14 
OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
Freedom of circulation .—Although, the grain of the rocks is an 
important factor in determining which beds the drainage follows, as 
the term “water sand” implies, an equally great factor in lenticular 
material is the freedom of inlet and outlet of the lens itself. It is 
evident that water will not circulate in a bed unless that entering at 
the outcrop can escape at the lower end of the lens, either into another 
lens or through some practicable channel. (See fig. 1.) If the upper 
end is open and the lower end sealed water will accumulate only up 
to the absorptive capacity of the sand, and if the hydrostatic head of 
Figure 1.—Diagram showing circulation or stagnation of the water in open and closed sand lenses 
before and after the lenses are penetrated by wells. 
the water thus trapped is not great enough to force an outlet move¬ 
ment will cease until the pocket is opened by the drill. The structure 
of the rocks may exercise an entirely similar effect in preventing 
circulation, so that water may be trapped in a syncline or a basin. 
If a lens is effectually sealed on all sides there can be no movement 
even when it is penetrated in drilling unless the rock pores are partly 
filled with air or gas under pressure, under which circumstances the 
expansion of the gases may displace some of the water. It is probable 
that most of the so-called water sands in the valley fields fall in one 
of these three classes, which are shown diagrammatically in figure 1. 
