64 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
MODIFIED GROUND WATER. 
Table 5 contains analyses of water, modified presumably by the 
action of the hydrocarbons, from the Coalinga, Midway, and Sunset 
fields. As already explained, the modified group is arbitrarily 
assumed to include only those waters that are primary alkaline in 
character and that contain a moderate to high proportion of sulphate. 
In all the analyses included in Table 5 the ratio of carbonate to sul¬ 
phate is greater than 1, and in several it is much higher. Most of 
these waters are characterized by a high percentage of primary 
alkalinity, but this figure is largely governed by the concentration 
of the chloride. 
Analysis 27 represents the type of water ttiat occurs in the Eastside 
Coalinga field some distance above the oil. It will be noted that the 
primary salinity is very low and primary alkalinity very high. The 
sulphate salinity ratio is fairly high, but this is due chiefly to the very 
small proportion of chloride present. When chloride is as low as it 
is in most of the waters from this district the relative proportions of 
sulphate and chloride are of less value than the comparison of sul¬ 
phate and carbonate. Analysis 28 shows a somewhat saltier water 
from the north end of the Coalinga syncline. The primary alkalinity 
is fairly high though the water still contains a large amount of sul¬ 
phate. No. 29 is a good analysis of the distinctly modified water 
that occurs just above the tar sand zone in the Westside Coalinga 
field. Despite the high sulphate salinity ratio and the low carbonate- 
sulphate ratio the fact that some of the sulphate has already been 
reduced is indicated by the presence of a considerable amount of 
sulphide, part of which is combined as hydrogen sulphide. Analysis 
30 represents the water used at the Coalinga Sulphur Baths, which 
is interesting because, although distinctly a modified type, it occurs 
600 feet below the oil. This water, however, is probably obtained 
from the Chico strata (Upper Cretaceous*) and is very different from 
that obtained in or just below the oil zone (upper Miocene), which 
is represented by analysis 64 (Table 10, p. 76). 
Analysis 31 probably represents a mixture of waters above and 
below the oil from a well on the extreme western edge of the Midway 
field. The sulphate is high in proportion to both chloride and 
carbonate, indicating a considerable admixture of upper water. It 
is also possible, because of the proximity of the outcrop of the sands 
and the fact that large quantities of water are being removed from 
them, that sulphate water is entering at the outcrop of the lower water 
sand faster than it is being altered. Analysis 32 represents an 
unusual type of water from Buena Vista Valley. Although occurring 
1 Arnold, Ralph, and Anderson, Robert, Geology and oil resources of the Coalinga district, Cal.: U. S. 
Geol. Survey Bull. 398, p. 223, 1910. 
