Amounts and ratios of hydrocarbons in nonreservoir rocks (potential source rocks) can be compared with associated oils if, in relatively old and stable geologic situations, the hydrocarbons reach or closely approach a distribution equilibrium between source rock and reservoir. A distribution-equilibrium equation makes possible the calculation of the composition of a hypothetical oil expected from the composition of the hydrocarbons in the nonreservoir rock and from the different tendencies of the hydrocarbons to be absorbed tendencies have been measured and the hypothetical oil compositions calculated and compared with those of the real oils.

The hypothetical and actual oil compositions agree very well in some relatively old and deep sedimentary deposits in which the source rocks and associated oils probably are related genetically. On the other hand, there is relatively poor agreement in some relatively young and shallow deposits, but the agreement appears to improve with increasing depth and age. One explanation for this is that the hydrocarbons in the relatively young and shallow oils may not be related genetically to associated young and shallow source rocks, but came from older and deeper ones. Alternatively the hydrocarbons in the young and shallow reservoirs and in the associated source rocks, in fact may be related genetically, but do not appear to be in distribution equilibrium because primary migration is still o curring.

These findings imply that certain petroleum components, particularly the saturated hydrocarbons, are generated and migrate over relatively long periods of time. The results also may imply that the generation of petroleum components in fine-grained sedimentary rocks causes primary migration. Perhaps it is unreasonable to assume that perfect equilibrium may be reached between source rock and reservoir, but this is the presupposition made when looking for close similarities in compositions of oils and rock extracts in order to correlate oils with source rocks.