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CONNAN, JACQUES, Elf Aquitaine, Pau, France

ABSTRACT: Advances in our Knowledge of Biodegradation of Hydrocarbons in Reservoirs

Biodegradation of hydrocarbons in reservoirs is a widespread phenomenon that is currently observed by petroleum organic geochemists in most sedimentary basins. This basic phenomenon is responsible for the occurrence of large, heavy oil deposits referred to as tar mats or tar belts.

Biodegradation of crude oils takes place in reservoirs in which oil-eating bacteria may thrive. For this reason, effective and present biodegradation effects are not observed at subsurface temperatures higher than 70-80 degrees C.

Significant compositional changes, especially at a molecular level, still remain linked to the aerobic biodegradation of crude

oils. Under favorable circumstances, both alkanes and aromatics are degraded, but when nutrients (N, P, O2) are impoverished, aromatics seem to be preferentially removed. Biodegradation extends also to sulfur-bearing aromatics with a preferential removal of alkylated structures (dialkylthiophenes, dialkylthiolanes, etc.).

Changes in molecular patterns are used to assess degrees of biodegradation in crude oils, the most readily consumable structures being the long chain alkylated ones viz n-, iso-, anteiso-, alkylcylohexyl-, alkylcyclopentyl-alkanes for alkanes, alkyl-, and alkyl-methyl- or alkyl-dimethyl- benzenes for aromatics. The most bacterially resistant structures are polycyclic ones, e.g., tricyclic terpanes, secohopanes, hexahydrobenzohopanes, 25-norhopanes, neohopanes in alkanes; benzohopanes, 8-14 monoaromatized secohopanoids, monoaromatic and triaromatic steroids in aromatics.

The in-reservoir biodegradation of hydrocarbons do not generate new hydrocarbons, e. g., 25-norhopanes as proposed by several authors. In fact, the selective removal of less resistant structures concentrates preexisting minor families that were not detected on the unaltered crude due to their low absolute concentration. Consequently, the molecular spectrum found in severely biodegraded oils may be considered as highly diagnostic of a part of the primary genetic spectrum of each oil.

In outcrop samples, biodegradation is associated with other complementary phenomena such as photooxidation, oxidation, inspissation, evaporation, water washing, etc. Of particular importance are weathering effects linked to oxidation, which entail drastic compositional changes, with neogenesis of resins, asphaltenes, and even insoluble residue.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.