OCCURRENCE OF WATER IN OIL FIELDS. 
17 
Where the oil occurs in several more widely separated sands water is 
usually encountered between them. 
In addition to the sands that are wholly filled with either oil or 
water there are sands that contain both, the oil occupying the up- 
slope and the water the down-slope portion. The topmost sand in 
the oil zone in the Westside Coalinga field, for example, contains 
water down the dip, and analogous conditions exist in several lo¬ 
calities in the Midway and Sunset fields. It is generally recognized 
that if any persistent oil sand in any of the fields could be followed 
far enough down the dip the oil would finally be found to be re¬ 
placed by water. 
The terms locally applied to waters in the three positions described 
arc respectively “top water,” “bottom water,” and “edge water.” 
It is natural for the driller, who is interested jirimarily in the position 
of the oil itself, to classify waters according to their position with 
regard to the oil, but his terminology is often ambiguous. The bot¬ 
tom water of an upper oil sand may be the top water of a lower, and 
edge water may be either top water or bottom water or both. These 
terms are perhaps too commonly used and too convenient to discard, 
but the term top water should be restricted to water above the highest 
oil sand, and the other terms should be used only when their meaning 
is perfectly clear. The terminology suggested by the writer (see p. 51), 
which is also based ultimately on the position of the water in relation 
to the oil, is believed to offer a basis for more rational distinctions. 
PHYSICAL CHARACTERISTICS. 
HEAD OR PRESSURE. 
In several fields of the San Joaquin Valley the water in many sands 
is under great pressure, and it has been necessary to devise special 
methods of shutting off this water in order to prevent its enter¬ 
ing the oil sands. In the McKittrick field, for example, several 
wells near McKittrick encountered water which is said to have 
flowed for a while over the derrick, and in the Midway-Sunset field 
many flowing waters have been struck at different depths, the 
greatest more than 3,500 feet. As a general rule, to which there 
are many exceptions, the pressure increases with the depth, and the 
waters below the oil zone are generally confined under considerable 
pressure. The head in most of the sands reported as water sands by 
drillers is more than 200 feet, and it has already been pointed out that 
many sands reported as dry probably carry water under a head of 
more than 50 feet. 
The pressure of the water in most sands, however, is not constant, 
but diminishes noticeably a short time after the sand is tapped. 
Waters that flow for months or years finally cease to flow, and the 
60439°—Bull. 653—17-2 
