The evolution characteristics and fractionation mechanism of carbon isotopes in the process of “multi-stage hydrocarbon generation”

• Published in: Acta geochimica Vol. 24; no. 1; pp. 1 - 8
• Main Authors: Xu, Yongchang, Liu, Wenhui, Shen, Ping, Zhang, Xiaobao, Huang, Difan, Song, Yitao
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.01.2005; State Laboratory of Gas Geochemistry, Institute of Geology, Chinese Academy of Sciences, Lanzhou 730000, China%Institution of Oil and Gas Exploration and Development of China Petroleum & Natural Gas Stock Co. Ltd. , Beijing 100083, China%Institution of Geosciences, Shenli Petroleum Administrative Bureau, Dongying 257015, China
• Subjects: Carbon; Carbon isotopes; Crude oil; Evolution; Fractionation; Geochemistry; Hydrocarbons; Isotope fractionation; Isotopes; Oil; Oils & fats; Rock; Rocks; Thermal degradation; Thermodynamics; carbon isotope fractionation; multi-stage hydrocarbon generation; immature oil
• ISSN: 1000-9426; 2096-0956; 1993-0364; 2365-7499
• DOI: 10.1007/BF02869683

The Jiyang Sag and the Liaohe Basin are the two important areas where immature oil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, R^sub o^, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of R^sub o^ values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e., light[arrow right]heavy[arrow right]light[arrow right]heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bond-breaking ratio of^sup 12^C-^sup 13^C also increases, making the relatively^sup 12^C-rich materials at the low mature stage evolve again towards^sup 13^C enrichment.[PUBLICATION ABSTRACT]