426 HISTORICAL GEOLOGY 



rarely indigenous to the rock containing them their origin has given 

 rise to much speculation. There are two principal theories of the 

 origin of oil, (i) the organic and (2) the inorganic, of which the 

 former is more generally held. The inorganic theory is based upon 

 laboratory experiments with metallic carbides, and holds that when 

 water percolating downward through the earth's crust reaches heated 

 rocks it becomes converted into steam which attacks the iron car- 

 bides, believed to exist there, generating hydrocarbons (oil). Ac- 

 cording to the organic theory, petroleum and its products are derived 

 from animal or plant remains or both, which were embedded in the 

 sediments and were later decomposed to oil. It is often stated that 

 oil and gas were derived from beds of shale, either underlying or 

 overlying the oil-bearing rock. 



Life of Oil Wells and Fields. — The amount of oil yielded by single 

 wells in various parts of the world in one year has exceeded 100,000 

 tons, but such an enormous production lasts but a few weeks at the 

 most. The oil wells of Pennsylvania have an average life of about 

 seven years, those of Texas about four years, and those of California 

 about six years. The average production of the wells of the Appala- 

 chian region was less than two barrels in 1907, and that of the Cali- 

 fornia field was forty-two and a half barrels. Since the discovery of 

 oil in the United States the production has increased from decade to 

 decade, but this increased yield has been the result of the sinking of 

 new wells and the discovery of new fields. The reason for the short 

 life of oil and gas wells is that, unlike water, there is no perennial supply. 

 The great spouting wells, or " gushers " are the fortunate tappings 

 of the accumulations of ages which, though enormously productive 

 when first opened, are also in about the same proportion rapidly 

 exhausted. 



Oil is more commonly found in the younger rocks than in the 

 older, although some of the richest " pools " are in the Ordovician 

 and Devonian. The reason for this seems to be that the older a for- 

 mation is, the more opportunity there has been for the escape of theoil 

 and gas (1) by faulting which permits the escape of the oil and gas 

 from the porous rock, and (2) by the erosion of the edges of the oil- 

 bearing strata when it is lost by evaporation. Only those Paleozoic 

 strata which have been deeply buried and sealed by newer formations 

 and have remained practically undisturbed are likely to yield large 

 quantities of petroleum. The Ordovician limestones of Ohio have 

 yielded large quantities of high-grade oil and gas; the Devonian 



