110 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
waters are found several hundred feet above the oil zone. In these 
fields, moreover, waters containing only small amounts of sulphate 
are found comparatively near the surface, and the change in the 
character of the water with depth is therefore less marked. It may 
be inferred by analogy that similar conditions exist in the Lost Hills 
fields and in the other structures of this type which are believed to 
contain oil. In the Eastside Coalinga field very different conditions 
prevail, and all gradations from a pure sulphate water to a pure car¬ 
bonate water may be observed. At the same time the waters near 
the oil measures in this field generally contain some sulphate, and 
may in fact contain more than the waters 500 feet above the oil in 
the Midway and Sunset fields. These local variations do not affect 
the broad principles involved, but they are evidently of great prac¬ 
tical importance, and only general conclusions can be drawn from an 
analysis until these variations have been worked out in the area in 
which the water occurs. 
In order to work out the detailed changes between successive 
water sands in each area it is necessary that as many reliable analyses 
as possible be placed on record. The analyses given in this report 
are sufficient to show the general type of the chemical changes that 
take place, but they are not intended to constitute a description of 
local conditions. It is believed by the writer that within small areas 
the chemical character of water from the same horizon is sufficiently 
constant to justify correlations on the basis of the analyses, and 
hence that when a sufficient number of analyses of reliable samples 
have been collected and made available for comparison the horizon 
of an unknown water may be closely estimated. Thus, one of the 
larger companies in the Midway field is making a practice of collecting 
water samples while drilling the well and preserving the samples, or 
analyses of them, for future reference. If the well later “goes to 
water ” a sample of this water is analyzed and the analysis is com¬ 
pared with those of the waters encountered while drilling. A marked 
difference in composition between the waters first above the oil 
measures and the water flooding the well is taken to indicate that the 
water flooding the well is from strata below the oil nr that it has in¬ 
vaded the oil sand at some other point and has migrated to the well. 
This use of water analyses is a matter of simple comparison and de¬ 
pends only remotely on the principles involved in the variation in the 
water as it approaches the oil zone. In the Midway-Sunset field, 
where variations in the character of the water are not as marked as in 
other areas and where a slight difference in sulphate content may 
indicate a difference in position of several hundred feet, it is probable 
that this method will be essential in the accumulation of an adequate 
amount of data regarding the character of the water in the 500 feet 
or so on either side of the oil measures. In the Westside Coalinga 
