Comparison of the A.P.I. gravity and percentage content respectively of Miocene, Oligocene, and Eocene Gulf Coast crude oils from depths of 3,000 to 5,000 feet with the oils of corresponding age from depths of 5,000 to 7,000 feet shows that the A.P.I. gravity of the fractions and the percentage content of the lower boiling fractions tend to increase with depth and age. The base of both the lower and the higher boiling fractions tends to change with increasing depth and age from naphthenic to intermediate, and almost to paraffinic. In order of similarity, those age-depth groups of crude oils are arranged: shallower Miocene, shallower Oligocene, deeper Miocene, shallower Eocene, deeper Oligocene, deeper Eocene. That variation of the Gulf Coast crude is consistent with transformation under the influence of time and some factor or factors proportional to depth below the surface but is inconsistent with alteration under the influence of time and some factor or factors proportional to nearness to the surface. The theory of evolution of the Gulf Coast crude oils from heavy naphthenic ancestral oils under the influence of time and some depth factor or factors therefore still is advocated. The evolution consists in, (a) decrease of the specific gravity of all but the lowest boiling fractions, (b) increase in percentage content of the lower boiling fractions, and (c) transformation of the base from naphthenic toward and almost to paraffinic. Cracking can not be the main reaction in the evolution. Methanation, possibly plus cracking, is regarded as the most plausible explanation of the evolution. This law of the variation of crude oil is only one of many; and occurrences of one type do not preclude the existence of other types.
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Donald Clinton Barton (1937) studied this question.