88 p. N. KROPOTKIN 



often lie in veins, which prove the vertical migration of hydrocarbons of the deep 

 oil type (Sadki deposits, Buguruslan district, and some pyrobitumen dikes on 

 prc-Cambrian cr^'stalline shields). 



The vertical distribution of petroleum does not show any connection with 

 the distribution of organic matter in the Earth's crust layers. The achievements 

 in the technique of prospect driUing to a depth of 4-8 km in the U.S.S.R. and 

 6-3 km in the U.S.A., have furnished much new data on the extent of the vertical 

 migration of hydrocarbons. It turned out that petroleum and gas are met in 

 large quantities below the layers rich in organic matter (for instance, the dark 

 clays of the Maikop series in the Caucasus), which only recently were considered 

 the mother formations (i.e. primary source rocks) of oil. With the increase of 

 drilling depth, in oil districts all over the world are discovered new oil-bearing 

 horizons in rocks having satisfactory porosity, down to the base of the non- 

 metamorphized sedimentary cover. 



The early Palaeozoic sedimentary rocks (Cambrian, Ordovician) at the base 

 of the sedimentary layers are oil bearing in the Mid- Continent, Appalachian, 

 and so-called Permian Basin of western Texas on the North American platform, 

 in Wyoming, U.S.A., on the Chinese platform, and in the southern Siberian 

 platform. The Devonian series, comprising the lower part of the sedimentary 

 cover, are oil-bearing in the Timan and Volga-Urals provinces on the Russian 

 platform. 



If the igneous and metamorphic rocks of the folding basement are sufficiently 

 crushed by faults and cracks and may serve as oil collectors, these rocks some- 

 times contain commercial reserves of oil (Kansas, the Edison oil field and others 

 in California; Mara-La Paz field in Venezuela) [19], Traces of petroleum have 

 been met in fractured granites or gneisses of the pre-Cambrian basement in 

 Wyoming, on the shores of Lake Baikal and the Red Sea, and in the pre-Cambrian 

 Ural-Volga oil-bearing districts (Shugurovo, Vyatskaya Polyana) [11, 20]. In 

 some Volga districts the impregnation of oil bitumens has been discovered in cores 

 of granite-gneissic rocks of the pre-Cambrian crystalline basement extracted 

 from boring wells. Bores in the metamorphic shales of the folding basement in 

 Timan have revealed oil and combustible gas to a depth of 400 m below the roof 

 (upper surface) of basement [21]. 



In the part of the Canadian pre-Cambrian shield bordering on the Michigan 

 and Appalachian petroleum- and gas-bearing provinces, combustible gases 

 appear directly from the pre-Cambrian granite-gneiss basement (the region of 

 the Lakes Superior and Huron, the Edwards Mine in the Adirondack pre-Cam- 

 brian uplift). At Fort William, for instance, combustible gas was obtained by 

 drilling in the pre-Cambrian basement at a depth of 335-400 m below its 

 surface [22]. 



Combustible gas comes from a considerable depth from the pre-Cambrian 

 rock of the Baltic (Fennoskandian) shield (Kola Peninsula, some Swedish mines) 

 and Australia (methane gases at the KalgoorUc Mines; gases containing 70% of 

 hydrogen and 7% of methane from a depth of 260 m on the York Peninsula) 

 [23]. This hst is enough to convince us that the sources of petroleum and gas 

 are deep down in the pre-Cambrian basement, corresponding to what the geo- 



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