ACCUMULATION OF PETROLEUM AND GAS 715 



In the Mid- Continent field there is no evidence that the present contain- 

 ing strata (the sands) are the original sources of the petroleum and gas; 

 bnt on the other hand the}^ are probably the porons bodies to which these 

 substances have been caused to migrate. It is believed that consolidation 

 of carbonaceous shale beds, with the resulting reduction in the total 

 amount of their pore space, due to compression of the rock into a lesser 

 volume, has been an important • factor in initiating the concentration of 

 the natural hydro-carbon distillates. 



In the early geological history of the carbonaceous beds, pressures that 

 changed them from clay to shale so reduced their volume that a move- 

 ment of their liquid contents, including hygroscopic, or connate water, 

 together with the liquid hydro-carbon compounds and gases, took place. 

 In reality this was a combination of distillation and concentration in 

 which the greater surface tension of water must have played an important 

 part in pushing the oils to the areas of greater porosity and thus begin- 

 ning the migratory movement to the nearest beds of sandstone. Most 

 carbonaceous shale will furnish some oil by destructive distillation, and 

 when one considers the slow exertion of pressure during a long period of 

 time, in which water moved through the shale, efliects similar to general 

 distillation may have been produced. It is true that such migration 

 would have a tendency to fractionate the oils except for the fact that they 

 have been moved by hydraulic force through pores already containing 

 water, and the results thus obtained might differ from fractionation by 

 adsorption of petroleum through dry shale or fullers' earth. In postu- 

 lating such migration it is plausible to consider that it was in upward 

 and lateral directions, which were usually those of least resistance, and 

 involved a great thickness of rocks in the time of their youth. In the 

 sandstone beds water moved on into the finer pores of the overlying shale, 

 as well as laterally toward the outcrop. 



A cross-formational movement of fluids of the character above men- 

 tioned would ultimately come to equilibrium, so far as the pressure is 

 concerned, after the deposits were sufficiently compressed or metamor- 

 phosed to support the overlying load of superimposed strata. Further 

 movement would result from capillarity alone, water passing into the 

 fine pores of the overlying shale, but small amounts of oil and gas re- 

 maining behind. The di7ness of some beds of sandstone may be ex- 

 plained by the process of capillary absorption of water into adjacent 

 shale. 



The oil and gas sands that now lie at drill able depths were formerly, 

 in many cases, buried under thousands of feet of overlying beds. Munn 



LIII — Bull. Geol. Soc. Am., Vol. 28, 1916 



