CLASSIFICATION OF THE OIL-FIELD WATERS. 
45 
sion to carbonate. A solution of calcium bicarbonate, for example, 
yields calcium carbonate on boiling. The bicarbonates of calcium 
and magnesium are relatively soluble in water, whereas the normal 
carbonates are almost insoluble; hence, when the bicarbonate solu- 
tions are boiled and, by release of carbon dioxide, normal carbonate 
formed, most of the normal carbonate is thrown out as a precipitate 
or “scale.” The solubility of the alkaline earth radicle in the pres¬ 
ence of the carbonate radicle therefore depends largely on the con¬ 
centration or “partial pressure” of carbon dioxide in the solution. 
As most surface waters can dissolve enough carbon dioxide from the 
air to form bicarbonate exclusively, it is generally assumed that prac¬ 
tically all the carbonate reported in analyses of surface water and 
shallow ground water represents bicarbonate in the solution. 
Carbonate and bicarbonate are subordinate in amount to sulphate 
in most of the surface water on the west side of the San Joaquin 
Valley, and in the normal ground water are generally present in 
minor amount. In the waters associated with the oil, however, 
they are generally present in larger amounts, and when chloride is 
absent carbonate and bicarbonate constitute the only acid radicles. 
As chloride increases, however, these weak acid radicles decrease, 
and in the strong brines they are present only in very small amounts. 
Carbonate and bicarbonate have been differentiated in 30 of the 
analyses in this report. In 19 of these waters there is no carbonate, 
and in the remaining 11 the average carbonate value is only one- 
fifth of the bicarbonate value. Three of these 11 waters may be 
disregarded, for the carbonate reported probably formed during the 
time the samples had stood before being analyzed. It is perhaps 
noteworthy that of the remaining 8, in which carbonate and bicar¬ 
bonate exist together, 3 are surface waters and shallow ground water, 
and 5 are deep waters, either from or below the oil measures. Appar¬ 
ently in the zone directly above the oil, in which the carbonate waters 
are believed to have formed there is a sufficient excess of carbon 
dioxide to prevent the formation of normal carbonate. 
Sulphide .—Sulphide occurs in small amounts in many of the waters 
above the oil zone, and the presence of hydrogen sulphide has long 
been recognized by drillers as indicating a top water. The manner 
in which the sulphide is held in solution is usually not determined, 
but it is highly probable that in primary alkaline waters some of the 
sulphide is held in equilibrium by alkalies and perhaps by alkaline 
earths. In some waters the acid sulphide (bisulphide) radicle (HS) 
is doubtless present. Both the normal and acid sulphide, however, 
tend to form the gas, hydrogen sulphide (H 2 S), which is easily recog¬ 
nized by its odor, and to which directly the term sulphur water 
is due. 
