Microbial processes sour oil, corrode equipment, and degrade hydrocarbons at an annual global cost to the oil and gas industry of nearly $2 billion. However, top-down control of these microbial processes can reduce their damage and enhance oil recovery. Here, we screened microbial communities from five oil wells in the Illinois basin and evaluated nutrient injection strategies to control metabolism and community composition. Molasses with molybdate supplementation stimulated gas and organic acid production while suppressing corrosive H 2 S formation in samples from two wells. These changes were accompanied with significant reshaping of the microbiome community. Simulations of field operations via a lab-scale mini-coreflood validated that oil well microbiomes can be engineered to inhibit deleterious H 2 S and shape oil hydrocarbon composition in situ. These pilot studies validate the economic potential and sustainability of top-down approaches for microbiome engineering to control microbes in oil extraction and enhance the economic viability of oil recovery.comprehensive 2D-gas chromatography, cultivation screening, microbial enhanced oil recovery, microbiome engineering, miniature coreflood, top-down design | BACKGROUNDFossil fuels are likely to remain a significant energy source for at least the next 30 years 1 as economies transition globally toward more renewable energy sources. 2 Extracting trapped oil from aging wells can not only increase the overall production of existing wells but also minimize the need to drill new wells and, ultimately, decrease the cost of extracting oil from existing reservoirs. 3 Expensive polymer and/or surfactant/polymer formulations that require large amounts of funding and research effort are used to extract oil not recovered by primary extraction techniques. 4 Additionally, native and exogenous microbes have been harnessed as a more cost-effective 5 alternative for secondary and tertiary oil recovery-an approach coined microbial enhanced oil recovery (MEOR). 6 However, microbial activity in oil wells, can also be disruptive to oil recovery and/or alter its composition. 6 For example, microbes can degrade and sour oil, and produce metabolites that corrode the well casing, flowlines, and pipelines. 7

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In this work, two methods were investigated for determining the composition of carbonate solvent systems used in lithium-ion (Li-ion) battery electrolytes. One method was based on comprehensive two-dimensional gas chromatography with electron ionization time-of-flight mass spectrometry (GC×GC/EI TOF MS), which often enables unknown compound identification by their electron ionization (EI) mass spectra. The other method was based on comprehensive two-dimensional gas chromatography with flame ionization detection (GC×GC/FID). Both methods were used to determine the concentrations of six different commonly used carbonates in Li-ion battery electrolytes (i.e., ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and vinylene carbonate (VC) in model compound mixtures (MCMs), single-blind samples (SBS), and a commercially obtained electrolyte solution (COES). Both methods were found to be precise (uncertainty < 5%), accurate (error < 5%), and sensitive (limit of detection <0.12 ppm for FID and <2.7 ppm for MS). Furthermore, unlike the previously reported methods, these methods do not require removing lithium hexafluorophosphate salt (LiPF6) from the sample prior to analysis. Removal of the lithium salt was avoided by diluting the electrolyte solutions prior to analysis (1000-fold dilution) and using minimal sample volumes (0.1 µL) for analysis.