PETROLEUM. 
OCCURRENCE. 
Because of its liquidity, petroleum differs markedly in geological 
occurrence from all other minerals. It appears on the surface in 
some localities in the form of oil seeps, but commercial quantities of 
petroleum are found only at depth inclosed within the rocks of the 
earth’s crust. Its occurrence is very similar to that of artesian water, 
with which, indeed, it is frequently associated. It saturates certain 
areas of porous rocks, such as beds of sand or sandstone, tending to 
accumulate where such strata occur beneath denser, impervious lay- 
ers. Occurring in this way under the pressure that obtains at depth, 
carrying immense quantities of natural gas in solution, and almost 
invariably associated with water, petroleum is capable of movement 
and in general migrates upward until it encounters a layer of imper- 
vious rock so disposed in structure as to impede further progress and 
impound the oil into a “ reservoir ” or “ pool ” 1 (see pi. 1 and fig. 12). 
The geology of petroleum, therefore, is the geology of rock struc- 
tures, and the skilful mapping of the surface disposition of rock 
formations gives the means for determining the structure at depth 
and hence the position of structural features favorable to the accu- 
mulation of oil. When this information is supplemented by careful 
records of the rock layers encountered as wells are drilled, a three- 
dimensional knowledge of the earth’s crust is obtained, remarkable 
for its detail and accuracjL Thus by the aid of geological methods 
the development of petroleum fields may be changed from a gambling- 
venture, to an exact science, and, if the scale of operations be suffi- 
ciently large, it may be figured rather closely how much oil can be 
obtained from a given expenditure of money. 2 3 Instead of represent- 
ing the most uncertain venture in the world, therefore, oil production 
can now be made as definitely an engineering project as the mining 
of a clay bank. 
The migratory character of petroleum, coupled with the general 
tendency of stratified rocks to occur in broadly undulating folds and 
shallow domes, gives peculiar significance to the underground dispo- 
sition of the oil deposit. Thus the process of winning the oil consists 
in puncturing the structural feature that holds it in restraint so as 
to give free scope to a movement upward to the surface. Accordingly 
the position of the oil grows highly unstable as soon as the deposit 
comes under exploitation and this variability affects the entire geo- 
1 These words are misleading in that they suggest great open spaces filled with oil 
rather than areas of oil-saturated rock. Water is sometimes lacking, and then the oil 
migrates in a different manner. 
3 Another interesting application of geology to oil exploration has been developed by 
the Standard Oil Co. of California and by the United States Geological Survey. It has 
been found that the chemical composition of the water encountered by the drill will 
give some indication of the proximity to oil and hence serve as a guide to a successful 
development. (See G. S. Rogers, Chemical relations of the oil-field waters in San 
Joaquin Valley, Cal., Bull. 653, U. S. Geological purvey, 1917.) 
