44 
OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
The decrease of sulphate in the waters as the oil measures are 
approached, and its absence from the waters most closely associated 
with the oil, are believed to be the result of chemical reaction with 
constituents of the oil or gas. The sulphate is probably reduced to 
sulphide or hydrogen sulphide, which may either escape as gas or 
undergo oxidation to free sulphur and so he lost by precipitation. 
The reduction of the sulphate is presumably attended by the oxida¬ 
tion of an equivalent portion of the hydrocarbon material to carbonate 
or carbon dioxide. These reactions are discussed in more detail on 
pages 93-99. 
A great predominance of sulphate in the shallower waters is char¬ 
acteristic of most arid regions. Gypsum (calcium sulphate) is dis¬ 
seminated through the rocks and furnishes an abundant supply to 
the scanty volume of water percolating through them. In addition, 
much of the “alkali” that occurs as an incrustation or efflorescence 
in arid regions is sodium and magnesium sulphate. Ocean water 
contains about 2,700 parts per million of sulphate, or 7.7 per cent of 
the total mineral content. 
Chloride .—Chloride is a widespread constituent of terrestrial waters, 
but its concentration varies greatly. As the chlorides of all the com¬ 
mon bases are highly soluble in water, they are not important as 
rock-forming constituents and are concentrated chiefly in the ocean. 
A high concentration of chloride in ground water usually indicates 
that the water is partly of oceanic origin or that it has leached saline 
deposits. 
As already explained, the chloride in the oil-field waters is undoubt¬ 
edly of oceanic origin, and its concentration depends on the extent 
to which the connate sea water has been admixed with meteoric water 
carrying sulphate or carbonate. This admixture is controlled largely 
by the freedom of the circulation. Where the geologic structure is 
such as to prevent circulation the water is very salty, and in some 
localities the concentration of chloride in it is as great as in ocean 
water. Near the surface and near the outcrop of the beds the con¬ 
nate water has been largely replaced by meteoric water, and the con¬ 
centration of chloride is therefore much lower. 
Carbonate and bicarbonate .—In most industrial analyses no distinc¬ 
tion is made between carbonate and bicarbonate, the two being 
grouped under the head of carbonate. Under ordinary conditions 
the two are more or less interchangeable; carbonate may be regarded 
as the primary radicle from which, in the presence of carbon dioxide, 
bicarbonate is derived. If a sufficient amount of carbon dioxide is 
present all of the carbonate radicle may be converted to bicarbonate, 
but a slight increase in temperature or decrease in pressure may drive 
part of the carbon dioxide from the solution and thus cause a rever- 
