GEOLOGICAL PAPE5S. 87 



the accumulating muds and silts of centuries. These were shut 

 off from the oxj-gen of the air and other destructive agents, so 

 that, eventually, a peculiar distillation converted the carbon, and 

 other constituents of the once living matter into oils and gases. 

 The distillation was a matter of long ages and the migration of 

 oil to more porous reservoirs than the limestones and shales, was 

 a secondar}^ step. 



This circulation and acctmiulation of oil, was accomplished by 

 agents such as capillarit)-, gravit)% and gas or rock-pressure. As 

 the minute distillation proceeded the oil was taken up under the 

 influence of gravity- and capillarit}-, and passed into the porous 

 sandstones. An important factor in the acctmiulation was the 

 "lay"' of the rocks. The sedimentary- strata were deposited tmder 

 water horizontally, or practically so. and the natural distillation 

 of oil probably began while the beds were in that position. Sub- 

 sequent folding of the strata formed series of arches or domes 

 offset by corresponding troughs or basins. The arches are known 

 as antichnes while the depressions are called s}-nclines. When 

 this folding took place, the water, petroleum, and gas within the 

 sandstone formations were forced to move according to the laws 

 of gTa\-itation and hence to distribute themselves according to 

 their specific gravities. The water was the heariest of the three 

 fluids, and, therefore, sought the s\-nclines as far as possible, de- 

 pending of course, upon the porosit}* of the sands. Its tendency 

 was to displace the oil and gas. forcing the oil to float on the 

 water and the gas to rise still higher. The oil w-as enabled to 

 rise as far as the water extended up the slopes of the s}-nclines, 

 while the gas was able to free itself and rise to the highest place 

 in the porous bed. usually the crest of the anticlines. The sur- 

 face of the oil sand on the anticline may be pitted or undulating 

 and this condition may affect an extensive area or only a few 

 acres of ground. Though the general accmnulation of oil and 

 gas is governed by the anticHne proper, covering many miles, the 

 segregation of pools may be caused by smaller folds on the large 

 one. This intricate system of s}-nclines and arches on the parent 

 fold is coupled with variation in the porosity- of the sands and 

 the two conditions greatly affect the distribution of oil and gas. 

 It is readily recognized that either factor may explain the local 

 presence of dr\^ holes within productive territorj-. 



A segregation of oil may take place along a bench or some- 

 times called a structural "terrace"', much in the same manner as 



