russell] PETROLEUM. 71 
it comes in contact, this tendency being greater in the heavier than 
the lighter varieties, and may be only in part or not at all displaced 
by water. If the oil refuses to flow under the pressure of water, 
however, it can not be separated except by methods which at present 
are commercially impracticable. 
Another important consideration in reference to the occurrence of 
petroleum and gas in the interstices of rocks, is the porosity of the 
rocks themselves. It has been shown by wide experience that water, 
petroleum, and gas, as they occur in the rocks, are generally contained 
in the small interstices and minute cavities between the grains or 
crystals, and not in caverns or open fissures. This is the nearly uni- 
versal rule, not only in reference to water in artesian basins, but also 
for petroleum and gas in practically all the areas where they occur. 
Although cistern-like bodies of these substances may exist, they are 
certainly rare,' and their presence beneath the surface can not be 
foretold. Their accidental discovery by the drill is a lucky chance. 
The porosity of rocks varies through all degrees, from loose sand, 
which may absorb one-half or more of its own volume of oil, to dense, 
compact quartzite, granite, and limestone, which are practically 
impervious. 
The three substances under consideration are contained in porous 
rocks in the same manner that water is held in a sponge or in a brick, 
and in the same manner, also, that water passes through a sponge or 
may be forced under pressure through a brick, so petroleum perco- 
lates or flows through porous rocks. 
The conditions under wh,ich water, petroleum, and gas exist in 
porous rocks may be illustrated by packing a bottle with sponges and 
pouring in water and petroleum; the water, being heavier than the 
petroleum, will evidently find its way to the lower portion of the recep- 
tacle, and the petroleum will gather above it. If the bottle charged 
in this manner is placed in a warm room, or a gentle heat be applied 
to hasten the process, some of the petroleum will evaporate and form 
a gas, which will rise above the oil. If the application of heat is con- 
tinued, more and more gas will be generated, and a strong gas pres- 
sure be produced. The application of the principle thus demonstrated 
will be considered later. 
The porosity of rocks varies, as stated above, but those most com- 
monly charged with petroleum or gas so as to act as reservoirs are 
sandstone, conglomerates, and certain open-textured limestones. It 
must not be concluded, however, that these are the only rocks that 
may act as storage reservoirs in this connection ; any rock that has 
an open texture, from whatever cause, may serve the purpose. A rock 
may be crushed and so filled with crevices as to permit of the ready 
percolation of liquids; even volcanic rocks with an open scoriaceous 
texture might fill the role. In general, also, the rocks which permit 
of the passage of petroleum and gas are permeable by water. 
