Anomalies and genetic mechanism of ethane carbon isotope in the Ordovician carbonate reservoirs in the Jingbian gas field, Ordos Basin, China

• Published in: Carbonates and evaporites Vol. 35; no. 1
• Main Authors: Han, Wen-xue, Chang, Xiang-chun, Tao, Shi-zhen, Yang, Wei-wei, Yao, Jing-li
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.03.2020; Springer Nature B.V
• Subjects: Anomalies; Carbon; Carbon isotopes; Carbonates; Components; Depletion; Earth and Environmental Science; Earth Sciences; Ethane; Fractionation; Gas production; Gases; Geology; Isotopes; Kerogen; Methane; Mineral Resources; Mineralogy; Natural gas; Oil and gas fields; Oil and gas production; Ordovician; Original Article; Reservoirs; Special Issue: Research in Karst Environments; Ordos Basin; China; Ordos basin; Ethane carbon isotope; High thermal maturity; Migration fractionation; Geochemical characteristics
• ISSN: 0891-2556; 1878-5212
• DOI: 10.1007/s13146-020-00558-9

The Ordos Basin is one of the most petroliferous basins in China, and the cumulative gas yield of the Jingbian gas field was 91.36 × 10 9  m 3 , accounting for nearly 30% of total gas production in the Ordos Basin. The ethane carbon isotopic value of gases in the Ordovician carbonate reservoirs in the Jingbian gas field is abnormally depleted (δ 13 C 1  > δ 13 C 2  < δ 13 C 3 ), which has directly led to the controversy over the source and genetic types of natural gas in the Jingbian gas field. To find out the genetic mechanism of anomalies of ethane carbon isotope, components and carbon isotopic values of natural gas in the Jingbian gas field were measured. On this basis, natural gas components and stable carbon isotopic values of the Wushenqi and Jingbian gas field were comprehensively measured and compared. The results show that higher thermal maturity leads to further cracking of the oil generated earlier and the residual kerogen. These secondary cracked gases mixed with the early generated gases lead to the anomalies of ethane carbon isotope. When thermal maturity reaches a critical value, it will have different effect on carbon isotopic values of methane and ethane. The carbon isotopic value of methane will continue to increase; however, that of the ethane will gradually decrease, which will eventually lead to the anomalies of ethane carbon isotope. Migration fractionation will also lead to the abnormally depleted of carbon isotopic values of ethane.