Affecting factors and application of the stable hydrogen isotopes of alkane gases

Abstract

To study the composition, affecting factors of the stable hydrogen isotopes of alkane gases and their application to identification of the natural gas origin and maturities, the chemical and isotopic compositions of 118 gas samples of Carboniferous- Permian in the Ordos Basin, and of Triassic in the Sichuan Basin, combined with 68 gas samples from the Sinian and Cambrian reservoirs in the Sichuan Basin, and Ordovician and Siliurian reservoirs of Tarim Basin, are analyzed comprehensively. The following conclusions are obtained: (1) Natural gases in the study area and strata of the Ordos and Sichuan basins are dominated by alkane gases, and the dryness coefficients and maturities of the Carboniferous-Permian gases in the Ordos Basin are higher than the gases in the Triassic Xujiahe Formation of the Sichuan Basin, while the hydrogen isotopes of the latter ones are much enriched in 2H than the former. (2) The δ2HCH4-C1/C2+3 genetic identification diagram of natural gas was drawn, and the diagrams of hydrogen isotopic differences between the heavy alkane gases and methane vs. hydrogen isotopes of alkane gases can also be used in natural gas genetic identification. (3) The δ2HCH4-Ro formulas of coal-formed gas in different areas of the two basins are given, and the δ2HC2H6-δ2HCH4 is a new index for maturity, and the (δ2HC2H6—δ2HCH4)-Ro formula of the coal-formed gas can be used to calculate the maturity of the natural gas. (4) The stable hydrogen isotopes of alkane gases are affected by parent materials in source rocks, maturity, mixing and the aqueous medium conditions, among which the aqueous paleo-salinity is the key factor. To sum up, the hydrogen isotopes of alkane gases are affected by multiple factors, and they are significant to the identification of the origin, and maturity of natural gas, and the water environment during the deposition of source rocks. Key words: alkane gases, maturity, hydrogen isotope, gas origin, aqueous salinity, Ordos Basin, Sichuan Basin, Tarim Basin