Composition of Crude Oils

Abstract

Petroleum originates from the bitumen of source rocks. Migration, however, and especially primary migration, is the cause of considerable changes in composition when bitumen is compared to petroleum. On the one hand, only a small amount of the total dispersed bitumen is mobilized and transferred into carrier or reservoir rocks, and an even smaller amount is accumulated in oil fields. In producing areas the ratio of reservoired oil to dispersed bitumen ranges from 1:10 to 1:10,000⁹. On the other hand, such drainage is selective, as shown by the gross comparison between the bitumen present in source rocks and the corresponding crude oil in reservoirs. The heaviest and most polar molecules, like asphaltenes, are strongly adsorbed on the source rock and can hardly be expelled into the reservoir. Therefore, the common distribution of petroleum constituents in crude oil parallels the adsorptive behavior of these constituents, i.e., the least polar saturated hydrocarbons are most frequent, then follow aromatics and benzothiophenes, and least abundant are the most polar and most easily adsorbed resins and the least soluble asphaltenes. Even within a given structural type like n-alkanes, light molecules seem to be favored compared to heavy ones.