COMPOSITION OF THE OIL-FIELD WATERS. 
67 
ALTERED GROUND WATER. 
Reversed type .—When a pure sulphate water is altered by the 
action of hydrocarbons it apparently becomes a pure carbonate 
water, a reversal from complete salinity to complete alkalinity 
taking place. However, there are few waters anywhere that do not 
contain some chloride, and in oil fields, where the underground cir¬ 
culation is normally restricted, a large concentration of chloride is 
generally found. It is probable therefore that oil-field waters of the 
reversed type, in which chloride is practically lacking, are rare, and 
so far as the writer knows the only fields in which they constitute 
the rule rather than the exception are the Eastside Coalinga and possi¬ 
bly the Kern River fields. (See p. 85.) Tables 6 and 7 show analyses 
of this type of water. The type is characterized by low primary 
salinity and high primary alkalinity, by a moderately low con¬ 
centration, and by a high ratio of carbonate to sulphate. Owing to 
the small amount of chloride present the sulphate salinity ratio 
(per cent of rS0 4 in rS0 4 -t-rCl) is much higher than in most altered 
waters, though generally below 10. per cent. In some waters, how¬ 
ever, the actual amount of sulphate seems to be abnormally large 
for a water associated with the oil, and it is possible that in some 
localities sulphate water is entering at the outcrop and percolating 
down faster than it is being altered. 
All the analyses in Table 6 and four of those in Table 7 represent 
water from the Coalinga field. No. 34 is a sulphur water which 
occurs a short distance above the oil measures at the upper end of 
the Coalinga syncline. Circulation is doubtless restricted in this 
locality and a considerable proportion of chloride has been retained. 
The primary salinity is 35 per cent and the primary alkalinity only 
52 per cent, and the water is therefore a mixed type, approaching 
that shown in Table 10, but characterized by a lower concentration. 
The presence of hydrogen sulphide together with a small amount of 
sulphate indicates that the alteration of this water is not entirely 
complete. Analyses 37, 38, and 39 represent waters from the 
deep territory on the east side of the Coalinga anticline near its 
southern end. These waters resemble 34 in that a considerable 
proportion of chloride is present, indicating an admixture of connate 
water. The properties of these waters are very similar, and all of 
them are characterized by a high proportion of carbonate as compared 
with either chloride or sulphate. However, a rather large amount 
of sulphate is present, especially in No. 39, which may be the normal 
condition or which may be due to an admixture of upper water in 
the samples analyzed. It may also indicate that these waters occur 
above the oil measures instead of in or below them, as generally 
believed. Analysis 42, which is very similar to these, represents 
