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.此前普遍认为，油藏中的生物降解作用主要发生在油水过渡带（oil–water-transition zone）。然而，页岩油藏的油柱下方并不存在含水层，因此生物降解是否能够发生尚不明确。本研究针对中国某页岩油藏的原油样品中的微生物群落展开了调查。我们在页岩油中发现了源自古地下水的微小水滴包裹体，每毫升原油中约含2×10^5个水滴，其细胞密度较页岩油藏的采出水高出100倍以上，且细胞活力与地下水中测得的水平相当。绝大多数完整细胞主要分布于水滴的油水界面处，而死细胞则留存于水相之中。基于测得的细胞密度，我们保守估算全球油内水滴中的微生物总种群量约占地下微生物总生物量的10^–8至10^–6。本研究揭示，水滴中存在以化能异养菌（chemoheterotrophs）为主的群落，这类微生物极有可能以溶解烃类以及源自生物量与碎屑的胞外物质为营养来源。研究结果表明，页岩油藏中的微小水滴蕴含着复杂且活跃的微生物群落，对生物降解过程具有潜在贡献。