VALUE OF WATER ANALYSES TO OIL OPERATOR. Ill 
field, however, fairly accurate estimates of the horizon of the waters 
can often be made on the basis of much less complete information. 
A factor which might militate against the success of this method 
in some regions is the possibility that a flow of water may change in 
character during lapse of time. When large quantities of water are 
removed by one well from a sand, water which may be different in 
composition is drawn toward the well from distant parts of the sand. 
The variation in character is most noticeable when the water with¬ 
drawn from the sand is salty and is being replaced by fresh water, 
which enters at the outcrop. Replacement by fresher water is of 
course most likely to take place in shallow sands, and in general 
little or no change in character takes place in flows from deep-lying 
sands. 1 This condition should hold especially in the oil fields, where 
usually no effort is made to produce water from the deeper sands 
and where their normal contents are therefore disturbed as little as 
possible. Changes in the concentration of the water may be ob¬ 
served, but the relative proportions of the several constituents prob¬ 
ably remain about constant, unless a considerable circulation is 
set up. 
The variations in the chemical character of the waters as oil is 
approached may also be used as a guide in wildcatting. In many 
prospect holes this evidence will probably be put to little practical 
use, for marked changes in the character of the water generally do 
not occur more than a few hundred feet above the first showings of 
tar or gas. Contingencies may readily arise, however, in which 
corroborative evidence of the probable presence of hydrocarbons 
below will be of value. For example, if a well reaches a depth of 
4,000 feet, and there encounters water very similar in character to 
that near the surface, and if at this point serious drilling difficulties 
involving great extra expense are encountered, the advisability of 
proceeding farther may be questioned. On the other hand, if the 
water at this depth contains no sulphate the prospect of finding oil 
a short distance below is decidedly more encouraging. It should be 
pointed out, however, that commercial quantities of oil or gas may 
not be necessary to produce an altered water. The writer has a 
number of analyses of water from prospect wells of the Standard 
Oil Co. in southern California, and although the water in all the 
wells which produced commercial quantities of oil is sulphate-free, 
that in several wells which gave only shows of oil also contains only 
small amounts of sulphate. However, in other unsuccessful wells 
the deep water, so far as the tests have been applied, contains more 
sulphate than the water near the surface. As far as commercial 
quantities of the hydrocarbons are concerned it seems probable that 
1 Sanford, Samuel, Saline artesian waters of the Atlantic Coastal Plain: U. S. Geol. Survey V ater-Supply 
Paper 258, p. 85,1910. 
