Hydrogen isotope characteristics of thermogenic methane in Chinese sedimentary basins

Abstract

The hydrogen isotope composition of methane is an important parameter in natural gas research and provides complementary information to that provided by carbon isotopes. The stable hydrogen isotope ratios of 313 natural gas samples from six of China’s sedimentary basins are used to evaluate the factors that influence the stable hydrogen isotopic composition of methane. An important factor is the δD of organic matter in hydrocarbon source rocks, which is influenced by the sedimentary environment and type of organic matter. Natural gases generated from sapropelic organic matter have relatively less negative δDCH4, while natural gases generated from humic organic matter have relatively more negative δDCH4. The other factor is thermal maturity. Increased thermal maturity leads to δDCH4 becoming less negative and there exists a two stage linear relationship between the δDCH4 and the logarithm of the vitrinite reflectance (Ro) value for natural gases generated from type III kerogen. The third factor is the environmental conditions of the aqueous medium in the original depositional environment and after sedimentation. Elemental hydrogen from water participates in biochemical processes as organisms grow and is exchanged during the sedimentation and diagenesis of organic matter as well as maturity process of kerogen to generate gases. The influence of the aqueous medium on δDCH4 after formation of natural gases can be ignored. Among these factors, the aqueous medium is the key constraining factor. The δDCH4 in natural gases can be combined with the use of stable carbon isotopic composition to identify the origins of natural gases and to aid gas-source correlations.