86 
OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
AREAL RELATIONS. 
The oil-field waters vary horizontally as well as vertically, but the 
horizontal or areal variation is due simply to the mingling in various 
proportions of meteoric and connate waters. No matter how effect¬ 
ually connate water may be trapped in a structural trough, it is evi¬ 
dent that near the surface of the ground and also around the edges 
of the trough where the rocks outcrop there will be a certain admix¬ 
ture of meteoric water. Although a few very salty waters have been 
found close to the surface (see No. 87, Table 14, p. 85) they are gener¬ 
ally confined to lower levels, and a marked increase of alkalies and 
chlorides with depth appears to be the rule. This phase of the mixing 
of fresh and salt waters has been sufficiently discussed in the preceding 
section, and the gradation from the outcrop toward the center of the 
trough will now be considered. Two phases of this gradation present 
themselves, first, the increase in chloride, which is a factor of simple 
mixing, and second, the change in the properties of the water, which 
is the net result of various chemical reactions. Since the chloride is 
not affected by hydrocarbons it furnishes a simple and convenient 
basis for comparing normal and modified as well as altered waters. 
The properties of reaction, however, furnish a more adequate basis 
for comparison and in studying waters of the altered group are espe¬ 
cially valuable. 
The examples given below are drawn from the Midway and Sunset 
fields, where it happens that the most complete data are available, but 
similar gradations have been observed in the Coalinga field and can 
doubtless be worked out elsewhere. 
Variation in chloride .—Normal connate water contains about 19,000 
parts per million of the chloride radicle and 16,000 parts of other con¬ 
stituents; ordinary surface water contains very little chloride and 
generally less than 3,000 parts of dissolved solids. The figures repre¬ 
sen ting chloride and total solids show the greatest contrast and are 
therefore convenient indices for comparing mixtures of the two types. 
The carbonate figure also shows considerable variation, being very 
low in brine and generally higher in meteoric water. 
In the accompanying table are shown a series of partial analyses 
of water from wells near Fellows in the northern part of the Midway 
field. The wells fall roughly along a line about coincident in direction 
with the dip of the strata. In the table they are arranged in order of 
their distance from the outcrop—American Oilfield Co.’s well 92 being 
about 1,600 feet from the outcrop and Associated Oil Co.’s well 3 
about 10,000 feet beyond well 92. The analytical figures given for 
the water from the first four wells represent partial analyses made by 
the American Oilfields Co.; the figures given for the water from the 
last two wells are taken from complete analyses made by Smith, 
