PRINCIPLES OF OIL ACCUMULATION 257 



barren shales are opened by joints or the oil shale layer faulted 

 against water strata.^ 



Capillary action is very slightly increased with the increase 

 of pressure,^ consequently the movements underground are not 

 materially different from those in the laboratory. The action, 

 however, varies with the absolute temperature of the liquids, but 

 in the upper 5,000 feet of sediment the temperatures would not 

 alter the actions as described. 



When the oil once reaches the sand or porous zone, it adjusts 

 itself to occupy the larger openings and remains there indefinitely 

 unless disturbed by some outside factor. 



It is impossible for oil to migrate any great distance in the 

 oil-bearing formations of the mid-continent field. A discussion 

 of this point is taken up in three parts: (i) difficulty of migration 

 from the mechanical standpoint; (2) negative experimental evi- 

 dence; (3) evidence of lenticular character of water zones. 



The movement of oil in water-soaked sediments is almost 

 entirely a problem of surface tension. Water, having a greater 

 surface tension and attraction for rock surfaces than oil, moves 

 into the smaller pores occupied by the oil and forces it into larger 

 openings. Such movement is characteristic as long as the open- 

 ings are less than o . i mm. Where the sand grains are unusually 

 large (3 mm. in diameter or over) or where induced openings have 

 been made locally in some hard formation, sorting due to specific 

 gravity is free to take place if the unbalanced weight overcomes 

 the friction of the movement. 



■Relative forces thus affecting oil movement can be figured 

 from the size of the openings in the rocks.^ In a shale at a depth 

 of 1,500 feet, where openings are o.oi micron, the capillary force 



' It is the popular belief that faults are detrimental to oil accumulation since 

 they afford passages for upward migration of oil, thereby allowing the oil to escape. 

 Such may be the case occasionally where the formations stand open along the fault 

 plane for hundreds of feet. In the oil fields of Oklahoma most faults probably never 

 reach the surface. Shales being so predominant in the section invariably fall together 

 along the fault plane, thus tightly sealing the majority of faults so that they are 

 not generally conductors of oil upward. 



- Johnston and Adams, Jour. GeoL, XXII, 9. 



^Ibid., p. 13. 



