74 
GEOLOGY OF SW. IDAHO AND SE. OKEGON, 
[BT7LL.217. 
represents a section of the earth's crust, say, 20 miles long and 1,000 
feet deep, where the rocks have been gently folded. In the section" 
two bodies of shale are shown, with an included layer of porous sand- 
stone. If the sandstone is charged with petroleum and gas generated 
in the shales beneath, or possibly in its own mass, it is evident that 
these light substances will in part be flooded out by water entering 
where the layer comes to the surface in the eroded fold at the right. 
Under such conditions indications of petroleum and gas should occur 
at a, of the nature of springs of these substances or as a general 
exudation of heavy oil, since the freedom of escape would facilitate 
evaporation of the petroleum and its consequent concentration. The 
petroleum in the porous layer beneath the unbroken fold b would be 
above the water and subjected to a pressure equal to the height of 
the water in the porous layer above the base of the petroleum. An 
"oil pool" would therefore form beneath the fold b, which would be 
subject to the pressure of the water in the higher portion of the same 
bed. The outcrop of the porous bed, where it receives water, maybe 
scores or hundreds of miles distant from the oil pool, and the height 
of the source of water supply scores or hundreds of feet above the 
Fig. 1.— An ideal geological section illustrating the manner in which petroleum and natural 
gas accumulate in anticlines, and also the origin of hydraulic pressure on such "pools." 
reservoir on which it presses. Gas generated from the petroleum in 
the oil pool beneath b would collect above it and form a gas pool, also 
under hydrostatic pressure. In computing the pressure exerted by 
water in cases like the one just cited, account must be taken of the 
density of the water, since, if highly saline, its weight will be materi- 
ally increased. Under the conditions just assumed the oil and gas 
pools beneath b may enlarge until their bases extend laterally below 
the downward fold between a and b, when a limit would be reached, 
as an avenue of escape through the porous bed to the outcrop at a 
would then be furnished. 
Under the conditions shown in fig. 1 a well drilled at b and reaching 
the porous layer would yield gas under pressure, which would escape 
without diminution of pressure until it was exhausted, providing the 
porous bed was sufficiently open textured to admit of the ready flow 
of water through it, otherwise the gas might escape more rapidly than 
the water could flow in, so as to maintain a constant pressure. After 
the gas had passed off the petroleum would rise in the well to a little 
above the level of a, depending on its specific gravity, and the gas 
well would be transformed into a " pumping" oil well. 
Great variations in the general conditions illustrated above occur in 
