Petroleum or Rock. Oil 287 



number of regular series. Each member of the series differs from 

 the preceding member by the addition of CH,. The petroleum 

 hydrocarbons begin with CH 4 , marsh gas or methane, and range 

 as high as the compound C :1 ,H 7 ,, technically known as Penta- 

 triacontane (a name which you do not need to try to remem- 

 ber!). It is interesting to note in passing that carbon and 

 hydrogen combine chemically directly when an electric arc 

 is formed between carbon terminals in an atmosphere of 

 hydrogen. 



It has been thought that oil rock oil or petroleum has 

 been distilled in some way from plant and animal tissues. Plants 

 and animals have lived upon the earth on land or in the seas 

 since very early in geologic time, but no one has yet explained 

 how the oil originated as oil. It has long been known that the 

 destructive distillation of organic matter, animal or vegetable, 

 under conditions which preclude the free access of air, will 

 produce hydrocarbons. Probably no subject in geochemistry 

 has been more discussed than that of the origin of petroleum. 

 Its nature is known. It is a compound of carbon and hydrogen. 

 It is lighter than water, hence is crowded out by water because 

 of the greater pressure of water. Under conditions such as 

 have existed and still exist in the rocks it collects in pools or 

 lakes deep in the ground. It may and does fill the interstices 

 in porous rocks. It may and does "drain" from one rock 

 horizon to another through a fracture or "fault" in the rocks. 

 It may be and is forced by the pressure of heavier water from the 

 rocks where it may have been formed to gather in a cavity, or 

 in another porous formation higher up. If a hole is drilled 

 from the surface and taps the pool or lake where the oil has 

 accumulated then a productive well results. Geologists have 

 come to regard some formations as oil producing formations, 

 as "oil sands." Such a "sand" may be located but may not 

 yield any oil when reached by the drill. The oil may have 

 "drained away" to some other location. Many rocks are not 

 oil bearing, and are known to be barren. Granite, quartzite, 

 many sandstones, and limestones, in California are barren of oil. 



