Conditions favorable to the formation of petroleum precursors have been in existence since early Precambrian time. Indigenous petroleum in commercial quantities is known today in strata ranging in age from Cambrian to early Quaternary. Discoveries of indigenous oil in the Precambrian can be anticipated where the limitations of space are met. Petroleum in space has no limitation in latitude, longitude, or present shoreline. It is limited to continental platforms and other sedimentary environments. It is also limited by a low tolerance for metamorphism. For this reason there is a "twilight zone" where oil and gas give way to gas only, both laterally in those basins that are bordered by tectonically disturbed belts, and with depth in deeper basins. Where coal is present, the degree of incipient metamorphism, or eometamorphism, can be determined roughly by carbon ratios, and more accurately by reflectance.

The lateral phase-out of oil caused by eometamorphism is found in many basins, including the Appalachian, Arkoma, and Alberta. Vertical phase-out occurs in the Gulf Coast, Permian basin, Anadarko basin, and the Baku district. During the last 16 years, 68 per cent of the new discoveries in the United States below 15,000 feet were gas (or gas and condensate); during the same period, only 30 per cent of the new discoveries above 15,000 feet were gas. Because of wide differences from place to place in thermal gradients and down-hole pressures, the depth of the oil "floor" changes considerably from place to place. In some areas oil phase-out can be expected from 15,000 feet (or above) to 17,000 feet; in others oil can exist a few thousand feet deeper. There is a distinct possibility that t ere is very little commercial oil below 22,000 feet.

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