CLASSIFICATION OF THE OIL-FIELD WATERS. 49 
I 
It is often difficult, owing to the wide variations in the chemical 
character of natural waters, to distinguish without careful study 
the significant differences from the more or less fortuitous, or more 
properly, the differences that are due to determinable and fixed 
conditions from those which result from changing or indeterminable 
conditions. The carbonate-sulphate ratio is a significant criterion, 
for it is apparently governed chiefly by the position of the water 
with regard to the oil; but the ratio of sodium to potassium, for 
example, is controlled by factors that are indeterminable or not 
well understood. These ratios are interesting and are valuable for 
some purposes, but they involve too much speculation to be of basic 
importance in a study of oil-field waters. Unfortunately, several 
investigators who have already undertaken the study of the Cali¬ 
fornia oil-field waters have deemed it unnecessary to make complete 
analyses, and have based their estimates of the position of a water 
upon such isolated criteria as the concentration of chloride, the 
ratio of magnesium to calcium, or the presence or absence of such rare 
constituents as lithium or iodide. Some of the conclusions based on 
these incomplete studies have proved erroneous, and the writer en¬ 
countered considerable skepticism among the oil operators as to the 
practical value of chemical studies of the oil-field waters. The more 
complete information now available furnishes a better basis for con¬ 
clusions, both as to the influence of hydrocarbon material on the 
composition of a water and also as to the genetic relations of the 
waters themselves. 
PROPOSED CLASSIFICATION. 
Since all natural waters are mixtures, it is impossible to classify 
them rigorously, and any system of classification is a matter of con¬ 
venience rather than of fixed principles. For practical convenience 
it is evident that oil-field waters should be classified as far as possible 
according to their position in relation to the oil. As waters near the 
oil measures differ in composition from those nearer the surface, a 
classification based directly on the chemical character of the waters 
and indirectly on their position in relation to the oil may be made. 
The sulphate-carbonate ratio furnishes a convenient basis for classi¬ 
fying waters that contain only minor amounts of chloride, but in 
most localities chloride must be taken into account. Thus, in the 
Midway field chloride may be far more prominent than sulphate in the 
shallow waters, and in the deeper waters may be present almost to 
the exclusion of carbonate. Under these circumstances the replace¬ 
ment of sulphate by carbonate is less striking. 
The relation of the various types becomes clearer when their origin 
is considered. The sulphate water, being the normal ground water 
60439°—Bull. 653—17-4 
