OIL AND GAS IN THE CENTRAL BASIN. 5 



The source of oil and gas. Concerning the origin of oil and 

 gas, there are those who think they are formed by chemical 

 reactions that take place within the earth, between different 

 minerals; and those who think they are formed by reactions 

 that take place as the result of organic matter in the rocks. 

 Most geologists, perhaps all, use the latter as a working basis. 

 That is to say, one of the first things they consider when a 

 prospective new field is brought to their attention is whether or 

 not any of the rocks beneath the surface ever contained a good 

 supply of organic matter. If so, this fact, in so far as it goes, 

 is encouraging. The organic matter was put down with the 

 rock beds in which it occurs. 



There are those who believe that oil and gas were formed 

 from the remains of animals that lived in the sea at the time 

 the beds were put down ; and others who think that they were 

 formed from the remains of plants that grew in the water 

 where the beds were formed, and still others who think the 

 plants were washed in by streams. Plant remains in beds now 

 being formed beneath water have both sources, and doubtless 

 have had ever since there were plants on the earth. The 

 reasonable supposition is that both have contributed to the 

 formation of oil and gas. Likewise, any given reservoir of oil 

 and gas might have had an animal origin or a plant origin, or 

 it might have come partly from the one source and partly from 

 the other. 



The reservoir. It is plain that oil and gas can not accumu- 

 late, either in large or small amounts unless the rocks are 

 capable of receiving and holding them. To receive them, there 

 must be cavities of some kind. These may be open spaces in 

 coarse-grained rocks, or joints in fine-grained ones. The 

 coarse-grained, porous rocks are usually sandstones or conglo- 

 merates, the former being composed of coarse grains of sand, 

 and the latter at least largely, of gravel, cemented together. 

 Rocks of this kind form the best reservoirs. As gas will move 

 more freely than oil, a reservoir for gas may be in finer-grained 

 rock than for oil. In the same way, a reservoir for very thin oil 

 may be in finer-grained rocks than for thick oil. Sandstones 

 and conglomerates are the common porous rocks, and so 

 frequently are they the rocks from which oil and gas are 



