Temperature and injection water source influence microbial community structure in four Alaskan North Slope hydrocarbon reservoirs

Abstract

A fundamental knowledge of microbial community structure in petroleum reservoirs can improve predictive modeling of these environments. We used hydrocarbon profiles, stable isotopes, and high-density DNA microarray analysis to characterize microbial communities in produced water from four Alaskan North Slope hydrocarbon reservoirs. Produced fluids from Schrader Bluff (24-27°C), Kuparuk (47-70°C), Sag River (80°C), and Ivishak (80-83°C) reservoirs were collected, with paired soured/non-soured wells sampled from Kuparuk and Ivishak. Chemical and stable isotope data suggested Schrader Bluff had substantial biogenic methane, whereas methane was mostly thermogenic in deeper reservoirs. Acetoclastic methanogens (Methanosaeta) were most prominent in Schrader Bluff samples, and the combined δD and δ(13)C values of methane also indicated acetoclastic methanogenesis could be a primary route for biogenic methane. Conversely, hydrogenotrophic methanogens (e.g., Methanobacteriaceae) and sulfide-producing Archaeoglobus and Thermococcus were more prominent in Kuparuk samples. Sulfide-producing microbes were detected in all reservoirs, uncoupled from souring status (e.g., the non-soured Kuparuk samples had higher relative abundances of many sulfate-reducers compared to the soured sample, suggesting sulfate-reducers may be living fermentatively/syntrophically when sulfate is limited). Sulfate abundance via long-term seawater injection resulted in greater relative abundances of Desulfonauticus, Desulfomicrobium, and Desulfuromonas in the soured Ivishak well compared to the non-soured well. In the non-soured Ivishak sample, several taxa affiliated with Thermoanaerobacter and Halomonas predominated. Archaea were not detected in the deepest reservoirs. Functional group taxa differed in relative abundance among reservoirs, likely reflecting differing thermal and/or geochemical influences.

Keywords: microbiology; petroleum; phylochip; reservoir; stable isotopes.

Figure 1

Schematic showing relative positions of geological formations in the Milne Point and/or Prudhoe Bay fields, Alaskan North Slope. Formations extend over many miles of the Milne Point and/or Prudhoe Bay fields and may tilt in regions, so depth range averages were used to depict the relative positions of the formations; the depth of each formation is not to scale. Prince Creek Formation water is used as make-up water supporting secondary recovery from Schrader Bluff and Kuparuk Formations. Prince Creek water is also used for artificial lift of Sag River produced fluids. Seawater injection is used to support secondary recovery from the Ivishak reservoir. Wells sampled for this study are labeled according to the formation from which they produce.

Figure 2

Hydrogen and oxygen isotope compositions of produced water from North Slope wells. Waters consist of a mix of reservoir formation waters, Prince Creek Formation water, and/or seawater that were injected to support oil production. North Slope seawater is strongly influenced by melting sea ice and runoff during the summer months.

Figure 3

Chromatograms for volatile fractions of North Slope oil from the Schrader Bluff reservoir (SB1 and SB2 are A,B, respectively), the Kuparuk reservoir (K1 and K2 are C,D, respectively) and the Sag River reservoir (E).

Figure 4

Cluster plots based on Bray-Curtis similarities of PhyloChip hybridization intensity data used to compare (A) archaeal and (B) bacterial microbial community structures. Hybridization scores were post-scale-normalized to total array intensity.

Figure 5

Carbon and hydrogen isotopic compositions of dissolved methane samples from Prince Creek Formation, and Schrader Bluff, Kuparuk, and Sag River reservoirs. A shift toward the left for Schrader Bluff samples indicates a significant portion of biogenic methane was produced from the acetoclastic (“fermentation”) pathway. This figure was adapted from Whiticar (1999).