NATURAL GAS 381 



found in the Appalachian, Lima, Indiana, and Kansas fields. 



East of the Mississippi river natural gas is found in the 

 rock formation of the Paleozoic age, extending from the high- 

 est carboniferous strata down to the stratum that is lower than 

 the Trenton limestone, while in the west and southwest the 

 fields containing the gas are much more recent in the geological 

 scale. 



The reservoirs in which natural gas is usually found are 

 composed of porous sandstone or limestone. In some cases a 

 limited quantity of the gas has been found in shales, but this 

 gas may be regarded as having gradually accumulated from 

 the underlying rock formation. Almost invariably the large 

 reserv^oirs have been developed in the strata on or near the 

 crests of the anti-clinal or rock waves, while petroleum has been 

 generally collected on the lower horizon; and frequently salt 

 water is found at a still lower level. Sometimes, however, the 

 gas fields are entirely isolated from the petroleum i)roducing 

 areas. There are three leaaing requisites necessary for the 

 accumulation of natural gas in merchantable quantity. These 

 are as follows: 



First. An open or porous strata capable of storing the 

 gas under pressure. 



Second. A slate or shale covering of this porous strata 

 to seal in the upper surface and the fractures of the strata 

 saturated with natural gas. 



Third. A sufficient flexure or relief of the strata to 

 enable the separation of the salt water and the petroleum 

 from the natural gas. 



In order that the production of the gas may be profitable, 

 the reservoirs must be packed with high pressure gas over a 

 large area. 



The original rock pressure has been found in very many 

 instances to equal the hydrostatic weight of a column of water 

 of a height equal to the distance from the point where the reser- 

 voir is pierced, to the surface. If it takes 2.3 feet of head of 

 w^ater to equal 1 pound to the square inch, or 43 pounds to the 

 hundred feet, then a depth of 1,000 feet will equal 430 pounds 

 to the square inch. This proportion of the depth to the pres- 



