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Chapter 6

NATURAL GASES AND CONDENSATES

6.1. COMPOSITION OF NATURAL GASES

Natural gases are a naturally occurring mixture of hydrocarbon and non-hydro-
carbon gases found in porous formations beneath the Earth’s surface, often in as-
sociation with crude oil. Their chemical composition and physical properties are
quite diverse. Most common among gaseous chemical compounds are hydrocarbons,
H 2 , CO 2 , CO, N 2 , H 2 S, NH 3 , O 2 , etc. On the basis of their origin, Belousov (1939)
classified gases into (1) biochemical; (2) aerial; (3) associated with natural chemical
reactions occurring at ambient or high temperature; and (4) radioactive. There are
many other classifications, which are either highly specialized (e.g., for volcanic
gases) or incorporate the above classification.
Primarily, natural gas is a mixture of hydrocarbon molecules belonging to the
paraffin series. The simplest hydrocarbon is methane, CH 4 ; followed by ethane,
C 2 H 6 ; propane, C 3 H 8 ; butane, C 4 H 10 ; and heavier components as shown in Table
6.1. These compounds have the chemical formula of C n H 2n+2 .
Natural gas is principally composed of methane with decreasing amounts of
ethane, propane, and heavier components. It normally is partially or completely
saturated with water vapor and many contain nitrogen and inert gases such as
helium and argon, and acid gases such as carbon dioxide, hydrogen sulfide, and
mercaptans.
In the Earth’s crust, gases are mostly dissolved (in water or in oil) or sorbed, and
often form free accumulations. Free gas accumulations may be formed by hydro-
carbons, sometimes by nitrogen, and rarely by carbon dioxide, but usually by mix-
tures (in various proportions) of these gases. The other compounds are present in the
form of an admixture, although sometimes a significant one (e.g., up to 20% of
H 2 S).
Hydrocarbon gases may contain a fraction of 1% to a few percentage points of
CO 2 (sometimes its content is much higher). The CO 2 content in the South Semi-
vodskoe gas-condensate field (West Siberia) is 73.1%, and that of the Braden South
Field (U.S.) is 86.6%. The CO 2 presence is typical in the solution gases of young
(Paleogene, Neogene) crude oils. Some increase in the CO 2 content is noted in the
natural gases of foredeeps. Average nitrogen content in hydrocarbon gases ranges
from 0.5 to 12%. The lowest nitrogen concentrations (fractions of 1%) are found in
the areas of deep basement subsidence, foredeeps, and intermontane depressions.
Gases on young platforms usually contain less than 5% nitrogen, and those on
ancient platforms (such as Russian or American) contain 5–7%. The nitrogen con-
tent in the natural gases drastically increases in the western Volga-Ural province,
where it may exceed 95%.