114 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
in the oil measures are practically free from sulphate. This decrease 
in sulphate is attended by a corresponding increase in carbonate, and 
in districts in which chloride is not abundant the waters near the oil 
measures are nearly pure carbonate waters. Where chloride is the 
predominating acid radicle, even in the shallower waters, carbonate 
is unimportant, and the chief change in character with depth is due 
to the disappearance of the sulphate. Sulphide and hydrogen sul¬ 
phide appear near the horizon at which sulphate begins to decrease 
and are present in many of the waters above the oil measures, but the 
complete disappearance of the sulphate and of the sulphide are coin¬ 
cident. The amount of sulphide in the deeper waters is roughly pro¬ 
portional to the amount of sulphate in the waters directly above 
them, or nearer the surface. 
Calcium and magnesium predominate in many of the surface wa¬ 
ters, but sodium and potassium greatly predominate in the deeper 
waters. Most of the waters associated with the oil are therefore 
variously proportioned mixtures of solutions of alkaline carbonates 
and chlorides, the proportion of carbonate depending chiefly on the 
extent to which meteoric water is able to enter at the outcrop. Be¬ 
sides the change in character with depth the oil-field waters therefore 
exhibit progressive variations with distance from the outcrop, the 
predominantly carbonate waters being found near the outcrop and 
the predominantly chloride waters farther away. The final stage in 
this gradation is represented by the very salty waters, which are 
believed to be comiate. Coincident with these changes in composi¬ 
tion is a great increase in the total amount of dissolved solids. 
The decrease of sulphate, appearance of sulphide, and increase of 
carbonate in the waters as the oil zone is approached all point to 
reactions between the oil-field waters and constituents of the oil or 
gas. It is probable that sulphate is reduced to sulphide by certain 
oil constituents whose carbon is simultaneously oxidized to products 
which eventually yield carbonate, though the stages involved in these 
reactions and the conditions under which they take place are not yet 
understood. These reactions not only alter the character of the 
waters but they also produce the gases carbon dioxide and hydrogen 
sulphide. Carbon dioxide, presumably thus produced, occurs in the 
natural gas of these fields in amounts ranging from 5 to more than 
30 per cent. The greatest proportions of carbon dioxide are usually 
found in gas near the outcrop, where sulphate water can most readily 
enter the beds and where the reaction would naturally be vigorous. 
A large part of the hydrogen sulphide formed is probably changed to 
sulphur, some of which apparently reacts again with the oil to make 
it heavier and more asphaltic. It is significant that the oils contain- 
