H kb Y n E ne£? D ] ACCUMULATION OF OIL. 141 
lions might be. Of all the conditions enumerated above, this is prob- 
ably the one which prevails over the largest areas. Wherever there 
are heavy deposits of sedimentary rocks, some of the beds generally 
contain organic material, either animal or vegetable, from which an 
abundant supply of hydrocarbons might be derived, and at some 
time in the history of such beds conditions have generally been favor- 
able for its conversion into petroleum. This is notably true of the 
many thousand feet of strata constituting the Cretaceous and Ter- 
tiary formations on the Gulf Coastal Plain. 
THE RESERVOIR ROCK. 
All granular rocks which enter into the composition of the earth's 
crust are to some extent permeable to liquids. This porosity, the 
vacant space between the rock particles, varies from less than one- 
half of 1 per cent in rocks like granite to 8 or 10 per cent in ordinary 
compact, fine-grained sandstones, and 25 per cent or even more in 
coarse gravel or cavernous limestone and dolomite. The porosity of 
a rock depends upon the shape of the grains, their uniformity in size, 
and the amount of cementing material. It is wholly independent 
of the size of the grains. Hence a fine-grained sand may have as 
great capacity for holding oil as a coarse gravel. The term ' ' oil pool " is 
in common use in most oil fields and is a convenient one, but is liable 
to lead to misapprehension. An oil pool is simply a restricted portion 
of any porous bed which is saturated with oil. It is limited both ver- 
tically and horizontally by some impervious barrier which prevents 
the escape of the oil. It does not generally contain any large fissures 
or caverns, the oil being contained in the minute spaces between the 
constituent grains of the bed. In exceptional cases, such as the 
Spindletop pool, where the reservoir rock is a limestone or dolomite, 
there are cavities of appreciable size, probably to be measured in 
inches and possibly in feet, in which the oil is stored as well as in the 
minute spaces between the constituent grains of the rock. 
The character of the reservoir rock does not determine the charac- 
ter of the oil, but does determine its behavior when the pool is 
tapped by the drill. When the rock is a firm, fine-grained sandstone, 
it yields its oil slowly, even when under great pressure, and the yield 
continues for a long time, steadily decreasing, however, as the supply 
is drawn from increasing distances. A cavernous dolomite, on the 
other hand, offers little resistance to the passage of the oil toward the 
well, and the flow from such a rock is consequently rapid and short 
lived. When the oil is held in an unconsolidated sand, the latter 
flows toward the exit along with the oil and quickly chokes the well 
unless held back by some straining device. 
THE IMPERVIOUS COVER. 
Since petroleum has a lower specific gravity than water, it always 
tends to rise when the two liquids are associated in the rocks, and 
