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Exploration and development of large gas fields is an important way for a country to rapidly develop its natural gas industry. From 1991 to 2020, China discovered 68 new large gas fields, boosting its annual gas output to 1 925×108 m³ in 2020, making it the fourth largest gas-producing country in the world. Based on 1696 molecular components and carbon isotopic composition data of alkane gas in 70 large gas fields in China, the characteristics of carbon isotopic composition of alkane gas in large gas fields in China were obtained. The lightest and average values of δ13C1, δ13C2, δ13C3 and δ13C4 become heavier with increasing carbon number, while the heaviest values of δ13C1, δ13C2, δ13C3 and δ13C4 become lighter with increasing carbon number. The δ13C1 values of large gas fields in China range from -71.2‰ to -11.4‰ (specifically, from -71.2‰ to -56.4‰ for bacterial gas, from -54.4‰ to -21.6‰ for oil-related gas, from -49.3‰ to -18.9‰ for coal-derived gas, and from -35.6‰ to -11.4‰ for abiogenic gas). Based on these data, the δ13C1 chart of large gas fields in China was plotted. Moreover, the δ13C1 values of natural gas in China range from -107.1‰ to -8.9‰, specifically, from -107.1‰ to -55.1‰ for bacterial gas, from -54.4‰ to -21.6‰ for oil-related gas, from -49.3‰ to -13.3‰ for coal-derived gas, and from -36.2‰ to -8.9‰ for abiogenic gas. Based on these data, the δ13C1 chart of natural gas in China was plotted. [ABSTRACT FROM AUTHOR]

By the end of the year 2010, a total of 15 large tight gas fields have been found in China, located in the Ordos, Sichuan, and Tarim basins. The annual production and total reserves of these fields in 2010 were 222.5×10 and 28657×10 m, respectively, accounting for 23.5% and 37.3%, respectively, of the total annual production and reserves of natural gases in China. They took a major part of all natural gas production and reserves in China. According to the analyses of 81 gas samples, the stable carbon and hydrogen isotopic compositions of tight gases in China have following characteristics: (1) Plots of δC- δC- δC, δC-C/C and δC- δC demonstrate the coal-derived origin of tight gases in China; (2) For the primary alkane gases, both carbon and hydrogen isotopic values increase with increasing molecular mass, i.e., δC< δC< δC< δC and δH< δH< δH; (3) The isotopic differences of δC- δC, δC- δC, δH- δH and δH- δH decrease with increasing R (%) and C/C; (4) There are seven causes for the carbon and hydrogen isotopic reversal, however, the carbon and hydrogen isotopic reversal of tight gases in China is caused mainly by multiple stages of gas charge and accumulation. [ABSTRACT FROM AUTHOR]

A number of mud volcanoes exist in the southern Junggar Basin. To date few systematic studies on natural gas geochemistry of mud volcanoes have been conducted in China. In June 1991 and August 2010, the authors investigated the mud volcanoes in the southern Junggar Basin twice, and the mud volcanism weakened gradually as seen from the variations such as the decrease of gas pressure and output, the downthrow and dry up of the mud pool. The volcanic intensity was significantly weaker than that in Taiwan. The natural gas from the mud volcanoes in the southern Junggar Basin has similar geochemical characteristics, indicating the same source or origin. The main component of the mud volcano gas is alkane gas with contents of 91.15%-97.49%, and the gas is high-quality commercial gas since methane dominates in the alkane. The δC values are −49.1‰-−40.6‰, which are in accordance with the peak δC frequency of mud volcano gas around the world, and the alkane gas displays positive carbon isotopic series, i.e., δC< δC< δC, suggesting typical thermogenic origin. The helium in the mud volcano gas is typically crust-derived due to the low R/Ra values of 0.011-0.054. The mud volcano gas is coal-derived since the δC values are all greater than −28‰, and C/C and δC values are in accordance with those of natural gas derived from the Lower-Middle Jurassic coal-measures. Therefore, alkane gas from mud volcanoes in the southern Junggar Basin is mainly sourced from the Lower-Middle Jurassic coal-measures. [ABSTRACT FROM AUTHOR]

Abstract: The compound specific hydrogen and carbon stable isotope ratios of alkanes in natural gases from the Xujiahe and Jurassic formations in the western Sichuan Basin were investigated to distinguish between coal-derived and oil-associated gases. All gases were determined to be coal-derived and sourced from the Xujiahe Formation. Thermal maturities of the source rocks are expected to vary from 0.8–1.8% vitrinite reflectance (%R o) according to the empirical relationship between the measured δ13CCH4 values and % R o, which is within the scope of the maturities of source rocks (0.7–2.1%). In particular, C1–3 alkanes of primary coal-derived gases which have not undergone any secondary alteration become more enriched in deuterium with increasing molecular mass and the δDCH4 values increase with increasing thermal maturity of source rocks. In the Sichuan Basin, δD values of coal-derived gases sourced from terrigenous source rocks of the Xujiahe Formation are more negative than those of oil-associated gases sourced from marine organic matter of the Sinian, Permian and Triassic formations. [Copyright &y& Elsevier]

Abstract: Coal-derived hydrocarbons from Middle–Lower Jurassic coal-bearing strata in northwestern China are distributed in the Tarim, Junggar, Qaidam, and Turpan-Harmi basins. The former three basins are dominated by coal-derived gas fields, distributed in Cretaceous and Tertiary strata. Turpan-Harmi basin is characterized by coal-derived oil fields which occur in the coal measures. Based on analysis of gas components and carbon isotopic compositions from these basins, three conclusions are drawn in this contribution: 1) Alkane gases with reservoirs of coal measures have no carbon isotopic reversal, whereas alkane gases with reservoirs not of coal measures the extent of carbon isotopic reversal increases with increasing maturity; 2) Coal-derived alkane gases with high δ 13C values are found in the Tarim and Qaidam basins (δ 13C1: −19.0 to −29.9‰; δ 13C2: −18.8 to −27.1‰), and those with lowest δ 13C values occur in the Turpan-Harmi and Junggar basins (δ 13C1: −40.1 to −44.0‰; δ 13C2: −24.7 to −27.9‰); and 3) Individual specific carbon isotopic compositions of light hydrocarbons (C5–8) in the coal-derived gases are lower than those in the oil-associated gases. The discovered carbon isotopic reversal of coal-derived gases is caused by isotopic fractionation during migration and secondary alteration. The high and low carbon isotopic values of coal-derived gases in China may have some significance on global natural gas research, especially the low carbon isotope value of methane may provide some information for early thermogenic gases. Coal-derived methane typically has much heavier δ 13C than that of oil-associated methane, and this can be used for gas–source rock correlation. The heavy carbon isotope of coal-derived ethane is a common phenomenon in China and it shed lights on the discrimination of gas origin. Since most giant gas fields are of coal-derived origin, comparative studies on coal-derived and oil-associated gases have great significance on future natural gas exploration in the world. [Copyright &y& Elsevier]