January 16, tgig 



NATURE 



387 



to contain the liquid material. To strengthen the 

 main argument, analogies are drawn by those in 

 favour of the project between the Palaeozoic oil- 

 fields of North America and the corresponding 

 strata in (ire. it Britain, and emphasis is laid on 

 the supposed association of coal and oil — an asso- 

 ciation which is the exception rather than the 

 rule, and is of very doubtful significance. Further 

 support is sought in the curious theory pro- 

 pounded by Mr. E. II. C. Craig that the Scotch 

 shale-fields an essential!) an old oil-field in which 

 the petroleum has become inspissated, has corn- 

 ed with the clay, and has produced the valu- 

 able seams ol shale which form the raw product 

 of the industry. 3 



At first sight this body of evidence appears by 

 no means negligible, and there are numerous oil- 

 fields which have been founded on more slender 

 data. It is only when we examine the negative 

 evidence that a true perspective is obtained and 

 that the reasons for a pessimistic attitude become 

 apparent. To begin with, the numerous evidences 

 ol petroleum in Britain are neither surprising nor 

 abnormal ; they are merely examples of the general 

 rule that petroleum in small quantities is almost 

 universal, particularly in sedimentary rocks. 

 Britain, with its wealth of strata, representing all 

 phases ol marine and terrestrial conditions, would 

 undoubtedly contain examples of such a wide- 

 spread material. There is a world of difference, 

 however, between a small patch of oil and a large 

 accumulation. One represents the result of a 

 process of organic decomposition which is not 

 uncommon ; the other entails this process on a 

 much larger scale, and involves in addition an 

 accompanying or succeeding phase of conditions 

 favouring the concentration and preservation of 

 this material. This association of essential inde- 

 pendent conditions on a large scale is necessarily 

 a much less common event. 



Secondly, the parallel drawn between the Penn- 

 sylvania!! and other Palaeozoic fields of the United 

 s and the British Carboniferous is not merely 

 misleading ; it is untrue. The cases are not 

 parallel ; they are opposite. For in all these oil- 

 fields in the older strata the distinctive feature 

 which requires explanation is not the formation 

 of the oil, but its preservation from the dispersive 

 effects of denudation. The outstanding feature 

 of these Palaeozoic oil-fields of America is the 

 absence of strong folds and faults ; the structures 

 are either terraced, or broad open folds with 

 dips generally below 5 . When the Appalachian 

 oil-fields approach the areas of more pronounced 

 folding the oil disappears. It is not suggested 

 that oil does not occur in strongly folded strata; 

 on the contrary, this is quite common in the 

 Tertiary oil-fields ; but it is maintained that such 

 structures are incapable of preserving the elusive 

 fluid in commercial quantities for long geological 

 periods when the rocks are jointed, faulted, and 

 folded, and the subsequent denudation is exten- 

 sive. The blanketing effect of the overlying forma- 



3 " Kcrocennnd Kerogen Shales." Journal of tht Institution of Petroleum 

 Technologists, vol. ii., pp. 238-73, 1916. 



NO. 2568, VOL. I02] 



' lions must be extremely effective to prevent the 

 dispersion of the oil, and in this particular the 



1 folded and faulted structures of the British Car- 

 boniferous and their extensive dissection render 



. them in no sense comparable with the altogether 

 abnormal structures of the Palaeozoic oil-fields of 

 North America. 



With the theory that the Scotch shale-fields are 

 relics of an old oil-field the writer does not pro- 

 pose to deal. It accords with neither the geo- 

 logical nor the chemical data, and does not require 

 s< rious controversy. 



That a true estimate of the whole problem may 

 Ik obtained, it is necessary to consider briefly the 

 primary conditions for the occurrence of oil in 

 commercial quantities. These involve three main 

 fat tors : the formation of large quantities of the 

 material, its migration into suitable reservoirs of 

 porous rocks, and its preservation in these reser- 

 voirs from complete dispersion by volatilisation 

 or by denudation. With reference to the forma- 

 tion of the material, it is now generally admitted 

 tli.it the origin of the oil is primarily organic; 

 further, it may be confidently stated that the 

 balance of evidence strongly supports a marine 

 habitat. Such marine conditions with abundant 

 remains of organic life decomposing under 

 anaerobic conditions have certainly occurred in the 

 geological history of these islands. Neither was 

 there any paucity of suitable reservoir rocks and 

 structures to guide the concentration of the petrol- 

 eum. It is the preservation of the material from 

 dispersion and loss which presents the whole diffi- 

 culty. This will be fully appreciated by those con- 

 versant with the folded and faulted character of 

 the Carboniferous rocks and the extensive 

 denudation to which they have been subjected 

 since their deposition. This denudation extended 

 over the long terrestrial period of the later Palaeo- 

 zoics and the early Mesozoics, and was followed 

 by a second period of erosion extending from early 

 Tertiary times to the present. This intense dis- 

 section of the Carboniferous folds, rifted as they 

 are with important fault systems, has given every 

 opportunity for the oil to effect its escape. 



In conclusion, it may be pointed out that in 

 general the widespread Tertiary oil-fields have 

 been largely folded by orogenic movements of 

 Middle Tertiary age. The subsequent erosion, 

 though in the geological sense quite short, has 

 been sufficient in many cases to allow already a 

 natural escape of the hydrocarbons to the surface, 

 and man is busy tapping the remaining resources 

 before Nature has completed the task. The 

 American Palaeozoic fields owe their preservation 

 to unique structure and protection from erosion. 

 The British Carboniferous strata, those in which 

 the prospects have been considered to be most 

 favourable, are much more strongly folded and 

 faulted than their equivalents in the American oil- 

 fields, and their dissection is much greater. The 

 idence of the geology, and the cumula- 

 tive experience of the smallness and ephemeral 

 nature of any oil occurrences which havi 

 noted, agree in indicating that large commercial 



