Biodegradation scales: Applications and limitations

Biodegradation schemes based on the sequential removal of hydrocarbons are commonly applied to indicate the level of degradation that a petroleum accumulation may have experienced, but inconsistent patterns of degradation are typically seen if large data sets are examined, and oil mixing greatly complicates any attempt at seeking a globally consistent biodegradation scheme. In the Athabasca oil sands province, degradation of steranes, diasteranes and hopanes can occur simultaneously along with the formation of 25-norhopanes, highlighting the difficulties towards assigning a degradation level to reflect the degree of compositional alteration experienced by a heavy oil or bitumen. In addition, the aromatic hydrocarbons, such as the methyl phenanthrene isomers, show differential susceptibility to degradation. Whilst the 9-methylphenanthrene (9-MP) may show selective resistance to degradation compared to 1-methylphenanthrene (1-MP), we have also seen that 9-MP may be removed prior to 1-MP. In some cases, both 9-MP and 1-MP may be strongly removed in comparison with the 3- and 2-methylphenanthrenes. The complex interplay of charge history, reservoir environment (e.g. presence/absence of water legs) and extent and mass transport mechanics of the basal biodegradation reaction zone are most likely to have led to the many and varied petroleum compositions that are a feature of the Athabasca tar sands and many other degraded oil provinces.