The isotopic composition of secondary calcites in 120 samples from biodegraded heavy-oil deposits of the Lower Cretaceous Mannville Group of Alberta ranged in ^dgr¹³C from -1.3 to +14.1 ^pmil PDB. The calcites postdate oil emplacement and are anomalously enriched in C¹³ in comparison with common sedimentary and diagenetic carbonates associated with conventional oil pools and authigenic carbonates of other shallow Cretaceous sediments of Alberta. In contrast to previously reported occurrences of heavy carbonates (Moneray Formation, Kimeridge Clay of Dorset, Caspian syncline), the heavy-oil calcites are not related to organic-rich clayey sediments and no coexisting low ¹³C-carbonates were found. The API gravities of the biodegraded oil corre ate with the ^dgr¹³C values of the carbonate, suggesting a relationship between the biodegradation process and C¹³-enriched cements. Different isotopic systematics were observed in 25 samples from other heavy-oil deposits (Indonesia, Malagasy, Brazil, Sicily, Zaire, California, Kentucky, Utah) whose ^dgr¹³C ratios ranged from -21.6^pmil to +1.1^pmil PDB.

An explanation may involve oxidation of the paraffinic oil components by microorganisms producing alcohols, organic acids, and ketones. A change to anoxic conditions causes fermentation of these biodegradation products, leading to the formation of ¹³C-rich CO₂, which precipitates as heavy carbonate. If anoxic conditions are not achieved, oxidation will persist and lead to the formation of light carbonate cements.

The detection and understanding of the formation mechanism of anomalously heavy carbonates can be useful in both future exploration strategy and exploitation of heavy oil deposits, particularly to enhanced recovery schemes based on CO₂ flooding.

End_of_Article - Last_Page 847------------