title: Environmental Drivers of Differences in Microbial Community Structure in Crude Oil Reservoirs across a Methanogenic Gradient.
abstract: Stimulatingin situmicrobial communities in oil reservoirs to produce natural gas is a potentially viable strategy for recovering additional fossil fuel resources following traditional recovery operations. Little is known about what geochemical parameters drive microbial population dynamics in biodegraded, methanogenic oil reservoirs. We investigated if microbial community structure was significantly impacted by the extent of crude oil biodegradation, extent of biogenic methane production, and formation water chemistry. Twenty-two oil production wells from north central Louisiana, USA, were sampled for analysis of microbial community structure and fluid geochemistry. Archaea were the dominant microbial community in the majority of the wells sampled. Methanogens, including hydrogenotrophic and methylotrophic organisms, were numerically dominant in every well, accounting for, on average, over 98% of the total Archaea present. The dominant Bacteria groups werePseudomonas, Acinetobacter, Enterobacteriaceae, and Clostridiales, which have also been identified in other microbially-altered oil reservoirs. Comparing microbial community structure to fluid (gas, water, and oil) geochemistry revealed that the relative extent of biodegradation, salinity, and spatial location were the major drivers of microbial diversity. Archaeal relative abundance was independent of the extent of methanogenesis, but closely correlated to the extent of crude oil biodegradation; therefore, microbial community structure is likely not a good sole predictor of methanogenic activity, but may predict the extent of crude oil biodegradation. However, when the shallow, highly biodegraded, low salinity wells were excluded from the statistical analysis, no environmental parameters could explain the differences in microbial community structure. This suggests that the microbial community structure of the 5 shallow, up-dip wells was different than the 17 deeper, down-dip wells. Also, the 17 down-dip wells had statistically similar microbial communities despite significant changes in environmental parameters between oil fields. Together, this implies that no single microbial population is a reliable indicator of a reservoir's ability to degrade crude oil to methane, and that geochemistry may be a more important indicator for selecting a reservoir suitable for microbial enhancement of natural gas generation.
authors.affiliation: Eastern Energy Resources Science Center, U.S. Geological Survey Reston, VA, USA.
authors.forename: Jenna L
authors.initials: JL
authors.lastname: Shelton
authors.affiliation: National Research Program-Eastern Branch, U.S. Geological Survey Reston, VA, USA.
authors.forename: Denise M
authors.initials: DM
authors.lastname: Akob
authors.affiliation: Eastern Energy Resources Science Center, U.S. Geological SurveyReston, VA, USA; Department of Hydrology and Atmospheric Sciences, University of ArizonaTucson, AZ, USA.
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: McIntosh
authors.affiliation: Department of Ecology and Evolutionary Biology, University of ColoradoBoulder, CO, USA; Cooperative Institute for Research in Environmental Science, University of ColoradoBoulder, CO, USA.
authors.forename: Noah
authors.initials: N
authors.lastname: Fierer
authors.affiliation: Department of Civil and Environmental Engineering, Colorado School of Mines Golden, CO, USA.
authors.forename: John R
authors.initials: JR
authors.lastname: Spear
authors.affiliation: Eastern Energy Resources Science Center, U.S. Geological Survey Reston, VA, USA.
authors.forename: Peter D
authors.initials: PD
authors.lastname: Warwick
authors.affiliation: Department of Civil and Environmental Engineering, Colorado School of MinesGolden, CO, USA; Hydrologic Science and Engineering Program, Colorado School of MinesGolden, CO, USA.
authors.forename: John E
authors.initials: JE
authors.lastname: McCray
keywords: Gulf Coast Basin; hydrogeochemical tracers; methane; methanogenic crude oil biodegradation; microbial ecology; oil field