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, test wells range from sparse in most Ordovician sections to almost nonexistent in Precambrian rocks. The prospects within these strata appear to improve with decreasing age, but the fact that the environment favorable for shelf sedimentation expanded progressively through the same span of time suggests that time is not the overriding factor and that no region of marine sedimentary rocks should be discounted merely on the basis of age.

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

In Asia and Australia pre-Silurian rocks have been tested very meagerly, and as yet production of petroleum is negligible. In Africa, although the presence of pre-Silurian petroleum has been established only recently, very significant production rates have already been achieved.

More than 90 percent of all oil produced from pre-Silurian rocks has come from North America, where the lower Paleozoic rocks have been important sources of petroleum for many years. Gas totaling trillions of cubic feet and oil totaling an estimated 5 billion bbl had been produced through the end of 1964 from pre-Silurian rocks of North America. The most significant area of pre-Silurian oil production is in 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 500 million bbl of oil and more than 1 trillion cu ft 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 the newly discovered petroleum from pre-Silurian rocks in Africa, Asia, and Australia will lead to intensive exploration programs and resultant significant discoveries in these old rocks.