298 V.a Illing—The Search for Oil. 



Palaeozoic fields are of an entirely different type. They are mere 

 broad undulations of the strata associated with epeirogenic movements, 

 and differ essentially from the folds immediately associated with 

 orogenesis. Pennsylvania has been chosen merely as a good example 

 of structures which are typical of all the American Palaeozoic oil- 

 fields, with perhaps the single exception of Southern Oklahoma 

 and North Texas, which will be alluded to later. 



The characteristic association of oil with relatively undisturbed 

 strata in the American Palaeozoic oilfields has been emphasized by 

 David White in a general law which he has formulated, that "In 

 regions where the progressive devolatilisation of the organic deposits 

 in any formation has passed a certain point, marked in most Provinces 

 by 65 to 70 per cent of fixed carbon (pure coal basis) in the 

 associated or overlying coals, commercial oil-pools are not present 

 in that formation nor in any other formation normally underlying 

 it, though commercial gas-pools may occur in a border zone of higher 

 carbonization ". 1 



The exact relationship of the composition of coals to earth stresses 

 is a disputed point, but it is certain that if White's law holds good, 

 and it is the result of wide experience in American fields, it 

 effectively eliminates the possibility of oil being found below most 

 of the British Coal-Measures. 



The fundamental truth which is contained in this generalization 

 appears to the writer to be largely concerned with the last of the 

 three necessary conditions for the formation of a commercial 

 oil-pool : — 



1. The formation of the oil. 



2. Its migration to suitable porous reservoir rocks. 



3. Its preservation from seepage and denudation by a thick cover 

 of impervious strata. 



The formation of the oil entails the entombment of organic matter 

 under marine anaerobic conditions, and probably includes certain 

 biochemical changes resulting in the production of hydrocarbons. 



The migration of the oil probably results from the compacting of 

 the sediment, the pressing out of the oil and salt water from the 

 compressible argillaceous sediments into and through the sandy and 

 dolomitic horizons whose spore space is not materially reduced 

 by pressure. 



With regard to the third essential, the rapid seepage of oil and 

 gas in the folded Tertiary oilfields teaches us how quickly such 

 material can be lost from the strata, and it is obvious that the most, 

 important factor in the preservation of oil over long geological 

 periods is the effectiveness with which it is sealed by the overlying 

 impervious rocks. This essential implies the absence of strong- 

 folding, important fracturing, and deep dissection of the strata. 

 Where this is not the case, the resultant migration during long 

 geological periods has been sufficient to allow complete loss of the 

 volatile gas and fluid from the oil reservoirs below. The same 

 tendency is of course true of the Tertiary oilfields ; the process of 



1 Journ. Wash. Acad. Sci., vol. v, p. 212, 1915. 



