12 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
are readily dissolved by the waters leaching through them. The 
waters of the west side, are therefore high in mineral matter and 
predominantly sulphate in character. Dole 1 has shown that an 
intermediate or axial zone may be distinguished near the center of 
the valley, where the waters are characterized principally by their 
high content of alkalies (sodium and potassium). 
The character of ground waters in San Joaquin Valley is, therefore, 
controlled primarily by geologic conditions, and it will be shown that 
in the oil fields local chemical influences have led to the development 
of still other types of water. Broadly speaking, the ground water of 
the valley enters it chiefly from the east side and flows slowly and 
against considerable resistance toward the north. It has been 
suggested that because of these conditions the meteoric water enter¬ 
ing from the Sierra side has produced sufficient hydrostatic head to 
force the oil to the west side of the valley and to trap it under the 
peculiar structural features locally existing there. In other words, a 
flow or pressure across the valley rather than in a longitudinal direc¬ 
tion is postulated. That underground waters have had much to do 
with the accumulation of the oil can not be doubted, and some 
support is lent this view by the fact that the water certainly encounters 
considerable resistance in its normal northerly course. The possi¬ 
bility of lateral pressure must be borne in mind in considering the 
accumulation of the oil, but it may be pointed out that the chemistry 
of the oil-field waters offers little support to this view; that if the 
theory is correct the anomalous position of the Kern River field on 
the east side of the valley must be explained; and that a theory 
involving the transmission of hydrostatic head through 20 to 40 
miles of lenticular materials, and with only a slight initial difference 
in head, involves grave difficulties. 
OCCURRENCE OF WATER IN THE OIL FIELDS. 
GENERAL DISTRIBUTION. 
In considering the ground-water system of the valley as a whole 
it has been tacitly assumed that the water exists as a continuous 
body saturating all the rocks to an indefinite depth. This conception 
is well founded and is valuable in a broad study, although the fact 
that the circulation of the water is confined largely to certain strata 
is of course of great practical importance. The question in which 
layers the water will circulate depends on a variety of factors, most 
of which are well understood. There is no intrinsic difference 
between a water sand and any other sand. 
1 Mendenhall, W. C., Dole, R. B., and Stabler, Herman, op. cit., p. 117. 
