The Possibility and Explanation of Complete Reverse Isotopi c Distribution of Light Hydrocarbons in Natural Gas

LI Chun yuan; WANG Xian bin; XIA Xin yu

Carbon isotope distribution among the C₁ to C₄ hydrocarbon is one of the most important criteria to identify the genesis of natural gas.Generally,bio genic natural gases possess a “normal distribution ”in carbon isotopes of C 1 to C₄ hydrocarbons and abiogenic natual gases a “reversal distribution”in carbon isotopes of C₁ to C₄ hydrocarbons.Some scholars consider that “isot opic reversals”can result from the mixture of two kinds of biogenic natural gas with “normal distribution” in carbon isotope.However,prevenient numerical sim ulation did not confim the viewpoint.By means of numerical simulation,the possib ility of “isotopic reversals” in C₁ to C₄ hydrocarbons resulted from the mixture of two kinds of biogenic natural gas was discussed in this paper. Computation in this article shows that a complete reverse carbon isotopic distri bution among C₁ to C₄ hydrocarbons (i.e. δ¹³C methane >δ¹³ C propane >δ¹³C n-butane) can be produced by the mixture of two kinds of biogenic gas with a normal carbon isotopic distribution (i.e. δ¹³C m ethane <δ¹³C ethane <δ¹³ propane <δ¹³C n-butane). While a normal carbon isotopic distribution in natural gas can also be produced by two e ndmembers with reverse distributions. As for the first case, the two endmembers should be of different source, different formation, or of different evolutional stage. It is unreasonable to explain the isotopic reverse distribution of the gas sampl es from several wells of the Changde pool, Songliao Basin by the mixture model, because the isotopically higher endmember needed to form this real gas is not fo und in Songliao Basin. So a better explanation to the isotopic reversal in this gas pool is to consider it to be abiogenic.

The Possibility and Explanation of Complete Reverse Isotopi c Distribution of Light Hydrocarbons in Natural Gas

Lanzhou Institute of Geology Chinese Academy of Sciences Lanzhou China 730000

Received Date: 1998-11-12

Publish Date: 1999-06-10

Key words:

light /
hydrocarbons /
&delta /
13C /
distribution /
numerical

Abstract: Carbon isotope distribution among the C₁ to C₄ hydrocarbon is one of the most important criteria to identify the genesis of natural gas.Generally,bio genic natural gases possess a “normal distribution ”in carbon isotopes of C 1 to C₄ hydrocarbons and abiogenic natual gases a “reversal distribution”in carbon isotopes of C₁ to C₄ hydrocarbons.Some scholars consider that “isot opic reversals”can result from the mixture of two kinds of biogenic natural gas with “normal distribution” in carbon isotope.However,prevenient numerical sim ulation did not confim the viewpoint.By means of numerical simulation,the possib ility of “isotopic reversals” in C₁ to C₄ hydrocarbons resulted from the mixture of two kinds of biogenic natural gas was discussed in this paper. Computation in this article shows that a complete reverse carbon isotopic distri bution among C₁ to C₄ hydrocarbons (i.e. δ¹³C methane >δ¹³ C propane >δ¹³C n-butane) can be produced by the mixture of two kinds of biogenic gas with a normal carbon isotopic distribution (i.e. δ¹³C m ethane <δ¹³C ethane <δ¹³ propane <δ¹³C n-butane). While a normal carbon isotopic distribution in natural gas can also be produced by two e ndmembers with reverse distributions. As for the first case, the two endmembers should be of different source, different formation, or of different evolutional stage. It is unreasonable to explain the isotopic reverse distribution of the gas sampl es from several wells of the Changde pool, Songliao Basin by the mixture model, because the isotopically higher endmember needed to form this real gas is not fo und in Songliao Basin. So a better explanation to the isotopic reversal in this gas pool is to consider it to be abiogenic.

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Reference (19)

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The Possibility and Explanation of Complete Reverse Isotopi c Distribution of Light Hydrocarbons in Natural Gas

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