Study on Carbon Isotopic Ratio of Individual Compound in Hopanes

SHI Ji yang; XIANG Ming ju; ZHOU You ping

The combined gas chromatograph-isotope ratio mass spectrometry(GC-IRMS) was employed to measure compound-specific isotope ratio of hopanes in five peat samples from Guangdong province and 3 oil samples from northwestern margin of Junggar basin. It is shown from the result that the isotopic ratio of carbon for hopanes in peats are -21.7‰～-34.4‰,which are relatively enriched in¹³C compared to that of the lacustrine Messel shale, Green River shale and the oils of Jun ggar basin, indicating that biological bacteria sourced hopanes of peats are di fferent from that of lacustrine shales and oils. It is also found from the result that the hopanes with different carbon number and configuration in peat XLI ha ve significant difference in isotopic composition of carbon, e.g. the δ¹³C values of C₂₇β hopane is -23.2‰,whereas the δ¹³C value of C₂₉β β hopanes is -34. 4‰. This demonstrates that various biological bacteria sourced hopanes may be present in the same depositional environment. As C₂₉βα, C₂₉ββ, C ₃₁βα hopanes found in peats are isotopically lighter than that of their C₂₉αβ, C ₃₀αβ co unterparts, it is suggested that C₂₉βα, C₂₉ββ, C₃₁βα hop anes are sourced f rom methanotrophic bacteria and that C₂₉αβ, C₃₀αβ hopans are sou rced from che moautotrophic bacteria. The aim to measure carbon isotopic composition of hopane s in oils of the northwestern margin, Junggar basin in this study is to explore δ¹³C values of hopanes for oils correlation because that the oils are pr odu ced from various strata in the northwestern margin area and that the strata are developed with many faults. As a result, it was found that δ¹³C values of oils fr om the west of northwestern margin area are quit different from that of the east of northwestern margin, i.e. δ¹³C value of hopanes for oils in the west of no rthwestern margin are -39.9‰～-53.4‰, whereas δ¹³C values of hopanes fo r oils in the east of northwestern margin are -33.1‰～-31‰.Since isotopic ratio of h opanes may be enriched in¹³C with increase of their thermal maturation, it is s uggested from this study that the age of rock sourced oils in the east of northwestern margin should be older than that sourced oils in the west of nort hwestern margin, i.e. the oils from the east of northwestern margin may be sourc ed from Permian, whereas oils from the west of northwestern margin may be sourc ed from Triassic and Jurassic.

Study on Carbon Isotopic Ratio of Individual Compound in Hopanes

1 Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640;

2 Agriculture University of Southern China, Guangzhou 510640

Received Date: 1999-08-10

Rev Recd Date: 1999-10-08

Publish Date: 2000-06-10

Key words:

Abstract: The combined gas chromatograph-isotope ratio mass spectrometry(GC-IRMS) was employed to measure compound-specific isotope ratio of hopanes in five peat samples from Guangdong province and 3 oil samples from northwestern margin of Junggar basin. It is shown from the result that the isotopic ratio of carbon for hopanes in peats are -21.7‰～-34.4‰,which are relatively enriched in¹³C compared to that of the lacustrine Messel shale, Green River shale and the oils of Jun ggar basin, indicating that biological bacteria sourced hopanes of peats are di fferent from that of lacustrine shales and oils. It is also found from the result that the hopanes with different carbon number and configuration in peat XLI ha ve significant difference in isotopic composition of carbon, e.g. the δ¹³C values of C₂₇β hopane is -23.2‰,whereas the δ¹³C value of C₂₉β β hopanes is -34. 4‰. This demonstrates that various biological bacteria sourced hopanes may be present in the same depositional environment. As C₂₉βα, C₂₉ββ, C ₃₁βα hopanes found in peats are isotopically lighter than that of their C₂₉αβ, C ₃₀αβ co unterparts, it is suggested that C₂₉βα, C₂₉ββ, C₃₁βα hop anes are sourced f rom methanotrophic bacteria and that C₂₉αβ, C₃₀αβ hopans are sou rced from che moautotrophic bacteria. The aim to measure carbon isotopic composition of hopane s in oils of the northwestern margin, Junggar basin in this study is to explore δ¹³C values of hopanes for oils correlation because that the oils are pr odu ced from various strata in the northwestern margin area and that the strata are developed with many faults. As a result, it was found that δ¹³C values of oils fr om the west of northwestern margin area are quit different from that of the east of northwestern margin, i.e. δ¹³C value of hopanes for oils in the west of no rthwestern margin are -39.9‰～-53.4‰, whereas δ¹³C values of hopanes fo r oils in the east of northwestern margin are -33.1‰～-31‰.Since isotopic ratio of h opanes may be enriched in¹³C with increase of their thermal maturation, it is s uggested from this study that the age of rock sourced oils in the east of northwestern margin should be older than that sourced oils in the west of nort hwestern margin, i.e. the oils from the east of northwestern margin may be sourc ed from Permian, whereas oils from the west of northwestern margin may be sourc ed from Triassic and Jurassic.

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Reference (10)

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