52 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
coincident with the “top water,” but in the Midway-Sunset fields sev¬ 
eral hundred feet of strata containing altered water intervene be¬ 
tween the base of the modified zone and the top of the oil measures. 
All the “bottom water” or “edge water” examined is of the altered 
type, and in general this type extends for several thousand feet 
below the oil measures. 
As most of these waters are mixtures of meteoric and connate 
water their degree of alteration as the oil is approached is different. 
Normal connate water would contain by reacting value about 9.2 
per cent of sulphate, so that a water of this class containing only 3 
per cent of sulphate might reasonably be referred to the modified 
group; a normal meteoric water, on the other hand, may contain 45 
per cent of sulphate, hence a water of meteoric origin containing 
only 15 per cent would naturally fall in the modified group also. 
Furthermore, the composition of normal ground water is different 
in different areas, and therefore the modified and altered types 
derived from it will differ in composition. 
Any limits assigned these groups evidently must be arbitrary and 
of only local value, but for the waters in the oil fields on the west 
side of San Joaquin Valley the following limits are suggested:. 
1. Normal group: 
Secondary salinity present, sulphate high; ratio of carbonate to sul¬ 
phate generally less than 1; alkaline earths may be prominent. 
2. Modified group: 
Chiefly of meteoric origin. Primary alkalinity present; sulphate 
salinity greater than 15 per cent of the total salinity; ratio of car¬ 
bonate to sulphate generally between 1 and 15; alkaline earths gen¬ 
erally subordinate to alkalies. 
Chiefly of connate origin. Secondary salinity present; sulphate salinity 
greater than 1 per cent of the total salinity; alkaline earths generally 
less than 10 per cent of total reacting value. 
3. Altered group: 
Reversed type. Primary alkalinity greater than 50 per cent; sulphate 
salinity less than 15 per cent of the total salinity; ratio of carbonate 
to sulphate generally greater than 15; alkaline earths generally less 
than 8 per cent of total reacting value. 
Brine. Secondary salinity present; sulphate salinity less than 0.5 per 
cent of total salinity; alkaline earths generally between 2 and 10 
per cent. 
Mixed type. Primary alkalinity less than 50 per cent; sulphate salinity 
less than 1 per cent of total salinity; alkaline earths generally about 
1 or 2 per cent. 
These limits have been observed as far as possible in grouping the 
analyses. The groups are not mutually exclusive, however, and 
when a water falls in two groups its classification has been decided 
by comparison with the waters above and below it. 
