OCCURRENCE OF WATER IN OIL FIELDS. 
23 
tends to change in character it can not be expected that fossil sea 
water will long retain exactly its original composition. Water 
entrapped in the older rocks, like the Paleozoic, may have changed so 
materially that its origin, so far as chemical evidence goes, may be in 
doubt, but some of the salt water in the rocks of San Joaquin Valley is 
so similar to sea water that there can be little doubt that it is fossil sea 
water. (See Table 8, p. 73.) This view is strengthened by the fact 
that the differences in composition between it and normal sea water 
may be adequately explained by fairly definite chemical reactions and 
is corroborated by the fact that geologic conditions in the areas in 
which it occurs are such as to prevent free circulation, which explains 
why it has not been leached out. Some geologists hold that connate 
water can not be retained long in rocks that have been elevated above 
sea level and that salt ground waters are due to the leaching of salt 
deposits. However, waters that have leached salt deposits would 
ordinarily differ in composition from ocean water in several respects 
and would probably be unlike any waters in the San Joaquin Valley 
oil fields that are known to the writer. Moreover, as geologic condi¬ 
tions in that region seem to be locally favorable to the retention of 
connate water, the view ascribing salt waters to the leaching of hypo¬ 
thetic saline deposits nee^d not be considered further. 
\ 
RELATION OF GEOLOGIC STRUCTURE TO DISTRIBUTION OF SALT WATER 
IN THE OIL FIELDS. 
Though much of the deeper water in the oil fields of San Joaquin 
Valley is very salty, in some localities it is practically fresh. This dif¬ 
ference is believed to be due largely to difference in structural condi¬ 
tions. All the fields of the west side are characterized by similar 
structure, being associated with the minor anticlines that border the 
edge of Temblor and Diablo ranges. The anticlines and the synclines 
behind them in all these fields have a general southward plunge. In 
considering hydrologic conditions each field may be regarded as occu¬ 
pying one or more of the following positions: The eastern flank of the 
anticline near its north end, where erosion has uncovered and trun¬ 
cated the oil-bearing strata; the summit and flanks of the anticline, 
where the oil-bearing rocks are deeply buried and do not outcrop; 
the trough between the anticline and the main slope of the valley; 
and the western limb of this trough near the outcrop of the oil-bearing 
beds. If San Joaquin Valley itself be thought of as a great open 
trough with these structural features perched high on its western rim 
it becomes evident that the synclines constitute traps in which the 
ground water may be effectually ponded. In the first position men¬ 
tioned, however, drainage down into the main valley should be 
relatively free, and in the second and fourth positions the freedom of 
drainage would depend on the completeness with which the waters are 
