82 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
Figure 5, B, shows these two analyses plotted according to the 
scheme followed in figure 5, A, except that the vertical scale used is 
only about half as great. Even on a smaller scale, however, the con¬ 
trast between the two figures is striking. The percentage values of 
sulphate and carbonate, instead of crossing, run practically parallel. 
The alkaline earths decrease greatly and the alkalies therefore increase. 
The percentage of chloride is about the same in the two waters. The 
concentration figure increases greatly, however, so that all these con¬ 
stituents except the alkaline earths increase in actual amount. 
Analyses of water from other deep wells outside the oil fields on the 
west side of the San Joaquin Valley indicate that these changes are 
representive and that the amount of sulphate does not decrease and 
generally increases with depth. 1 In general, the concentration in¬ 
creases and in many localities chloride shows a marked increase with 
depth, but this does not affect the relative proportions of sulphate 
and carbonate. 
The analyses given above show that in the Coalinga field the altera¬ 
tion of the waters as the oil zone is approached is extensive, but in 
the Midway field the changes are less striking. Analyses 81 and 82, 
Table 13, represent waters from the same well at depths of 1,380 
and 1,947 feet, respectively. A gas sand occurs at about 1,810 feet 
and an oil sand a short distance below the lower water, so that the 
section represented is roughly comparable to that shown in the lower 
part of figure 5, A. These waters, however, are of partly connate 
origin and the proportion of sulphate even in the upper water is small- 
The properties of reaction show only a slight change, primary 
alkalinity increasing from 28.6 to 31.8 per cent and the sulphate 
salinity ratio decreasing from 2.3 to 0.2 per cent. The carbonate- 
sulphate ratio shows the most pronounced change, from 20 in the 
upper water to 165 in the lower. There is also a marked difference 
in concentration, that of the lower water being over twice that of the 
upper. It is evident that these waters differ in composition along 
the same fines followed by the Coalinga waters but that the total 
changes are much smaller. In the Midway field water 600 feet apart 
may show only a slight difference in sulphate content, whereas in the 
Coalinga field waters in the same relative positions usually differ 
widely. 
1 Mendenhall, W. C., Dole, R. B., and Stabler, Herman, Ground water in San Joaquin Valley, Cal.: 
U.S. Geol. Survey Water-Supply Paper 398, 1916. 
