Hydrogen isotope of natural gas from the Xujiahe Formation and its implications for water salinization in central Sichuan Basin, China

Abstract

Based on the analysis of the molecular composition as well as carbon and hydrogen isotopes of gases from the Upper Triassic Xujiahe and other formations in the Sichuan Basin in China, by combining previous research results and regional geological background, the gas origin, gas source, and significance of hydrogen isotopes are studied. The results show that gas from the Triassic Xujiahe Formation in the Sichuan Basin is mainly hydrocarbon gas. The methane content ranges 67.89%–98.05%, whereas the content of heavy hydrocarbon gas (C2+) ranges 0.42%–16.62%. Meanwhile, the non-hydrocarbon gas content (CO2, N2) is low. Except for the relatively high content of N2 and CO2 in Well Yue121, the average content of N2 and CO2 are 0.82% and 0.26%, respectively. The gas from the Xujiahe Formation in the Yuanba area has a high average dryness coefficient of 0.991, which indicates that it is dry gas. It is also worth noting that gases from other areas of central Sichuan have a dryness coefficient of less than 0.95, which is indicative of wet gas. The δ13C1 value varies from −43.8‰ to −29.2‰, the δ13C2 value ranges from −33.5‰ to −20.7‰, δ13C3 value ranges from −33.6‰ to −19.3‰, and the δ13C4 value ranges from −27.2‰ to −22.2‰. The δD1 value ranges from −191‰ to −148‰, δD2 value ranges from −165‰ to −115‰, and δD3 value ranges from −153‰ to −107‰. Methane and its homologues (C2-4) become more enriched in 13C and D as the carbon number increases (δ13C1 < δ13C2 < δ13C3 < δ13C4, δD1 < δD2 < δD3); the aforementioned is consistent with the carbon and hydrogen isotopic characteristics of thermogenic gases. The thermal maturity RO of the natural gas from the Xujiahe Formation in the Yuanba area ranges 1.09%–1.78%, whereas the RO value of the natural gas from the Xujiahe Formation in other areas of central Sichuan ranges 0.64%–0.92%. The natural gas from the Xujiahe Formation in central Sichuan is mainly from the coal measure source rocks of the Xujiahe Formation. The gas from the 2nd Member of the Xujiahe Formation (Xu-2 Member) in the Yuanba area is mixed gas; it is a mixture of the high-mature coal-derived gas from the coal measure source rock of the Xujiahe Formation and the oil-cracked gas from the Lower Cambrian (and Lower Silurian). The δD value of methane from the Xujiahe Formation in the Sichuan Basin is relatively high as it is higher than −200‰. Compared to the coal-derived gas from the source rocks formed in limnetic facies with freshwater in the Taibei Sag, Turpan-Hami Basin, though at similar thermal evolution stage, the difference of the δD values of methane can be as high as 90‰, indicating that the source rock of the Xujiahe Formation is formed in an environment with water salinization.