Geochemical characteristics and reasons for the carbon isotopic reversal of natural gas in the southern Jingbian gas field, Ordos Basin, China

• Published in: Geological magazine Vol. 157; no. 4; pp. 527 - 538
• Main Authors: Han Wenxue, Han Wenxue, Chang Xiangchun, Chang Xiangchun, Ma Weijiao, Ma Weijiao, Tao Shizhen, Tao Shizhen, Yao Jingli, Yao Jingli, Hou Lianhua, Hou Lianhua, Yang Weiwei, Yang Weiwei
• Format: Journal Article
• Language: English
• Published: Cambridge Cambridge University Press 01.04.2020
• Subjects: aliphatic hydrocarbons; alkanes; Asia; C-13/C-12; Carbon; Carbon isotopes; carbonate rocks; China; Economic geology; energy sources; Ethane; Far East; Gases; Geochemistry; giant fields; hydrocarbons; isotope ratios; Isotopes; Jingbian Field; lower Paleozoic; Maturity; methane; Natural gas; Oil and gas fields; Ordos Basin; organic compounds; Oxidation; Paleozoic; petroleum; reservoir properties; reservoir rocks; sedimentary rocks; Sediments; spatial variations; stable isotopes; thermal maturity; Ordos Basin; China
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756819000682

The carbon isotope value of ethane in the southern part of the Jingbian gas field is lower than that in the northern part, indicating a carbon isotopic reversal in the southern Jingbian gas field (δ13Cmethane > δ13Cethane). Through comparing the geochemical characteristics of gases in the southern and northern parts of the gas field, the reasons for the carbon isotopic reversal in the southern Jingbian gas field were determined to be high thermal maturity and mixing action. When thermal maturity reaches a critical value, the carbon isotope value of ethane becomes relatively more depleted with thermal maturity. Although the carbon isotope value of methane increases with thermal maturity, the extent is relatively smaller. Finally, the rare phenomenon of δ13Cmethane > δ13Cethane occurs. High thermal maturity leads to the secondary thermal cracking of gases. Mixing of the cracked gases and primary gases also leads to carbon isotopic reversal. Both of the above mechanisms share a common premise, which is high thermal maturity.