Distribution Pattern of Carbon Isotope (N-Alkanes) and Its Applications to Identify the Source Matter Origin

Zhongping Li¹, Liwu Li¹, Minxin Tao², Xiaobin Li², Zuodong Wang¹, and Li Du^1
1Key Laboratory of Gas Geochemistry, Institute of Geology and Geophysics ,Chinese Academy of Sciences, Lanzhou, China
²College of Resources Sciences and Technology, Beijing Normal University, Beijing, China

The carbon isotope distribution patterns can be used not only to calculate some geochemical parameters of oil generation, migration and accumulation, but also to provide some valuable geochemical information about organic matter source. There exist various views to interpret the zigzag pattern of carbon isotope values of n-alkanes. but no scholar had explained the phenomenon from the aspect of internal molecular structure. In view of above reasons, single-origin-source samples chosen were analyzed, through the carbon isotope(n-alkanes) values of 16 samples selected from four different locations in the southern region of Gansu province, China. We found, with the carbon number increase, the carbon isotope values show a zigzag pattern with the values decreasing from -28.2‰ to -39.4‰(PDB). Further studies show the average correlation coefficient value of consecutive neighboring odd-number carbons is 0.83, it is 0.69 for the consecutive even-number carbon, yet it is only 0.24 for consecutive neighboring carbons. Combining with hydrogen isotope values and the molecular biomarker parameters, the authors argued that the isotope fractionation in the decarboxylation process can account for the zigzag pattern phenomenon.

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