russeli..] PETROLEUM. 75 
nature. The locality a may be much higher than the surface at b, in 
which case the gas first escaping would be followed by a How of petrol- 
eum, and when that failed, water would follow and form an artesian 
well. Under the conditions shown in fig. 1, wells put down between 
c and a, with the hope of finding either petroleum or gas, must evidently 
result in failure. 
Another source of pressure which may be brought to bear upon 
fluids and gases in a porous rock has been suggested, namely, the 
weight of the overlying rocks, but this hypothesis may evidently be 
discarded unless, after the rocks have been charged with water, petrol- 
eum, or gas, the weight of the superimposed strata causes compres- 
sion, which diminishes the volume of the interstices in the porous bed. 
Such a change might evidently take place, but in general must be of 
such small amount in comparison with other causes of pressure on the 
contained fluids and gases that it need not be considered. 
Geologists have found that it is not a scarcity of petroleum or gas 
that makes the occurrence of commercial quantities of the substance 
so valuable, but their concentration into what are termed "pools" or 
natural reservoirs. In this concentration the position in which the 
Fig. 2.— An ideal geological section illustrating the manner in which petroleum and natural 
gas may be concentrated in horizontal beds. 
layers of rock in the earth's crust occur is in the highest degree 
important. 
Stratified rocks, when first formed, were in nearly all cases essenti- 
ally horizontal, but, owing to subsequent movements in the earth's 
crust, are now in many instances variously inclined, folded, and broken. 
In strictly horizontal beds it is evident that the conditions for concen- 
trating the petroleum and gas they may contain are unfavorable. 
This, however, is not equivalent to saying that horizontally bedded 
rocks are of necessity destitute of oil and gas pools. For example, 
as in the section shown on fig. 2, the upper surface of a porous bed 
inclosed between two sheets of shale may be irregular, and the eleva- 
tions in its upper surface might supply conditions similar to those 
furnished by an upward fold. In the diagram, oil and gas might 
accumulate at a and b under gas pressure and be of sufficient volume 
to yield valuable returns. Again, in horizontal, impervious beds 
there may occur irregular, porous, masses of rock — as sandy patches 
in shale or porous regions in limestone, for example — which would 
serve as reservoirs, and on becoming charged with petroleum and gas 
yield one or the other of these substances, according to the shape of 
the pool and the place where it chanced to be penetrated in boring. 
An illustration of such a possibility is shown to the right in fig. 2. 
