296 V. C. Illing—The Search for Oil. 



endeavour to compare the geological conditions in Britain with those 

 of the American oilfields, with a view to discovering whether these 

 conditions are sufficiently similar to warrant the assumptions which 

 have just been stated. 



Before proceeding with the comparison it will be necessary to make 

 a few preliminary statements on the general geological occurrence of 

 petroleum in the world. In the first place, the oilfields may be 

 divided into two main groups — those of Upper Cretaceous and 

 Tertiary age, and those of Palaeozoic age. This somewhat sweeping 

 assertion overlooks a few small fields in Europe and America, but 

 is in the main correct. The Upper Cretaceous and Tertiary fields 

 form a belt encircling the world : Galicia, Rumania, Caucasus, 

 Persia, Burma, Sumatra, Java, Borneo, Japan, California, the West 

 Indies, and Peru. In all these fields the strata are in the main 

 strongly folded, often faulted, and sometimes thrust. Usually surface 

 indications of asphalt, oil, and gas are numerous, and productive 

 wells are largely located on the anticlinal folds. 



The Cretaceous-Tertiary fields of Mexico, the Gulf States, and 

 Alberta are exceptions to this general assertion ; the structures are 

 not folded to the same extent and usually the surface indications are 

 not so numerous. The Palaeozoic oilfields on the other hand are at 

 present completely confined to North America, and particularly to 

 the United States. The more important oilfields range in age from 

 the Upper Ordovician to the Pennsylvanian, most of the oil being 

 found in the Devonian, Mississippian, and Pennsylvanian. As con- 

 trasted with the Tertiary oilfields, the older fields do not occur on 

 belts of mountain-building movement. They are not even affected 

 by well-defined folds, but occur on broad open undulations with such 

 gentle dips that the structures would not be apparent on sections 

 drawn to scale. The oil-pools are situated on terraces, minor flexures, . 

 and gently dipping sand-lenses. To speak of these minor flexures as 

 anticlines and synclines tends to give a wholly exaggerated idea of 

 their importance, for the dips rarely exceed 2°, and are usually only 

 from 20 to 50 feet in the mile. As a general rule surface indications 

 of petroleum are rare in these fields. Asphaltic sandstones are found 

 in Oklahoma and Texas, and oil and gas seepages to a small extent in 

 Western Pennsylvania and New York State, but such occurrences 

 are infinitesimally small compared with the immense supplies of oil 

 and gas which are found underground. 



Now there is no peculiar significance in geological age which would 

 lead us to infer that all new oilfields will necessarily conform in their 

 stratigraphical position to those at present known. Yet there are 

 distinct lithological portions of the crust Avhere it would be futile to 

 search for petroleum, i.e. the pre- Cambrian and the British Trias. 

 The strongly folded and deeply dissected Lower Palaeozoics, the Old 

 Bed Sandstone, the Mesozoics, and the Tertiaries of Britain are all 

 for varying reasons ruled out as possible strata containing oil-pools. 

 There remains the Carboniferous system, the strata of which 

 have yielded the most abundant signs of bitumen, and which have 

 been more extensively mined than any others in the geological 

 column. 



