T. 0. Bosivorth — Outlines of Oilfield Geology. 



55 



applied to sexeral of the cases already described and to all cases where 

 the oil-rocks crop out. Most of the gas and much of the lighter 

 constituents of the oil have generally escaped from outcropping oil- 

 rocks, though U7ider certain circumstances valuable accumulations are 

 found. But oil-sands outcropping far from the anticlinal axis seldom 

 contain much oil. 



1. Oil-sands sealed at Outcrop hy Bitumen. — The escaping oil on 

 evaporation has often left behind oxidized residues which block up 

 the pores of the oil-sand and prevent the escape of the remaining oil. 



Fig. 11. Oil in outcropping beds, near anticline, sealed in by bituminous 



residues. 



Large quantities of oil are obtained from such outcropping rocks in 

 many oilfields by drilling to strike them at depth, but the yield per 

 well usually is not great, because owing to the gas having escaped 

 there is but little pressure (Fig. 11). 



2. Oil-sand Sealed at Outcrop hy Infiltrated Mineral Matter. — Surface- 

 waters bearing calcium carbonate or other mineral matter sometimes 

 have deposited cement in the pores of an outcropping sandstone and 

 thus stopped the escape of the oil ascending towards the outcrop. 



C. Oil Accumulations determined by Peteogeaphical Circum- 

 stances. — When the rocks are not appreciably folded or denuded 

 the characters of the rocks have been the chief factor in determining 

 the sites of oil-pools. 



Fig. 12. Oil accumulation in horizontal sand lenticle. Length of section, say 



1 mile. 



1. In Lenticles. — In undisturbed strata big lenticles of sandstone, 

 with impervious clay above, are ideal places for the accumulation 

 of oil (Fig. 12). Being convex they possess much the same advantage 

 as a dome, for the hydrocarbons moving upwards are trapped and 

 concentrated under the apex of the dome-shaped clay roof. Good 

 examples occur in the Oklahoma Oilfields.^ 



^ "The Origin of Oil and Gas," byL. L. Hutchinson: Petroleum Eeview 

 June 3, 1911. 



