Eometamorphism (early metamorphism) limits the distribution of indigenous deposits of oil and gas in both time and space. However, in regions, such as parts of Australia, where Precambrian sedimentary rocks are not metamorphosed, there is no limitation at the lower end of the geologic age column, for it is now known that life existed at least as long ago as 2 billion years. There does appear to be a limitation at the upward end of the age column inasmuch as indigenous oil and gas in commercial quantities have not been found in late Pleistocene deposits. This is probably the result of inadequate eometamorphism of the petroleum precursors in the sediments.

Eometamorphism limits the occurrence of hydrocarbons in space both laterally, in basins bordered by mobile rims, and at depth, in deeper basins. The agent are addition of heat and increase in pressure. The recorders of eometamorphism are coal, shale, reservoir rocks, and hydrocarbons in the rocks. The degree of eometamorphism by heat depends upon both the temperature and the length of exposure ("cooking time") involved. Temperatures recorded at depth reach a known maximum in excess of 500°F., and pressures range from atmospheric to an estimated 17,500 psi. In most areas, the downward increase in temperature below the first 3,000 feet of depth is from 1°-2° Fahrenheit per 100 feet.

Coal records the extent of eometamorphism by carbon ratios and reflectance. Oil and gas tend to phase out, and gas-only fields occur where the fixed carbon in nearby coal beds is more than 60. Reflectance, which is considered to be a more accurate index of eometamorphism than carbon ratio, is the measure of the ability of glossy coal to reflect light. This parameter also indicates the twilight zone where commercial oil deposits disappear. Shale records eometamorphism by increase in density and hardness.

Petroleum in reservoir rocks is extremely responsive to heat and pressure, with conversions to lower density compounds beginning at about 250°F. The end product (prior to graphitization) is dry gas. A census of fields below 15,000 feet discovered between 1949 and 1965 has shown that nearly four fifths produce gas or gas condensate, whereas only one fifth of all fields discovered during the same period produced these lighter hydrocarbons exclusively.

It is estimated, based on past experience and therefore subject to modification in the future, that the commercial oil floor ranges from a depth of about 14,000 feet where the thermal gradient is 2° Fahrenheit per 100 feet, to 27,500 feet where the gradient is 1°. The commercial gas floor is an "assay floor;" it is the level at which the volume of gas obtainable from reservoirs with decreased porosity does not yield a profit. It is concluded that commercial deposits of oil and gas do not extend to the basement rock in deeper basins.