T. 0. Bosworth — Outlines of Oilfield Geology. 59 



Sometimes mud volcanoes act with such violence as to throve up 

 tons of clay and rock 20 or 30 feet into the air, and in tvpo knovrn 

 instances small islands, hundreds of feet across, have heen built up 

 off the coast by submarine mud volcanoes situated on well-known 

 anticlines.^ Mud volcanoes occur chiefly along the crests of anticlines 

 or along fault lines. 



Conclusion. 



Petroleum or other bituminous matter is common in small quantity 

 in sedimentary rocks of all ages, and in certain cases where the 

 structure is favourable it is present in large amounts. 



Some idea of the quantities of oil found imprisoned in the earth's 

 crust is obtained from a study of oilfields' statistics. The world's 

 annual output during the last few years has exceeded 40 million 

 tons, whilst from only 6 square miles of country near Baku 11 million 

 tons of oil were extracted in one year, and it is estimated that the 

 oil which has been obtained from one 27 acre property there would 

 fill a tank of that area to a depth of 270 feet.^ In a number of 

 instances in various parts of the world the amount of oil yielded 

 in one year by a single well has exceeded 100,000 tons. At the 

 present time immense quantities of petroleum oil and gas are escaping 

 naturally at the earth's surface almost wherever Tertiary strata 

 have been upraised and exposed to denudation. This is a natural 

 process of decarbonization ; shallow-water sediments which probably 

 have formed the main bulk of the marine deposits of every age 

 originally contain a great amount of carbonaceous material disseminated 

 in their muds, and this carbon is capable of undergoing chemical 

 change into volatile hydrocarbons, which must be eliminated ere 

 the composition ,of the rocks is stable. 



Thus we do not now commonly find oil in the older rocks, 

 except where they have been deeply buried and sealed up by 

 newer formations practically undisturbed. Under such circumstances 

 occur vast stores of oil in the Silurian, Ordovician, Devonian, and 

 Carboniferous formations in the Appalachian fields of the United 

 States. But in the majority of oilfields the oil-rocks are of Tertiary age. 



The decarbonization of the freshly upraised Tertiary sediments is 

 taking place in the earliest stage of their denudation, and long ere 

 the next marine transgression has overlaid the area with new 

 permanent deposits, it is likely that further denudation will have 

 entirely obliterated all records of the process, leaving in the Tertiary 

 as little indication of petroleum as in the old formations now. It is 

 therefore not improbable that the older formations were as petroliferous 

 as the Teitiary, and that the decarbonization process at present 

 witnessed on the surface of the newly upraised sediments has been 

 usual on similar occasions in the past; or, in other words, many of 

 the old planes of discordance have been the scenes of phenomena 

 similar to those described above. 



^ A. Beeby Thompson, Petroleum Mining, 1910, pp. 102, 103. 

 2 Ibid., p. 86. 



