Gases of the Earth's Crust

Gases of the Earth’s Crust 

gases found in the earth’s crust in a free state, dissolved in water and oil, and as sorbates of various rocks, especially coal.

The quantity of gases in the earth’s geospheres increases with depth (see Table 1). Gases of the earth’s crust may be divided into nine genetic groups, depending on the type of gas-generating processes involved. The most important gases are of catagenetic, metamorphic, volcanic, biochemical, or radioactive origin, or they come from the atmosphere. Gases produced by nuclear reactions, gases originating in radiochemical processes, and subcrustal gases are of but secondary importance in the earth’s crust.

Gases of catagenetic origin are formed as a result of the transformation of organic matter contained in sedimentary rocks when the rocks are carried downward and simultaneously exposed to an increase in pressure of from 10 to 200-250 millinewtons per sq m (from 100 to 2,000-2,500 atmospheres) and an increase in temperature of from 25°-30° C to 250°-300° C. Most fuel gases are catagenetic.

When temperature and pressure are further increased, metamorphic gases are formed, and when the rocks melt, gases of regeneration appear. These gases include water vapor, carbon dioxide, carbon monoxide, hydrogen, sulfur, sulfur dioxide, methane, nitrogen, and, rarely, inert gases and volatile chlorides.

Volcanic gases come primarily from the depths of the earth. They result from the degassing of the mantle.

Biochemical gases are formed during bacterial decomposition of organic substances and, more rarely, during reduction

Table 1. Quantity and total composition of gases in the earth’s geospheres (according to V. A. Sokolov)
Geospheres	Weight of geospheres (in 1016 tons)	Total weight of gases (in 1015 tons)	Average gas content (%)	Weight of various components (in 1012 tons)
				O2	N2	CO2	CH4	H2	H2S + SO2	HCL + HF	He	Ar
Sedimentary stratum ................	2.5	0.214	0.0097	2	76	92	43	0.2	0.8	—
Granite and basalt										)	28)	600
strata ................	26	7.8	0.03	—	500	6,300	15	115	200	600)	)
Upper mantle ................	—	435.0	—	—	13,000	210,000	—	8,600	210,000	83,000

of mineral salts. These gases include methane and its homologues (ethane among others), carbon dioxide, hydrogen sulfide, nitrogen, oxygen, and, rarely, hydrogen and other gases. This group constitutes a large part of the gases that are released into the atmosphere or that accumulate in the uppermost parts of the earth’s crust.

Radioactive gases are formed when radioactive elements decay. This group includes helium and some short-lived emanations of radium and thorium. These gases do not form independent accumulations.

Gases from the atmosphere penetrate the earth’s crust mainly in water solutions. Nitrogen, oxygen, and the inert gases argon, krypton, and xenon are included in this group.

According to their chemical composition, gases of the earth’s crust may be divided into three principal groups: hydrocarbon gas, nitrogen gas, and carbonic acid gas. The special property of gases—their ability to move about easily both in a free state and dissolved in water—leads to the formation of mixtures of gases of different origins and at the same time to their wide distribution in nature (see Table 2).

Table 2. Chemical composition of gases of different origin (in percent)
Location	CO2	CO	CH4	C2H6 and over	H2	SO2	N2	Ar	H2S
1 composition of gases extracted from crushed rock
Mt. Etna ................	28.8	0.5	1.0	—	16.5	34.5	(18.7)		—
Kislovodsk, Narzan ................	92.13	0.37	—	—	—	—	7.3	0.129	—
Noril’sk, gabbrodiabase1 ................	34.2	—	30.7	—	—	—	35.1	—	—
Noril’sk, porphyrites1 ................	23.6	—	8.9	—	51.3	—	16.2	—	—
Mud volcano of Bog-Boga (Apsheron Peninsula) ................	1.6	0.4	94.7	0.29	0.3	—	2.7	—	—
Gas deposit at Karadag, stratum Vll-a (Azerbaijan) ................	0.19	—	97.72	2.09	—	—	—	—	—
Gas deposit at Lacq (France) ................	9	—	74	2	—	—	—	—	15
Casinghead gas from Mesozoic deposits in western Ciscaucasia ................	7.68	—	84.57	6.54	—	—	1.2	0.52	0.01

Most fuel (hydrocarbon) gases are found dissolved in underground waters. The average methane content in the strata waters of the western Kuban’ depression ranges from 1-10 cu m per cu m. The total quantity of methane dissolved in underground waters is many times greater than its reserves in gas and oil deposits, and it constitutes some n × 10¹⁶ cu m, according to L. M. Zor’kin.

A substantial quantity of hydrocarbon gases is contained in organic substances that are dispersed in sedimentary rocks or form coal. Coal may contain 50 cu m or more of methane per ton. Gases may be released from underground waters and may accumulate in dry form only when the elasticity of the dissolved gases exceeds the water pressure at a given depth. Hence, it is chiefly the gases of catagenetic origin that form free concentrations.

REFERENCES

Kozlov, A. L. Problemy geokhimii prirodnykh gazov. Moscow-Leningrad, 1950.
Sokolov, V. A. Geokhimiia gazov zemnoi kory i atmosfery. Moscow, 1966.

N. B. VASSOEVICH