Page 134 - Geology and Geochemistry of Oil and Gas

ISOTOPE COMPOSITION OF NATURAL GASES 105
Mixing of nitrogen in sedimentary rocks with the nitrogen coming from the
mantle is quite possible, but cannot be proved at the current level of analytical
techniques. Prasolov (1990) who studied the problem mentioned that the mantle
nitrogen in most cases cannot be identified against the background of the crustal and
atmospheric nitrogen.

6.2.5. Inert gases

Inert gases of a predominantly radioactive origin may form in the Earth’s crust as
well as in the upper mantle. According to Prasolov (1990), the mantle helium is
present in the natural gases of the Earth’s crust. Its content is minimal (o1%) in the
gases of sediment cover. The mantle helium dominates only in gases associated with
the present-day volcanism and rifting. The same writer estimated the current gas
composition in the upper mantle taking into account the vapor pressure and isotopic
composition:

CO 2 /He CH 4 /He N 2 /He 40 Ar/He 36 Ar/He 3 He/He
100 10 4 1 10-4 10-5

On the basis of the above, the mantle CO 2 may be expected in gas accumulations
of the sediment cover, in addition to negligible amounts of methane and other gases.
The age of the gas can be determined from the He/Ar ratio, taking into account
their radioactive origin. For instance, Fig. 6.1 shows the evolution in time of the He/
4
40
40
Ar ratio. Table 6.3 illustrates distributions of He/ Ar B and 40 Ar P / Ar B in gas
accumulations of tectonic units of different age.

40
4
Fig. 6.1. Variation of He/ Ar p ratio with time. 1 — In gases being formed in rocks t years ago; 2 — in
gases formed between 4.5 billion years ago to time t; 3 — in gases formed between the time t and present
time. t — abscissa. (After Prasolov, 1990.)