PETKOLEUM. 31 



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 " ^ (see pi. 5 and fig. 13) , 



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. Wlien 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 accuracy. 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.^ 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. 



2 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 hag 

 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-fleld waters in San 

 Joaquin Valley, Cal., Bull. 653, U. S. Geological Survey, 1917.) 



