286 ECONOMIC GEOLOGY 
empty, and only an uncertain percentage of the oil is re- 
coverable. It is generally considered that under present 
conditions only from 10 to 20 per cent. of the oil in a bed is 
obtained, and that most fields, except under heavy water- 
pressure, do not yield more than 10 per cent. The life of 
a field can therefore be prolonged by an improvement in 
the recovery factor, either by more vigorous pumping, by 
the explosion of a torpedo at the bottom of a well to open 
up the fissures and to secure the better drainage of the ad- 
jacent beds, or by the increase of gas-pressure either by 
raising the temperature or by forcing down compressed air. 
The second, and more reliable method of estimating the oil 
reserves of a field is by the decline in yield of the wells. 
(Cf. Tables by C. H. Beal, U.S. Bur. Mines, Bull. 177, 1019). 
The output of a well is usually greatest when it is first opened ; 
the fall in yield is rapid during the first few weeks ; and the 
quick fall during the first year passes into a gradual decline. 
In the Oklahoma field, e.g. if the yield of a well in the first 
year is taken as I, in the second year it would be from 23 
to -75, and the total yield would be 2 or 3. In California 
if the yield of a well in the first year is I, in the second year 
it is +75, and the total is 5. For most fields a diagram can 
be prepared showing that as the well approaches exhaustion 
the decline is very slow, so that after the large yield a small 
supply can be obtained for years by pumping at appropriate 
intervals, say for an hour a day or at intervals of a week. 
The future supply from a field can be estimated from its 
** decline curves.” 
The different oilfields of the world are under such different 
conditions that a geographical summary is more useful than 
a systematic classification. 
THE O1LFIELDS OF THE UnrTED Stares—The oilfields of the 
United States had yielded by June, 1926, over 9000 million 
barrels of oil, and they contribute about 70 per cent. of the 
world’s supply. 
The eastern or Appalachian fields were the first discovered. 
Petroleum had long been known there to the Indians and 
a little was found in Pennsylvania while boring for brine 
between 1790 and 1820. The modern development of the 
American oil industry dates from 23rd August, 1859, when