petroleum possibilities oe elorida 
47 
a rule uniformly porous. Locally the pore space in the sandstone may 
be partly or wholly filled with a cementing substance, thus reducing or 
destroying the storage capacity of the rock. Limestones not infrequently 
store large quantities of oil either in the pore space incident to shell 
breccias or in solution cavities; dolomitic limestones are often notably 
porous rocks. On the other hand, shales which probably are the source 
of much of the oil have themselves but limited storage capacities, owing 
to the fine texture of the rock. 
GEOLOGIC STRUCTURES EAVORABLE TO THE ACCUMULATION OE PETROLEUM 
The migration of oil and gas under the influence of specific gravity, 
capillarity or otherwise, into porous rocks, is not of itself sufficient to 
bring about accumulation of oil in large quantities except in localities 
where the structural conditions in the rocks are favorable. 
If a rock is uniform in porosity and horizontal in position, it may 
contain a considerable quantity of oil and gas and not afford favorable 
conditions for segregating either oil or gas in commercial quantities. 
In nature, however, it is rarely true that a rock formation is either en¬ 
tirely uniform in porosity or entirely horizontal in position. On the con¬ 
trary, variation in porosity in the rocks and departure more or less pro¬ 
nounced from the horizontal in the rock stratum is the rule rather than 
the exception. Favorable geologic structures in this sense are those 
structural conditions in the rocks which bring about segregation of oil 
or gas in commercial quantities. 
The conditions in the rock that may bring about storage of oil or gas 
are so varied as to be extremely difficult to classify or enumerate. It is 
not practicable to treat this phase of the subject exhaustively. However, 
a number of the best known structural conditions favorable to the ac¬ 
cumulation of oil may be described. 
AN INCLINED POROUS STRATUM 
It has already been stated that rock formations rarely if ever lie en¬ 
tirely horizontal in the earth, and that inclination from the horizontal is 
a common condition. The effect of departure from the horizontal in a 
formation containing oil will be apparent. If the stratum contains both 
oil and water, the oil will be forced by the water up the dip. In such in- 
