84 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
I 
Field assay of water occurring between 1,030 and 2,609feet in Petroleum Development Cods 
deep well, sec. 4, T. 29 S., R. 28 E. 
Parts per million. 
Bicarbonate (HC0 3 ). 132 
Sulphate (S0 4 ). 5 
Chloride (Cl). 15,100 
Total hardness calculated as CaC0 3 . 2, 300 
Total solids. i . 25, 000 
Although definite groups of analyses showing the vertical gradation 
in other fields are not available, there is every reason to believe that 
the order of succession is the same. The total extent of the alteration 
may differ in different fields, and the point at which alteration is com¬ 
plete may be higher in some localities than in others, hut these are 
merely local variations which remain to be determined in each field. 
In some localities an incompletely altered water may he found below 
the oil measures, but such occurrences are doubtless due to unusual 
geologic conditions. Thus, analysis 30, already discussed, indicates 
an only partly altered water occurring 600 feet below the oil measures, 
but this water is in Cretaceous strata and has probably had a very 
different history from the altered water more closely associated with 
the oil (analysis No. 64). However, as the migration and accumula¬ 
tion of oil in the fields of the San Joaquin Valley has been greatly 
influenced by angular unconformities in the strata, it is conceivable 
that surface water might enter at the outcrop of a lower formation, 
barren of oil, and migrate through it without complete alteration to 
a point below the oil measures. In a region of such complex geologic 
structure some apparent irregularities in the alteration of the water 
are to be expected. 
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