The compositions of the hydrocarbons in oils change as they mature. Several of these changes may be used to estimate the lengths of time required to attain measured hydrocarbon compositions and hence to estimate the ages of these hydrocarbons. The information needed to calculate the hydrocarbon ages includes detailed chemical analyses of the oils, stratigraphic information for the section with which the oil samples are associated, and temperature information from which geothermal gradients may be calculated. The hydrocarbon ages calculated by this method are usually in good agreement with geologically interpreted ages of the oils. The agreement commonly is better for oils from Cenozoic reservoirs than for oils from older reservoirs, and it is better for oils from clastic eservoirs than for oils for carbonate reservoirs.

Applications of the calculation method include the recognition of hydrocarbon assemblages having ages different from those of the reservoirs in which they are found and the recognition of oils that have migrated along faults or across unconformities. Thus, these calculations complement other geochemical methods of relating oils to their probable sources.

The changes of hydrocarbon composition occur both in the C₅-C₇ and C₁₅ + ranges of molecular size. We were able to correlate these changes with time-temperature histories for suites of selected oils believed to be indigenous to the reservoirs in which they are found. For "unknown" oil samples, the correlations, together with stratigraphic and temperature information for the section in which the oil possibly may have its origin, provide the basis for calculations of the ages of the samples' hydrocarbons, and thus the ages of the beds that may have originated the oils. The ages of single samples commonly can be determined with some confidence because the least-squares correlations are based on groups of 29 and 56 indigenous oils from beds of widely diffe ent ages and temperatures.