Characteristics of Carbon Isotopic Composition of N-alkanes during Hydrocarbon Generation and Expulsion and Its Significance

Key words: 
• hydrocarbon gaaeration /
• expulsion expelled oil /
• residual oil /
• n-alkane /
• GC-IRMS

Abstract: Gas chromatography isotope ratio mass spectrometry (GC IRMS) has been widely applied in the various fields of organic geochemistry, e.g. identifying organic source, correlating oil with possible source rocks, and reconstructing paleoenvironment and paleoclimate. The carbon isotopic composition of n alkanes from oils and extracts has played an important role in oil and natural gas geochemistry. Previous studies indicate that the hydrocarbon generation and expulsion has obvious effects on the distribution of n alkanes and other biomarkers, e.g. steranes and terpanes. This paper tries to reveal the characteristics of carbon isotopic composition of n alkanes during hydrocarbon generation and expulsion by determining the δ ¹³ C values of the n alkanes in the pyrolysates (residual oils and expelled oils). The results indicate that the liquid n alkanes are mainly derived from the primary cracking of kerogen during the early hydrocarbon generation, so,whether in the expelled oils or in the residual oils, their isotopic compositions have no obvious variation (less than 2‰) with increasing temperature, and display a similar distribution. It suggests that they can be used in oil/oil correlation. However, at the peak of hydrocarbon generation, the second cracking of those heavy hydrocarbons formed at the early stage, such as asphaltene, NSO fraction, and n alkanes with high carbon number, makes the remaining n alkanes markedly riched¹³ C in carbon isotope, especially for the heavily weighed fraction. Significant differences (1 to 4‰) in the carbon isotopic compositions are observed between the n alkanes in the residual oils or the expelled oils generated at this stage and those formed at the former stage. Since thermal maturation has effects on the carbon isotopic compositions of n alkanes due to different generation mechanisms may occur at the different stages, therefore, to explain the significant difference when taking the oil/oil and oil/source correlations, we also need consider thermal maturation other than organic source. The conclusion of heterogeneity should not be made easily. In addition, the experimental results show that the hydrocarbon expulsion has no considerable effect on the carbon isotopic composition of the liquid n alkanes.