BIODEGRADATION OF LOUISIANA WILCOX GROUP CRUDE OILS: EVIDENCE FOR INCREASED MICROBIAL METHANOGENESIS?

The Olla Oil Field, a producing field in the Wilcox Group (Paleocene–Eocene) of north-central Louisiana, has shown positive indicators of methanogenesis (high alkalinities and positive δ¹³C-DIC values of formation waters) compared to adjacent fields also producing from the Wilcox Group. This research evaluated whether the indicators of enhanced methanogenesis were due, in part, to biodegradation of formation oils, increasing available organic substrates for methanogenesis. Initial results confirm that Wilcox oils collected for this study probably share a similar source (down-dip Wilcox Group rocks) and exhibit similar geochemical trends (e.g., similar δ¹³C-C_n values) as documented in extant Wilcox-sourced oil data. However, Olla wells with the strongest indicators of methanogenesis are not the wells with the strongest indicators of biodegradation (e.g., loss of n-paraffins, lowest API gravity values, and highest sulfur contents, indicative of biodegradation). The Olla Field is, at most, biodegraded only slightly compared to fields located up-dip, which show heavy to severe biodegradation and lower indicators of methanogenesis. Oil biodegradation (i.e., the presence of additional methanogenic substrates) therefore may not be the rate-limiting step in microbial methane generation, or high alkalinities should not be used as an indicator of enhanced methanogenesis.