14 
OIL AND GAS; OHIO, WEST VIRGINIA, PENNSYLVANIA. 
lary action and great rock pressure may be suggested as causes which 
aid in forcing the petroleum out from the shaLes, but there are not 
sufficient data on this subject to justify any scientific explanation. 
It matters little what is the ultimate source of the oil; the important 
facts are its occurrence now in the porous sandstones, its circulation 
through the rocks, and the conditions leading to its accumulation in 
commercial deposits. 
MOVEMENT IN POROUS ROCKS. 
The porous rocks into which the oil and gas enter may be dry or 
they may be completely saturated with water. In most cases it is 
probable that a combination of these two conditions exist—that the 
porous rocks are completely saturated with water up to a certain level, 
but above that point they are dry. The movement of the hydrocar¬ 
bons through the rocks will not be the same in the two cases, and 
therefore each condition must be considered separately. 
If small quantities of oil and gas enter a dry porous rock at different 
points the oil will flow down as long as gravity is sufficient to over¬ 
come the friction and the capillary attraction. The gas will diffuse 
with the air or water vapor contained in the pores of the rock. 
Oil and gas entering a porous rock that is completely saturated 
with water will be forced up to the top of the porous stratum by the 
difference in the specific gravity of the hydrocarbons and the water. 
Here the oil and gas will remain if the porous stratum be perfectly 
level, but if it has a dip sufficient to overcome the friction the par¬ 
ticles of oil and gas will gradually move up this slope, the gas with 
its lower specific gravity occupying the higher places. 
In case the porous rocks are partly saturated a combination of 
these two actions will take place. The oil entering above the line 
of complete saturation will flow down to that line and the oil entering 
o 
below will be forced up to the top of the completely saturated portion. 
I he statements given above are based on the assumption that the 
oil-bearing rock is homogeneous throughout and that the oil will 
move with the same degree of freedom in every direction. This is 
rarely the case. Sandstones are noted for their irregularity in com¬ 
position, as regards both the size of the individual grains of sand and 
also the material which cements the grains together. It is obvious that 
any fluid will move more rapidly through a coarse conglomerate im- 
peifeetl\ cemented than through a dense, fine-grained sandstone the 
particles of which are thoroughly coated and all the interstices filled 
with impervious cement. If the oil-bearing rock contains areas prac¬ 
tically impervious, these areas, according to their size and position, 
\\ dl be more or less perfect barriers against the movement of the 
oil or the gas. 
