UNDERGROUND MIGRATION OF OIL AND GAS 
159 
Beyond doubt, the first stages of formation of organic material 
into bitumens are contemporaneous with burial in the silt or 
ooze of the basin floor. Synthesis of oil and gas is at a maximum 
in the midst of muds and silts, and is at a minimum in sandy 
beds. Compacting of the former drives out the fluids and gases 
to a great extent. Some of these escape upward into the water 
or air above the accumulating sediments, but much of them 
must first pass through the pores and openings of interbedded 
sandy layers. Here differential surface tension and viscosity 
tend to hold the newly formed petroleum, and may effectively 
segregate it in the coarser beds. Thus at the very start of the 
process of oil formation this transfer of oil from its place of origin 
in the muds to its place of storage in the sands begins. 
Continued accumulation of debris in the lodgment basin 
means deeper burial and increased compactness of the successive 
strata. More and more of the fluid content of the muds and 
shales is forced out, and more impenetrable become the over- 
lying strata which prevent the escape of the increasing volume 
of oil or gas from the sandy reservoirs into which it has been 
driven. Eventually the muds, silts, and clays become shale 
and the sands become sandstone. Loose debris is now “solid 
rock” and compacting of sediments becomes henceforth a quanti- 
tatively minor matter. Shale and sandstone alike acquire suf- 
ficient rigidity and strength to support the weight of a consider- 
able thickness of overburden. 
Nevertheless, compacting does not entirely cease when the 
sediments are consolidated into “rocks.” Somewhere below the 
surface is a zone below which voids cannot exist within the 
rocks; deformation there takes place by rock flowage rather 
than by rock fracture. The thickness of overburden necessary 
to produce sufficient pressure to close all openings is generally 
believed to be about eleven miles, and the notion is prevalent 
that a “zone of flow” exists at some such depth as that. It 
should be noted, however, that the eleven-mile column is neces- 
sary to deform the strongest rocks, while the weaker rocks will 
“flow” under much less pressure. The “zone of flow” is not a 
true “zone,” but a condition in which different rocks are found 
