88 OIL-FIELD WATEKS IN SAN JOAQUIN VALLEY, CAL. 
Relations of the mixed type .—Besides causing a simple variation in 
the chloride content the mingling of connate and meteoric waters 
leads to a series of reactions between the other constituents and pro¬ 
duces corresponding changes in the properties of the waters. These 
changes may he observed in the waters of any one zone along a line 
leading away from the outcrop, but may best be studied in the waters 
of the altered group, whose complete alteration by the oil affords a 
uniform basis for comparison. The fact that the altered waters along 
the western or shallow edge of the Midway and Sunset fields are of 
the chloride-carbonate type, whereas those farther east are secondary 
saline brines, is due to the entrance of meteoric water at the outcrop, 
S W. Distance between wells In feet NE. 
Figure 6. —Diagram showing increase in salinity of waters in the northern part of the Midway field with 
distance from the outcrop. 
and the gradations that may be traced between this mixed type and 
the brines afford an interesting study of the mixing of meteoric and 
connate waters. The relations and derivation of the mixed type 
may more easily be understood if some of the reactions involved are 
first considered. 
The reactions involved in the conversion of a sulphate water into a 
carbonate water, which forms the basis of the vertical gradation 
already described, are discussed on pages 94-99. Let it be assumed 
for the present that the process is a simple substitution of carbonate 
for sulphate. In unaltered sea water sulphate and chloride exceed 
the alkalies in value and are therefore partly in equilibrium with the 
alkaline earths; hence, if carbonate is substituted for the sulphate the 
alkaline earths will be partly in equilibrium with carbonate. Only a 
