Wellhead Samples of High-Temperature, Low-Permeability Petroleum Reservoirs Reveal the Microbial Communities in Wellbores

In order to understand the microbial processes in petroleum reservoirs, most liquid samples are collected directly from wellheads because this method is convenient and causes no interruption to oil production. However, the wellhead fluids include the microorganisms inhabiting the wellbore, which could distort the understanding of the community in the reservoir. Therefore, the wellbore community as a possible contaminator should be investigated beforehand. A new method is proposed in this paper to exploit the extreme conditions of selected reservoirs, including low permeability (<40 × 10^–3 μm²) and high temperature (>120 °C), to retain and inactivate the microbes in strata, so that the wellbore community can be cost-effectively investigated by directly sampling wellhead fluids. In three selected reservoirs, the results of 16S rRNA gene clone libraries show that both bacterial and Archaeal domains were dominated by Firmicutes, Proteobacteria, and Euryarchaeota with low richness (number of operational taxonomic unit, 10–15), while no hyperthermophiles (>110 °C) were detected. Combining the environmental adaptability and the significant dissimilarity between the three communities, it is suggested that in these reservoirs the wellhead samples could represent the communities inhabiting the wellbores, instead of the reservoirs. The coexistence of aerobes and anaerobes indicate that petroleum production processes (such as drilling and workover) were continuously introducing substances (biomass and oxygen) from the upper strata and surface into the wellbore. Of these exogenous microorganisms, mesophiles and aerobes could gradually become dominant over time because growth rates are faster than in thermophiles and anaerobes. Therefore, after decades of oil production, the wellbore community (providing a significant temperature variation along the wellbore) would have a great possibility to become distinct from the reservoir community. This could explain some common problems frequently appearing in field studies, such as the unexpected detection of aerobes and/or mesophiles in the wellhead fluids from anaerobic and thermophilic reservoirs.