14 BEREA GRIT OIL SAND IN CADIZ QUADRANGLE, OHIO. [bull. 198. 
Mode of accumulation of oil and gas. — The slope or dip of the oil- 
bearing formations can have an influence on the accumulation of oil 
and gas only through the differences in specific gravity between these 
two substances and the water and air with which they come in con- 
tact. Gravity, acting in the manner to be explained below, is the 
force that has caused the segregation and collection of oil and gas in 
economic quantities. 
The thick shales which constitute an important part of the sedi- 
mentary rocks of the oil and gas territory are probably to be looked 
upon as the source of the oil and gas. These two hydrocarbons, form- 
ing in very small quantities over a large territory, have been forced 
out of the shale into an adjacent porous stratum whenever such porous 
formation lies in juxtaposition to the oil-bearing shale. These porous 
strata are saturated with water, and from the nature of their formation 
must have been so at the time of their original deposition. The porous 
strata usually consist of sandstone, but in some cases they are com- 
posed of limestones which have been under conditions of crystallization 
that give the rock an open structure. 
Assuming a bubble of gas and a drop of oil to have been forced from 
the underlying shale into a porous sandstone already saturated with 
water, the natural course of the two particles, owing to their lesser 
speciiic gravity, will be to force their way up through the sand rock 
until a roof or impervious stratum is reached, and here to remain if 
the cap or cover to the porous stratum is perfectly level. If, how- 
ever, the cap rock has a slope, there will be a tendency for the hydro- 
carbons to creep along, always up the slope of the stratum, until a 
reverse or counter dip is met, at which point an accumulation will be 
formed, the gas occupying the highest points, with the oil directly 
beneath it and resting upon the water. 
In assuming the existence of a continuously porous stratum entirely 
filled by water, we are not entirely supported by fact. The sand 
rocks are of all degrees of porosity, a single bed frequently varying 
from a coarse conglomerate, with good-sized pebbles having large 
spaces between them through which any fluid may flow with but little 
friction, to the finest grained sandstones, through which the passage of 
a liquid is slow and difficult. At other points the sand grains may be 
cemented together by lime, iron oxide, or silica into sandstones that 
are practically impervious to fluids. These differences in the reser- 
voir strata may at any time cause results entirely at variance with the 
conditions to be expected from the anticlinal theory. 
Although these sedimentary beds must have been completely satu- 
rated with water when first deposited, they are not always so at 
present. Some porous beds are freely saturated through their whole 
extent; others are saturated only in the lower portions of the folds, 
while the portions which occupy anticlines are almost entirely free 
