The reservoirs in which natural gas is usu ally found stored when pierced by the drill 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 re garded as having gradually accumulated from the underlying rock formation. Almost invari ably the large reservoirs 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 producing areas. There are three leading requisites neces sary for the accumulation of natural gas in merchantable quantity. These are as follows: 1. An open or porous strata capable of stor ing the gas under pressure, generally sand stone or limestone.
2. A slate or shale covering of this porous strata to seal in the upper surface and the frac tures of the strata saturated with natural gas.
3. A sufficient flexure or relief of the strata to enable the separation of the salt water and the petroleum from the natural gas, which is almost invariably found in the higher portion of the strata.
These gas reservoirs have been accumulating for ages the gas they contain.
Original Pressure.— The original pressure of natural gas reservoirs has been found in many cases equal to the hydrostatic balance, or in other words, to the weight of a column of water equal to the vertical distance between the reservoir and the surface of the ground. Al lowing 2.3 feet for each pound, or about 43 pounds to the 100 feet, a reservoir at a depth of 1,000 feet should show 430 pounds rock pressure per square inch. This hydrostatic pressure has been equalized to a c6rtain extent by a large number of minute vents that have permitted the escape of the lighter hydrocar bons to the surface. These vents are of com mon occurrence throughout the Appalachian gas field from northwestern central New York to central Tennessee and along the great Cin cinnati uplift from central Kentucky to north ern Ontario.
These vents have during past ages allowed the gas to escape in immense quantities, amounting to unnumbered millions of cubic feet, so that the supplies from which we are now drawing are presumably but a small frac tion of what has been produced in the earth.
Reserves of Natural Gas.— Many of the original natural gas fields have been practically exhausted and a large number of the produc ing companies have had to seek new localities, where a fresh supply could he secured. This
has been often accomplished by drilling deeper wells in more remote regions. The gas fields adjacent to Pittsburgh having been used up, most of the supply for that city now comes from the deep natural gas sands of south western Pennsylvania and West Virginia, dis tant from 80 to 100 miles. The gas pressure in the original pool in northwestern Ohio is nearly exhausted. That of central Indiana has only about 15 per cent of the original pressure and volume remaining. The newly developed field in eastern central Ohio has recently sup plied a large and increasing quantity to the in habitants of that State. West Virginia has, during recent years, supplied a constantly in creasing quantity of natural gas to Ohio and Pennsylvania. New York and Ohio have also been supplied by Pennsylvania. In all the natural gas fields it requires constant drilling of new wells and their connection with the main lines to keep up the supply, which is con tinually being depleted. Owing to the decrease in pressure at the wells and the desire to de liver large quantities of natural gas to distant consumers without increasing the size of the main lines, the companies have erected power ful compressing plants in convenient localities. These plants are models of mechanical engi neering skill. The compressors are, in many cases, operated by large internal combustion engines of from 500 to 1,500 horse power each. About nine cubic feet only of natural gas is required to develop one horse power; and 1,000 cubic feet of natural gas at a pressure of 0 may be compressed to 270 pounds, by con suming 33% cubic feet (or 3% per cent), where very close to double this quantity is required if the natural gas be consumed under boilers and the steam used in condensing en gines. The deep sand reservoirs in south western and central Pennsylvania and those of West Virginia should keep up a fair supply for many years to come. The West Virginia field is estimated as good for 50years at its present rate of utilization. What may result from the deeper drilling of wells in localities where structural conditions are favor able is a problem for future determination. The Caddo field of Louisiana is regarded as the most extensive and prolific of all. Its utilization was begun only in 1913. The Cali fornia field, developed in 1910, has not yet reached its maximum.