--> ABSTRACT: Isotopic Correlation of Helium, Argon, Nitrogen and Carbon in Natural Gases, by Eduard M. Prasolov; #91020 (1995).

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Isotopic Correlation of Helium, Argon, Nitrogen and Carbon in Natural Gases

Eduard M. Prasolov

Isotopic composition is the single direct geochemical criterium of the origin of natural gas. So the profitable and uncontradictory interpretation of empirical isotopic data is very important, and the utilization of different isotopic systematics is very appropriate. In this work the isotopic data on helium, argon, molecular nitrogen and carbon of methane in gas and oil fields in former Soviet Union (many hundreds of isotopic analyses) are discussed jointly.

In the majority of 480 gas and oil fields the value 3He/4 He does not differ from crust's one. The contribution of mantle helium is more than 0.1 in only 8% of cases. According to helium isotopic data the contribution of mantle methane is less than 0.05 always and than 0.001 in the most of cases. Some other noble gases ratios must depend on temperature or depth of gas phase forming. In the 370 gas pools the average value R=4He/40 Arrad=8 that corresponds to~200°C. There are positive empirical correlations between R, 40Aratm/40Arrad (or36Ar/40Ar),40Aratm and negative between 3He/4He and R.

The presence of negative correlations ^dgr13C of methane with R, 40Aratm/40Arrad,40Aratm and positive one with 3He/4He has revealed. Temperature of methane generation calculated according to the value R and the correlation curve of R and ^dgr13C is similar to the same, but calculated in conformity with the thermodynamic (isotopic exchange) effect There are not any correlations between ^dgr15N and discussed above isotopic ratios. This fact counts in favour of the little influence of isotopic migration effects.

Revealed correlation s show an existence of the main genetic reason of isotopic variations. I think it is temperature and/or depth of natural gas forming. On the previous data natural gases were formed on wide range of depths, but the most part on the great depths (>150°C). The depths differ in various regions and basins first of all according to neotectonic activity.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995