LOSING WEIGHT: UNDERSTANDING THE METHANOGENIC METABOLISM OF HEAVY HYDROCARBONS IN FOSSIL ENERGY RESERVOIRS

Fossil energy reservoirs harbor thriving microbial communities that can greatly influence the chemical composition of the entrained hydrocarbons. For example, it is believed that over geological time, deep subsurface methanogenic communities have biodegraded hydrocarbon components in light crude oils, resulting in the generation of lower quality heavy crudes that are more difficult to recover. However, understanding the microbial metabolism of hydrocarbons within crude oils can have positive applications related to enhancing energy recovery from mature reservoirs or to treating problems (e.g., paraffin accumulations). While the methanogenic degradation of lighter alkanes (up to C₁₈) and up to 2-ringed aromatic hydrocarbons have been shown, little is known about whether heavier crude oil components can also be biodegraded. Given that heavy oil reservoirs are globally abundant, it is important to understand how microbes proliferate in and thus influence such subsurface environments.

Thus, we sought to determine whether heavier hydrocarbons are susceptible to methanogenic decay, using a 3-ringed aromatic (phenanthrene, PHEN) and a waxy paraffin (C₂₈H₅₈, octacosane, OCT) as model compounds. In PHEN-amended cultures, including enrichments from Alberta’s heavily biodegraded oil sands, enhanced levels of CH₄ and hydrocarbon depletion were observed relative to controls, providing evidence for methanogenic PHEN degradation. GC-MS analysis of PHEN-amended cultures revealed putative metabolites that were not present in controls, providing clues as to how phenanthrene may be metabolized. In OCT-amended enrichments, significantly enhanced CH₄ was also seen compared to substrate-free controls, suggesting methanogenic degradation of C₂₈H₅₈. Microbial community sequencing of this culture revealed syntrophs often associated with anoxic alkane metabolism and diverse methanogens. Further, the detection of the assA gene (the key gene for anaerobic alkane metabolism) substantiated methanogenic OCT biodegradation. Overall, we found that heavier hydrocarbons are susceptible to methanogenic biodegradation, not only broadening the range of hydrocarbons that can be metabolized, but offering promise for the application of microbial technologies to a broader range of fossil energy reservoirs.