Page 135 - Geology and Geochemistry of Oil and Gas

106 NATURAL GASES AND CONDENSATES

TABLE 6.3
40
4
40
Distribution of He/ Ar B (A) and 40 Ar P / Ar B (B) in Gas Accumulations (After Prasolov, 1990)
Sample Parameter Ratio Sample Parameter Ratio
Oil and gas basins worldwide A 10 Young platforms A 7
8.5–12 5–9
B 0:65 B 0:06
0.6–0.75 0.45–0.75
Ancient platforms A 16 Folded regions A 1:1
12–21 0.8–1.6
B 1:0 B 0:23
0.7–1.3 0.16–0.32
Note: Denominator shows variance range with confidence probability of 0.95

6.3. PHYSICAL PROPERTIES OF NATURAL GASES

6.3.1. Gas density
3
3
Gas density (r in kg/m , g/cm , lb/cu ft) is a mass m of a unit volume of gas V or
the ratio of the gas molecular mass M to the volume of a mole:
r ¼ m=V (6.1)
or
r ¼ M=22:4 (6.2)
Molecular mass of a substance (a non-dimensional value) is the ratio of the mass
of a substance’s molecule to 1/12th of the mass of the 12 C-isotope atom mass. The
amount of a substance equal to its molecular mass expressed in grams is called gram-
mole. The density of a gas mixture is determined from the density of the mixture
components taken under identical conditions:
X
r ¼ (6.3)
i
mix r n i
where r and n i are the density and molar fraction of the ith component in the gas
mix
mixture.
3
Specific weight g of a gas is the weight of 1 cu m (kg/m ) or 1 L (g/l) of the gas at
01C and 1 atm:
g ¼ rg (6.4)
where g is the gravitational acceleration.
Gas density is determined by its chemical composition (proportions of various
components), molecular weight of its components, pressure, and temperature. As
temperature increases, the gas density decreases. As the molecular mass and pressure
increase, the gas density grows. Sometimes, the specific gravity of a natural gas is