Water Droplets in Crude Shale Oil Harbor an Active Heterotrophic Microbial Community Feeding on Both Oil and Necromass

Biodegradation in oil reservoirs was generally thought to take place at the oil–water-transition zone. In shale oil reservoirs, however, there is no aquifer below the oil leg, and it is unclear if biodegradation can take place. Here, we investigated the microbial community in crude oil from a shale oil reservoir in China. We found small water droplets trapped in the shale oil (approximately 2 × 105 droplets/mL oil) originating from ancient groundwater. These droplets contained over 100-fold greater cell density than the production water from shale reservoirs and a high cell viability comparable to those measured in groundwater. Most intact cells were mainly distributed at the oil–water interface of the water droplets, whereas dead cells were located in the water phase. Based on the determined cell density, we conservatively extrapolate the global microbial population in water droplets in oil to be 10–8 to 10–6 of the total microbial biomass in the subsurface. Our study revealed a community of chemoheterotrophs in the water droplets that are most likely feeding on dissolved hydrocarbons and extracellular substances derived from biomass and detritus. The results indicate that small water droplets in shale oil reservoirs harbor a complex and active microbial community, contributing to biodegradation.