Geochemistry of Resins and Asphaltenes.

The heavy constituents of crude oil (resins and asphaltenes) play an important role in generation and accumulation of petroleum, and also in production by conventional and enhanced oil recovery processes. Asphaltenes and resins are considered here as small fragments of kerogen, with a comparable overall structure: they may act as intermediate compounds in oil generation by thermal breakdown of kerogen. Furthermore, pyrolysis of asphaltenes separated from a degraded crude oil is able to generate a new saturated hydrocarbon fraction which duplicates the original one, now degraded, thus providing a new correlation tool for crude oils. Heavy constituents are not favored, as compared to hydrocarbons, during migration from the source rock to the reservoir, where they are less abundant. The occurrence of a macrostructure of asphaltenes and resins, such as micelles or aggregates, is probably responsible for the high viscosity of heavy oils. A better understanding of the macrostructure may result in new techniques to lower viscosity and improve recovery of heavy oils.