OCCURRENCE OF WATER IN OIL FIELDS. 
13 
WATER SANDS. 
Texture .—Water flows most readily through rocks of uniformly 
coarse grain, and hence tends to circulate chiefly in the sandy layers. 1 
The amount and pressure of the water in a sand depends largely on 
local conditions, but if these were uniform water would normally be 
found in greater available quantity and under higher head in coarse 
sand than in tight sand or sandy shale. In other words, the character 
of the rock has much to do with its content of available water, 
which in a thick sandy stratum is usually concentrated in one or 
more “pay streaks” just as oil is concentrated in certain parts of an 
oil sand. 
The rapidity of circulation of the water through a stratum is not, 
however, an index of the amount of water that the rock contains. 
Coarse open sand, through which flow is most rapid, has an average 
pore space or absorptive capacity of 32 to 37 per cent; unsorted or 
tighter sand, of 38 to 42 per cent; and clay, of 44 to 47 per cent. 2 
The important consideration for practical purposes is, however, that 
the sand yields its water readily because mor£ fluid can easily take 
its place, whereas movement is slower in the clay and much of the 
water is probably held by capillary attraction. A sandy shale or a 
tight sand may also contain considerable water which will not be 
apparent in drilling because it escapes too slowly from the bed. If 
all the strata penetrated by an ordinary oil well could be carefully 
examined, those below the ground-water level probably would be 
found to contain more or less water, although the water content of 
certain strata might be far less than the absorptive capacity of the 
rock. Most of the beds, however, are too fine grained to permit 
perceptible movement of the water in them, and consequently most 
of the drainage passes through a few layers. 
It is thought by some drillers that water sands have a peculiar 
texture by which they may be distinguished from oil sands. It is 
variously held that the grains are sharper or more angular, or that 
they are smaller, or that water sand contains grains of mica, whereas 
oil sand does not. Any of these distinctions may be valuable locally, 
and some sands may be traced for a considerable distance by their 
texture and mineral character, but to assume that water sands and 
oil sands are essentially different in grain or mineral composition is to 
overlook entirely the principles governing the movement of water 
and oil. 
1 Slichter, C. S., The motions of underground waters:,U. S. Geol. Survey Water-Supply Paper 67, 1902; 
Field measurements of the rate of movement of underground water: U. S. Geol. Survey Water-Supply 
Paper 140, 1905. 
2 King, F. H., Principles and conditions of the movements of ground water: U. S. Geol. Survey Nine¬ 
teenth Ann. Rept., pt. 2, pp. 209-215, 1899. 
