Biodegradation characteristics of bitumen from the Upper Devonian carbonates (Grosmont and Nisku formations) in Alberta, Canada

In this study, we investigated the biodegradation processes of bitumen from the Upper Devonian carbonates (Grosmont and Nisku formations) in Alberta using GC and GC-MS analyses of four drilling cores. The analyzed samples contain notable rich extracted bitumen, which is predominantly composed of NSO and asphaltene compounds and has lesser amounts of saturated and aromatic hydrocarbons (HCs). The GC and GC-MS results for both saturated and aromatic HCs indicate that there are considerable variations among the samples, which are primarily attributed to the differences in the degree of biodegradation involving microbial activity. The sulfur content and its isotope values for the samples also support biodegradation. Three groups can be classified based on the biomarker distributions as follows: (1) in Group 1, C 19 -C 25 tricyclic terpanes are most abundant, particularly the C 23 compounds; (2) Group 2 has no C 30 -C 35 αβ hopanes and more abundant 25-norhopanes than the other two groups; and (3) Group 3 is similar to Group 1, but generally has less abundant C 19 -C 25 tricyclic terpanes than Group 1. The reservoir alteration processes have notably influenced the biomarker properties of the samples. The most pronounced effects are the reduction of the amounts of C 30 + hopanes and increasing demethylated hopanes, particularly in Groups 1 and 2, but not in Group 3. For Group 3, unlike Group 1, the degradation of hopanes apparently does not result in demethylated hopanes, indicating that there is another pathway to produce hopanes and/or destroy demethylated hopanes. These findings indicate that the biodegradation processes that occurred in the Grosmont and Nisku formations have been spatially and vertically varied due to the different environmental conditions and microbial processes.