Technical Abstract: Hydrogen sulfide is associated with myriad problems in the oil industry including, reservoir souring, contamination of fuel gas and oil, corrosion of metal surfaces, and the plugging of reservoirs and consequent reduced oil recovery due to the precipitation of metal sulfides. A major source of sulfide is the metabolic end product of sulfate reducing bacteria (SRB). These organisms reduce sulfate to sulfide at the expense of the oxidation of a wide range of organic substrates and hydrogen. If sulfate is available, significant amounts of sulfide can be generated posing an environmental risk and undesirable economic consequences. We propose to explore the use of nitrate as a preferred electron acceptor, for the control of sulfide formation in oil fields. This approach is technically feasible because denitrifying bacteria (DNB) tend to outcompete SRB for electron donors when both nitrate and sulfate are available to the resident microflora. In addition, some organisms can oxidize sulfide at the expense of nitrate. The goal of this project is to investigate the relevance of this approach to Oklahoma oil industry. Specifically, we will determine, where the maximal rates of sulfate reduction occur in the Oklahoma oil recovery process. This would include an analysis of reservoir fluids, oil-water transport systems, the oil-water separators, produced water handling systems, and oil storage facilities. This will help identify where the major sulfide production problem is located and help focus the nitrate treatment where it will likely have the most benefit. We will also determine if sufficient microbial potential is inherent to the aforementioned oil recovery facilities to make the nitrate treatment a viable option. That is, it may not be reasonable to presume that SRB, DNB, or sulfide oxidizing bacteria are ubiquitously distributed throughout these facilities. If insufficient microbial diversity is present, then steps can be taken to rectify the situation. In case of insufficient microbial diversity, we will explore conditions for the selective enrichment of DNB, a sulfur oxidizing population, or both and how best to maintain these organisms in the desired locations. If necessary, inoculation with desirable bacterial species (e.g. Thiobacillus denitrificans) may also be considered. Armed with this knowledge, we will scale up the treatment process to demonstrate its effectiveness at an oil field in Oklahoma.