48 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
samples of oil-field water as submitted for analysis contain a small 
amount of oil floating on the surface, and as this oil is also included 
in the determination it is evident that the figure has no exact value. 
It has therefore been omitted from the tables of analyses published 
in this report. 
CRITERIA FOR COMPARISON. 
Natural waters differ widely in the chemical character of the sub¬ 
stances dissolved in them, and several different types of water may be 
found in nearly every region. However, the different varieties tend 
to mingle and react through constant circulation, until the pure 
types may be less common than the gradations between them. 
Moreover, if the rocks differ irregularly in composition, the waters, 
whose character reflects that of the rocks with which they come in 
contact, must bo similarly different. In many areas, therefore, the 
natural waters present such infinite variation in chemical composi¬ 
tion that even though it is recognized that a certain type is normally 
characteristic of a certain locality or formation it is impossible to 
draw a definite line of demarcation. 
In the oil fields of San Joaquin Valley, however, special condi¬ 
tions have led to the formation of very distinct types of water. As 
the transitions from one type to another occur within relatively 
short distances fairly definite zones or localities that are characterized 
by the same type of water can be established. The development of 
these distinct types has been due partly to the influence of the oil 
and gas, and the oil-field waters therefore present contrasts that are 
lacking in the waters of most areas. 
The three groups of salts most commonly dissolved in natural 
waters are chlorides, sulphates, and carbonates, and in the oil fields 
there are waters which contain any one of the three almost to the 
exclusion of the others. The concentration of chloride has already 
been shown to be governed by physical conditions, such as freedom 
of circulation; and since these conditions are more or less local and 
irregular the content of chloride furnishes the least satisfactory basis 
for the comparison of waters. The relative proportion of sulphate 
and carbonate, on the other hand, is influenced by fairly definite 
chemical reactions that take place only near accumulations of oil and 
gas; hence the carbonate-sulphate ratio may serve as an indication 
of the position of the water with respect to hydrocarbons. Study of 
the carbonate-sulphate ratio in waters in a developed field, in which 
the relations of oil, gas, and water have been discovered, forms an 
excellent criterion for differentiation and a tangible basis for § system 
of classification; in an undeveloped field, on the other hand, it may be 
used as an indication of the presence or absence of hydrocarbons 
near by. 
