Microbial communities involved in reservoir souring and microbiologically influenced corrosion of shale oil and gas fields are still poorly known. An evaluation of the in situ microbial communities obtained from shale oil and gas reservoirs are necessary to determine souring risks. Notably, our study using samples of injection, produced and facility waters from a Canadian shale oilfield reservoir indicated possible souring risks at various salinities tested. However, when injected with nitrate at high salinity, souring can be prevented. Thus, reinjection of high salinity produced water may be preferable over the current practice of removing produced water by disposal and using low salinity, high sulfate injection water piped in from another subsurface location for oil recovery.