Geological–Geochemical Models and Isotope Fractionation Laws and Control Factors of Thermogenic Coalbed Gas in Panxian, China

Abstract

Coalbed gas (CBG) is a typical self-generated and self-stored gas and is one of the most ideal materials for studying the geological–geochemical models of natural gas and its source rock. However, studies show that CBG is usually a mixed gas consisting of thermogenic gas and secondary biogenic gas which make its original geochemical compositions change. We collected nine CBG samples and the corresponding coal samples from a coal core drilled in a CBG well in the Panxian Basin. The gas samples were measured using a MAT-271 trace-gas mass spectrometer for the molecular components and a Delta Plus XP stable isotope mass spectrometer for the carbon and hydrogen isotope values. The coal R max values were measured by a LEICA DM2500P and MSP-II microscope photometer. The main geochemical characteristics of the CBG samples are as follows: CH4 concentrations from 97.28 to 95.16%, C1/C1–n from 0.995 to 0.999; δ13C1 from −33.2 to −41.9‰, δD CH4 from −170.6 to −123.5‰, δ13C2 from −29.7 to −23.2‰, and δ13CCO2 from −24.6 to −20.0‰. The characteristics reflect that Panxian CBG is a pure thermogenic gas, thus, it is more suitable for studying the geological–geochemical model system of thermogenic natural gas. Some important and new discoveries are presented in this paper: a discriminant system, δ13C1–Δδ13CCO2–C1 , is proposed, which could be used to effectively distinguish between biogenic and thermogenic CBG, and the reasonable boundary values of δ13C1 and Δδ13CCO2–C1 may be −50‰ and 25‰, respectively; a series of models referring to isotope values, R max, and depth, such as δ13C1 = 31.58R max – 100.02 (r = 0.913), δD CH4 = 248.68R max – 625.73 (r = 0.946), δD CH4 = 7.5228δ13C1 + 148.2 (r = 0.990) and δ13C2 = 20.722R max – 66.596 (r = 0.681), were established for the first time; the carbon isotope fractionation degree of CBG components decreases with the increase in their relative molecular mass. The temperature and the molecular mass of CBG components play important roles in controlling the isotope compositions and variations of CBG. The models and the isotope fractionation laws may be more reliable and precise because of the self-generating and self-storing characteristics of CBG. The results have not only great application value in the fossil energy fields but also important scientific significance in gas geochemistry.

Gas geochemistry and hydrochemical analysis of CBM origin and accumulation in the Tucheng syncline in western Guizhou Province

“…Uncertainty regarding the CBM enrichment mechanism has made selection of CBM wells unsuccessful, so that exploration and development of CBM have not been commercialized in this area. According to Tao et al (2020), CBM is of thermogenic origin throughout the entire Panxian mine, but its δ 13 C-CH4 values are very similar to those found in the San Juan Basin, where secondary biogenic methane has been found (=-44.13‰; Zhou et al, 2005). Conversely, coal maturity is lowest in the research area 5 of this study, the Tucheng syncline, with Ro=0.8%~1.2% (Li et al, 2015b).…”

“…Formation of secondary biogenic gas is often related to the uplift of coal bearing strata. With the infiltration of surface fresh water, methanogens and other microorganisms inoculated into the coal-bearing strata, allowing secondary biogenic gas generation (Brinck et al, 2008;Golding et al, 2013;Tao et al, 2007Tao et al, , 2020. However, the sealing effect of surface fresh water on structural fissures is also an important factor in CBM preservation.…”

“…3d, the Tucheng data points are distributed in the thermogenic region only, indicating that CBM is the thermal origin under this model. Tao et al (2020) noted that δ 13 C-CH4 is positively correlated with coal rock maturity (Ro), using the thermal simulation regression equation (δ 13 C-CH4= 22.42logRo-34.8; Liu and Xu, 1999) to explore the δ 13 C-CH4 characteristics of CBM from adjacent synclines. Based on the previous, the Ro values of Tucheng coal are 0.8%~1.2%, with δ 13 C-CH4 values of -35.0‰~-33.0‰ produced by the foregoing equation.…”

Geological control on carbon isotope equilibrium and kinetic fractionation of CH4-CO2-HCO3− in microbial coalbed and shale gas systems

“…Samples were analyzed at the Key Laboratory of Gas Geochemistry (Lanzhou), Institute of Geology and Geophysics, Chinese Academy of Sciences. The molecular components of the gas samples were analyzed by a MAT 271 trace gas mass spectrometer (Tao et al, 2007(Tao et al, , 2020.…”