THE ORDOVICIAN PERIOD 



425 



(p. 3 16) which have burst through 

 the strata (Fig. 395 C), raising 

 them and producing a dome-like 

 structure which thus brings about 

 favorable conditions for the con- 

 centration of oil. (5) Oil some- 

 times accumulates also when 

 porous strata are faulted (Fig. 

 395 E) (California). The oil 

 ascending along an inclined, 

 porous layer is prevented from 

 escaping to the surface by a 

 fault which has displaced the 

 strata to such an extent that the 

 porous layer is sealed by an im- 

 pervious one. (6) In Oklahoma 

 big lenses of sandstone covered 

 over by impervious beds (Fig. 

 395 D) sometimes yield large 

 quantities of oil. 1 



The size of oil-producing re- 

 gions is usually comparatively 

 small. 



Origin of Oil and Gas. — Pe- 

 troleum has not a definite chemi- 

 cal composition, but is made up 

 of a large number of substances 

 (hydrocarbons), ranging from 

 gases to solids, the gas in oil wells 

 being merely a liquid that is 

 volatile at low temperatures. 



Since oil and gas are probably 



Fig. 395. — Diagrams showing the more important modes of the occurrence of oil 

 and gas. Oil is represented in solid black. A, the oil-bearing stratum (dotted) con- 

 tains water, and the oil and gas are consequently near the crest of the anticline 

 (Pennsylvania and Illinois). B, the oil-bearing stratum is devoid of water. The 

 oil is in the syncline. C, the strata are bent upward around a volcanic neck, and the 

 oil has accumulated around the latter (Mexico). D, oil and gas occur in lenses of 

 sandstone (Oklahoma). E, oil is accumulated as a result of faulting (California). 



1 Bosworth, T. O., — Outlines of Oilfield Geology: Geol. Mag., Vol. 9, 1912, pp. 16-24, 53~6o. 

 Clarke, F. W., — Data of Geochemistry: Bull. U. S. Geol. Surv. No. 491, 1911, pp. 681-704. 

 Ries, H., — Economic Geology, 3d ed., pp. 50-100. 



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