AN INTERPRETATION FOR OIL AND BITUMEN RE-OS AGES

The research and application of Re-Os isotope system have achieved a lot over the past 25 years. ¹⁸⁷Re-¹⁸⁷Os radioactive isotope geochronometer can yield precise deposition ages of organic-rich sedimentary rocks (ORS). When applied on oil and bitumen, the ages we get are interpreted as the time for oil generation and migration. However we have not got a clear idea of which specific event we are dating when the Re-Os method is conducted on oil and bitumen. This paper tries to explain the mechanisms that control Re-Os behavior during various geological processes by focusing on the types of the geological processes. And based on the analysis we can infer that the ages we get from oil and bitumen are probably the time for oil generation rather than migration.

Geological processes are divided into two types, physical processes and chemical processes.Physic processes may change the abundances of the elements but the Re/Os ratio can remain constant because they are hosted in the same organic matter. Biodegradation and water washing mostly remove the light fractions of the oil. The progressive precipitation of petroleum asphaltenes removes the heavy fractions of the oil. They have little effect on Re and Os isotope system.

While the chemical processes refer to the reactions with other substances, for example, thermochemical sulfate reaction (TSR) and hydrothermal fluid flushing. Chemical changes are different because the new generated substances may react with Re and Os. Products of TSR include solid bitumen, mental sulfides, carbonate minerals and various organosulfur compounds in the oil. The temperature and chemistry of TSR fluids may consume the Re and Os hosted in organosulfur ligands by the oxidation of the ligands, or lead to the enrichment of Re and Os by generating new organosulfur ligands. Similarly, hydrothermal fluid is rich in various metals and some may replace the Re and Os in organic matters because of the similar chemical properties. As a consequence, the fraction of Re/Os happens within the interface between the hydrothermal fluid and the shales.

In the oil generation progress, the organic matters mainly experience chemical changes. While the migration process mainly includes physical changes. This suggests that the Re-Os isotope ages we get from oil and bitumen are actually the time for oil generation.