Biomass in fractured shales: Sources, persistence, and implications for hydrocarbon recovery

The role and impact of microbial activity during energy extraction from hydraulically fractured deep shale environments is poorly understood. However, in other hydrocarbon reservoirs, microorganisms can catalyze a range of deleterious processes including souring, microbially induced corrosion of infrastructure, and clogging of pores and fractures. Therefore, determining microbial signatures and impacts in black shales is important for optimizing the efficiency of gas and oil recovery. We have sampled input waters, flowback fluids, and produced waters from a series of hydraulically fractured wells across the Marcellus and Utica shales in Ohio and Pennsylvania. While geochemical data has revealed a relatively harsh environment rich is dissolved solids and metals, microbial populations derived from input fluids have been detected at all time points, suggesting that biocides are not fully effective. Using assembly-based metagenomic analyses of recovered DNA, we have reconstructed microbial genomes and used this information to isolate dominant community members in the laboratory. Halanaerobium is one such species that can account for up to 95% of the microbial community in many produced water samples. Metabolic characterization of this isolate has revealed the ability to drive sulfidogenesis through the reduction of thiosulfate, and to form biofilms under high pressure and high temperature conditions characteristic of deep fractured shale environments. Overall, these data reveal that microbial biomass persists in hydraulically fractured shales over extended periods of time. The potential for these microorganisms to drive sulfide generation, bio-corrosion, and biofilm formation emphasize the importance of effective microbial control in these ecosystems.