46 
FLORIDA GEOLOGICAL SURVEY—I 4 TH ANNUAL REPORT 
while the oil as found in nature is usually segregated into relatively re¬ 
stricted areas. 
MIGRATION OE OIL 
It is a commonly accepted conclusion that oil and gas, as obtained 
commercially, have migrated more or less from the original source. The 
forces that influence migration include specific gravity, capillarity, and 
possibly some others not well understood. The well-known difference in 
specific gravity between oil, gas and water are probably among the im¬ 
portant causes of separation of oil and gas. As oil is lighter than water, 
it naturally rises to the top, and hence tends to segregate in the rock 
above the water-line. Gas likewise accumulates at a level above the oil. 
In the case of strata inclined from the horizontal, and capped by an 
overlying impervious stratum, the oil and gas, for the same reasons, 
migrate up the dip to a level higher than the water. 
The capillary attraction of liquids is a force that may be of impor¬ 
tance in the migration of oil. ‘The capillary attraction of oil is less than 
that of water. Hence, it is assumed that water, through capillary at¬ 
traction, may force oil from rocks having small pores, such as shales, 
into rocks of larger pore space, such as sandstones. 
It is probable that the compacting of sediments by its own weight is 
an important factor in compelling movement of included fluids. Other 
causes mentioned are increased temperature as sediments accumulate, 
affecting the fluidity of oil. A reduction in volume of voids in the rock 
in the course of time through cementation processes would likewise 
compel migration of included liquids and gases . 1 
THE STORAGE OF OIL 
The migration of oil in the rock will effect its segregation in com¬ 
mercial quantities only where the storage conditions are suitable. The 
rocks are capable of storing a gas or a liquid in proportion to their 
porosity, and any rock that is porous may under favorable conditions 
serve as a reservoir. As a rule, sands and sandstones are relatively high 
in porosity and much of the oil secured is taken from sands or sand¬ 
stones. However, not all sands or sandstones are sufficiently porous 
to serve as important reservoirs. Nor is the same sandstone horizon as 
1 Johnson and Huntley, Oil and Gas Production, p. 46. 
