24 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
ponded in the trough. It is perhaps noteworthy that the accumula¬ 
tion of oil in the first position seems to be unusual and is illustrated 
only in the northern part of the Eastside Coalinga field. 
These ideas seem to be supported by the character of the water 
in the four structural provinces distinguished. So far as data are 
available, all the synclines are characterized by more or less salty 
water, and the proportion of chloride at any point in the trough is 
apparently governed by its depth and distance from the outcrop. 
, The water near the axis of the Midway syncline, for example, is very 
salty, whereas, nearer the outcrop to the west, where fresh water 
may enter the beds, it is much less so, and every gradation between 
the two extremes may apparently be’ found. (See figs. 6 and 7 and 
discussion, pp. 86-91.) On the other hand, in the northern part of the 
Eastside Coalinga field, which is on the eastern slope of the southward- 
pitching anticline, the waters contain only a small proportion of 
chloride, many of them far less than is perceptible to the taste. (See 
analyses 35 and 36, Table 6, p. 69.) The pitch of the anticline has 
caused the oil-bearing strata in this locality to outcrop a short dis¬ 
tance west of the field, and as there is apparently no structural bar¬ 
rier to the east to prevent circulation down into the main valley, 
practically all the connate water has been driven from the beds. 
The removal of chloride has been even more complete north of the 
limits of the productive field, where the structure becomes truly 
monoclinal and the dip somewhat steeper. Chloride is much more 
abundant toward the southern end of the field, however, or down the 
pitch of the fold where the strata are deeply buried. 
A similar contrast is apparent if the Kern River field on the east 
side of the San Joaquin Valley is compared with the Midway field. 
The structure in the Kern River field is very much more gentle than 
that typical of the fields of the west side, and apparently no struc¬ 
tural barrier exists to pond the water. Furthermore, owing to the 
much greater abundance of water on the east side of the valley, the 
chlorides have been in large part removed, and such other salts as 
are derived from the rocks usually form only a dilute solution. The 
writer knows of only two waters whose analyses show a concentra¬ 
tion greater than 2,000 parts per million, and most of the waters 
contain less than 1,000 parts of dissolved mineral matter. (See p. 85.) 
The period of subsidence during which the salt water now present 
in the oil fields became entrapped in the rocks can not be determined. 
The Cretaceous and older Tertiary deposits were laid down beneath 
the sea and were once saturated with salt water, and some of the 
later Tertiary formations are also to a large extent of marine origin. 
However, much of the water originally entrapped in the Cretaceous 
sediments was probably leached out during later periods of eleva¬ 
tion, and some of the salt water now found in theso rocks doubtless 
