112 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
when the evidence of the water is adverse it can be taken with more 
assurance than when it seems favorable. 
The remarks in the preceding paragraph concerning the use of 
water analyses in prospecting are intended to apply only to the San 
Joaquin Valley region. If the writer’s conception of the principles 
involved is correct, then, as oil is approached, similar variations in 
the character of the water should occur also in other regions. So 
far as is known, however, these variations have never been studied 
elsewhere, and in advance of definite knowledge it must be recognized 
that an extension of this method to other regions is based wholly on 
theory. 
In using water analyses to predict conditions in unexplored dis¬ 
tricts a complication arises from the fact that the rate of change in 
the composition of the waters and the distance above the oil measures 
at which sulphate disappears are different in different areas. The 
chemical character of the oil itself may be involved in these differ¬ 
ences, but observations in the San Joaquin Valley fields indicate 
that stratigraphic and structural conditions are of prime importance. 
If the bedding is sufficiently irregular to permit transverse migration 
of either the water or the hydrocarbons the water may be con¬ 
siderably altered some distance above the main body of oil. If the 
course of the water is influenced by fault planes or unconformities 
the variations in its character may be irregular. Structural con¬ 
ditions influencing the freedom of circulation are also very important, 
for they determine in a measure the quantity of sulphate that is 
brought down to be acted upon by the oil. In other words, a given 
quantity of oil may be able to reduce the sulphate in a large body of 
stagnant water, but if the water is in circulation and fresh supplies of 
sulphate are constantly penetrating to the oil measures the reduction 
may not be complete. Hence, if structural conditions in the area 
in question are such as to prevent circulation, a high concentration 
of chloride is to be expected; sulphate will probably be present in 
rather small amount even in the shallower waters and may be entirely 
absent in the waters several hundred feet above the oil zone. If, on 
the other hand, the circulation is relatively free and the outcrop of 
the beds not far distant, chloride will probably be low and a consid¬ 
erable concentration of sulphate may be present in the waters close 
above the oil measures. 
In comparing the analyses of two different waters all the constitu¬ 
ents must be taken into account; their relative values as criteria 
for comparison have been indicated elsewhere. General comparisons 
may be made of analyses reported in hypothetic combinations, but 
much more satisfactory results are obtained if the analyses are recal¬ 
culated and the comparison made on the basis of the reacting values 
of the radicles themselves. The practice of attempting to estimate 
