SOME EFFECTS OF CAPILLARITY ON OIL 

 ACCUMULATION 1 



A. W. McCOY 

 The University of Oklahoma 



All rocks in the upper crust of the earth contain pore space. 

 The percentage by volume of this space varies from a fraction 

 of i per cent in the case of most fresh crystalline rocks 2 up to 40 

 per cent in some sandstones. 3 Below ground-water level these 

 openings are more or less saturated with water, which moves 

 about from points of higher to points of lower pressure. 



The movement of water thus entombed does not exactly follow 

 hydrostatic laws, as can be observed by the small loss of head in 

 artesian flow. For example, an instance is cited ; by Van Hise 4 

 where water traveled under ground 150 kilometers with a loss of 

 only 50 m. in head. This shows that the movement was very slow 

 (perhaps a few feet per year), for the friction through the porous 

 stratum was almost nothing. In the case of water moving in 

 large openings, such as pipes, friction is an important factor. A 

 somewhat similar example was observed by the author in Missouri, 

 where the loss of head by flow in the Roubidoux sandstone was 

 about 200 ft. in 75 miles. A theoretical means of comparison with 

 the observed facts is to note the size of the openings in the rocks. 

 All tubular openings less than 0.508 mm. 5 are capillary. There- 

 fore, by geometrical proof, it can be shown that sandstones with 

 uniform rounded grains of less than 2 mm. in diameter, would 

 contain mainly capillary openings. Rocks with uniform rounded 

 grains, regardless of the size of grain, contain about the same 



1 A paper read before the Geologic Conference of Oklahoma, January 7, 1916, 

 at Norman, Oklahoma. 



3 Van Hise, Monograph, U.S.G.S. 47, p. 125. 

 3 G. P. Merril, Rocks, Rock-Weathering and Soils, p. 198. 

 * Monograph, U.S.G.S. 47, p. 587. 

 s Alfred Daniell, Text Book of Physics, p. 315. 



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