LINKING MICROBIAL COMMUNITY PRESENCE AND DIVERSITY TO FORMATION WATER GEOCHEMISTRY, CRUDE OIL BIODEGRADATION, AND METHANOGENESIS

Engineering microbial communities in the subsurface to recover crude oil as natural gas may be one strategy for recovering additional natural gas after traditional recovery operations. Yet, little is known about what geochemical parameters drive microbial population dynamics in biodegraded, methanogenic oil reservoirs. To investigate microbial community impacts and methanogenic crude oil biodegradation, 22 wells along a subsurface hydrogeochemical gradient in Louisiana, USA were sampled for microbial community analysis and geochemical analysis of produced water and crude oil. Microbial communities were characterized using the Illumina MiSeq 16S iTag next-generation sequencing platform. Archaea dominated the communities in the majority of the wells ranging from 30 to 85% of total reads, with methanogens being numerically dominant in every well. The methanogen populations included hydrogenotrophic and methylotrophic organisms. The dominant groups of Bacteria were Pseudomonas, Acinetobacter, Enterobacteriaceae, and Clostridiales, which have been identified in other microbially-altered oil reservoirs. The only groups of Bacteria shared between all wells were Alicyclobacillus spp. and Methylobacterium komagatae. A statistical comparison of microbial community structure to formation fluid geochemical parameters revealed that relative degree of biodegradation, salinity, well depth, and spatial location within the Gulf Coast basin are the major drivers of microbial diversity. There was no correlation between the community composition and relative methane production, assessed using geochemical indicators and methane production data. Despite the abundance of methanogens, methanogenic activity was not predicted solely based on the microbial community composition. Crude oil biodegradation did appear to be correlated with community composition and produced water geochemistry. Together, this suggests that a single microbial population is not an indicator of a reservoir’s ability to degrade crude oil to methane. However, methanogenic populations are abundant and available to respond to the microbial cascade of crude oil biodegradation.