Marine sedimentary rocks of Precambrian, Cambrian, and Ordovician ages constitute a major frontier for petroleum exploration. In regions where appreciable thicknesses of such rocks exist, the distribution of test wells ranges from sparse in most Ordovician sections to virtually non-existent in Precambrian rocks. The prospects for petroleum occurrence within these strata appear to improve with decreasing age. However, the fact that the environment favorable for shelf sedimentation expanded progressively through the same space of time suggests that time is not the overriding factor and that no region of marine sedimentary rocks should be discounted simply on the basis of age.

Petroleum hydrocarbons in apparently commercial quantities are known from pre-Silurian rocks of four continents: North America, Africa, Asia, and Australia.

In Asia and Australia, pre-Silurian rocks have not been tested adequately, and production of petroleum to date is negligible. In Africa, although the presence of pre-Silurian petroleum has been established only recently, very significant production rates already have been achieved.

Approximately 94 per cent of all oil produced from pre-Silurian rocks has come from North America, where the lower Paleozoic rocks have been important petroleum reservoirs for many years. Trillions of cubic feet of gas and an estimated 4.8 billion barrels of oil had been produced by the end of 1963 from pre-Silurian rocks of North America. The most significant area of pre-Silurian oil production is a belt occupying parts of Kansas, Oklahoma, Texas, and New Mexico, where productive beds are found in the Arbuckle, Ellenburger, and Simpson. Elsewhere in North America, the Trenton Limestone of Ohio, Indiana, and Michigan has yielded more than a half billion barrels of oil and more than a trillion cubic feet of gas.

Only in North America has the pre-Silurian section been extensively explored, and it is in North America that most of the known pre-Silurian hydrocarbon accumulations have been found. It would seem reasonable to anticipate that newly discovered petroleum from pre-Silurian rocks in Africa, Asia, and Australia will lead to intensive exploration programs and result in significant discoveries.

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