Natural gas constituent and carbon isotopic composition in petroliferous basins, China

• Published in: Journal of Asian earth sciences 2014-02, Vol.80, p.1-17
• Main Authors: Zhu, Guangyou, Wang, Zhengjun, Dai, Jinxing, Su, Jing
• Format: Article
• Language: English
• Subjects: China; Gas source; Isotope; Natural gas; Origin
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2013.10.007

•Most deep gases in east China are consisted of coal-type and oil-type gases.•Isotopic reversal among gas hydrocarbon components is very common in eastern Chinese.•Coal has high gas generation potential and major contribution to deep gas reservoirs.•Faults and unconformities are major emigrational pathways of deep gases in east China. There are abundant gas resources in petroliferous basins of China. Large to midsize gas fields are found in Eastern, central and Western of China. However, origin, constituents and isotopic composition of natural gas in different gas fields are varied distinctly, and some present strong chemical secondary alteration and show variation both in age and space. Based on the systematic analysis of constituents and carbon isotope of a large number of gas samples, combined with the geological characteristics, this paper classifies the origins of the gases, explores the gas isotope characteristics and evolutionary regulation with the variation time and space, and further discusses the distinctive geochemistry of the gases in China. These gases are dominated by dry gas, its methane carbon isotope values range from −10‰ to −70‰, ethane from −16‰ to −52‰, propane from −13‰ to −43‰, and butane from −18‰ to −34‰. The carbon isotopes of most gases show the characteristics of humic-derived gas and crude oil cracked gas. In addition, large primary biogenic gas fields have been discovered in the Qaidam basin; inorganic-derived alkane gases have been discovered in deep of the Songliao Basin. Half of these gas fields are characterized by the alkane carbon isotope reversal in different degrees. Research indicates there are several reasons can result in carbon isotope reversal. Firstly, gas charge of different genetic types or different source in one gas reservoir may cause carbon isotope reversal. Besides, high-over mature evolution of gas can also lead to the carbon isotopic reversal of alkanes. Thirdly, secondary alteration of hydrocarbons may also result in abnormal distribution of carbon isotope, isotope transforms to unusual light and heavy.