waliT ri>ir.;'; in >li;ilum .\\'> has driwii the oil and }^as into the ui)i)fi' 

 liorlion of the stratum where they have arranged themselves accord- 

 iiif^ to their rclatixc specifir ^ra\ity. Unless the line of faulting is 

 filled with lime ov i la\ nv otlicr inipcr\'ious material there would be 

 an oil spring or asphaltuni bed at point C — the probabilitx' is that 

 most of the gas would have escaped along the fault line. 



Oil. POOLS I'OK.MEI) THROUGH CHANGE IX PERME- 

 .MULITY OF STRATA. 



There doubtless are instances where a change in the permeability 

 of strata have occasioned oil pools or a variation in the yield of wells 

 situated near one another, and the occurrence of dry holes in other- 

 wise producLi\e territory may often be attributed to this cause. In 

 Fig. 7 stratum AB is shown to change from sand to clay and the oil 

 and gas being pressed up by the water have percolated through the 

 porous strata until they reached the barrier of clayey material in 

 front of whicn they form an oil pool. It is evident that while C will 

 be a productive well, D will be a dry hole. 



The illusirations accompanying this article show types of geolog- 

 ical structures which have been found favorable to the accumulation 

 of petroleum. 



The most important of these is the anticline, to which the greater 

 number of the important oil fields of the world may be referred. 

 When there are sufficient rock exposures in a locality the geologist can 

 generally determine the character of the structure but of course it is 

 impossible for him to demonstrate whether or not any particular 

 structure contains an oil pool. He can, however, eliminate many of 

 the risks of prospecting for oil by selecting territory where there are 

 structures favorable to the accumulation of petroleum and where oil 

 yielding horizons may be reached by the drill. 



Fig. 4 — Contour Sketch of Dome. 



36 



