation Characteristics during the Oil Shale Water Extraction Process and Its Implications

“…Based on the carbon isotope fraction between CH 4 and CO 2 , the area of oil from generation to the maximum yield can be constrained. To quantify the ORR evolution process during this main oil-generation stage, we further analyzed the relationship between the carbon isotopes and the ORR (Figure ), the method always used to control the shale gas-accumulation process and the thermal mature evolution character of organic matter …”

“…To quantify the ORR evolution process during this main oil-generation stage, we further analyzed the relationship between the carbon isotopes and the ORR (Figure 8), the method always used to control the shale gasaccumulation process and the thermal mature evolution character of organic matter. 46 The correlation analysis shows that there exists no obvious correlation between δ 13 C C 2 H 6 (δ 13 C C 3 H 8 ) and ORR (Figure 8b,c). However, δ 13 C CH 4 exhibits completely different characteristics relative to the carbon isotopes of heavy hydrocarbon gas (Figure 8a) and has a strong correlation with the ORR (R 2 = 0.96, 0.99, and 0.66 for 350, 365, and 385 °C, respectively).…”

Simulation Experiment Study on Impacting the δD Value of Aromatic Compounds

“…Methane δD values in the Altay and Zhanjiang samples were significantly lower than those for PAHs (Table 4). The difference between the methane and average PAH δD values at 350 °C and 400 °C was -219‰ and -201‰ for the Altay sample, respectively, whereas Zhanjiang sample is -230‰ and -198‰, respectively [25]. At 350 °C and 400 °C, the average δD values of n-alkanes were -173‰ and -156‰ in the Zhanjiang sample and -201‰ and -156‰ in the Altay sample, respectively [24].…”

“…The peat was then crushed and dried again naturally for several days. Similar to those previously described, methods for peat pyrolysis, extraction, and instrumental analysis for aromatic compounds were used [24,25]. In short, the experiment on the peat sealed in a quartz tube was conducted in a muffle furnace under anhydrous condition.…”

“…Based on our previous results on hydrogen isotopes of hydrocarbon gases and n-alkanes in the studied samples [24,25], the thermodynamic fractionation of hydrogen isotopes between the PAHs and both the hydrocarbon gases and n-alkanes was studied by comparison. At 350 °C and 400 °C, the δD values for methane obtained after pyrolysis were -318‰ and -272‰ in the Zhanjiang sample and -336‰ and -291‰ in the Altay sample, respectively [25]. Methane δD values in the Altay and Zhanjiang samples were significantly lower than those for PAHs (Table 4).…”