Page 133 - Geology and Geochemistry of Oil and Gas

104 NATURAL GASES AND CONDENSATES

both hydrogen and carbon in natural gases depends on their origin (Shen et al., 1988;
see also Khilyuk et al., 2002).

6.2.3. Sulfur
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Numerous studies in the former Soviet Union showed that the d S content in
natural gases ranges between 18 and 20%. The gases in younger deposits (Neogene,
Paleogene) are richer in the heavy sulfur isotopes than the sulfur in crude oils,
whereas in the older (Carboniferous) gases the isotope content of gases and oils is
similar or even lower in gases. This fact casts doubt on the conclusion that there is a
genetic relation between the sulfur present in oil and in gas. Interesting results were
obtained in the experiments on microbial reduction of sulfates. It was found that the
resulting H 2 S might have sulfur of different isotope composition depending on the
amount and speed of the introduction of sulfates into water. When the sulfate supply
is constant and high, the sulfur in the H 2 S is lighter. When the rate of sulfate
introduction is equal to that of its destruction, the formation water may become
almost totally sulfate-free and the isotope composition of sulfur in the H 2 S may be
equal or slightly lower than in the introduced sulfate. Krouse et al. (1988) had found
that the sulfur in the gas accumulations rich in H 2 S are enriched in 34 S.

6.2.4. Nitrogen

Nitrogen isotope composition in the natural environments ranges between 30
15
and +30%. The d N content in the natural gases ranges from 15 to 30%.
15
Prasolov (1990) reported the following d N variations for large tectonic units (%):
Russian Platform 10 (Kolliyar, C 1t ) to +26.5 (Orenburg, P 1 )
Timan-Pechora +0.4 (Dzhebed, D 3 ) to +26.9 (Vuktyl, C 1 )
North Caspian Basin +27.8 (Listvinskoye, P 1 )
Dnieper-Don Basin 6.5 (Rybalskoye, P 2 ) to +0.4 (Glinsko-Rozbyshev, C 1 )
Siberian Platform 5.3 (Kuimba, R) to 1.3 (Ust’ Vilyuy, R)
Vilyuy Syneclise 8.6 (Middle Tyunga, P 2 ) to 1.8 (Mastakh, P 2 )
West Siberia 14 (Urengoi, K 1 ) to 3 (Kharstovey, K 1 )

15
Considerable variations in the d N content in natural gases are due to (1) the
presence of intrinsic nitrogen in the Earth’s crust and (2) fractionation (and mixing
15
of gases with various d N content) during migration. The latter has been established
15
by Hoering and Moore (1958) — the d N becomes lighter in the direction of mi-
gration.
Apparently, sedimentary rocks, which are the richest in nitrogen, serve as
the major source of nitrogen. These rocks include nitrogen in their minerals and in
the organic matter. Metamorphic rocks, coals, crude oils, volcanic rocks, and the
atmospheric air also may have been sources of nitrogen. Compared to the associated
crude oils, nitrogen in the natural gases is lighter by 3–15%.