Methods and systems relate to the in-situ removal of heavy metals such as mercury, arsenic, etc., from produced fluids such as gases and crudes from a subterranean hydrocarbon-bearing formation. A sufficient amount of a fixing agent is injected into formation with a dilution fluid. The fixing agent reacts with the heavy metals forming precipitate, or is extracted heavy metals into the dilution fluid as soluble complexes. In one embodiment, the heavy metal precipitates remain in the formation. After the recovery of the produced fluid, the dilution fluid containing the heavy metal complexes is separated from the produced fluid, generating a treated produced fluid having a reduced concentration of heavy metals. In one embodiment, the dilution fluid is water, and the wastewater containing the heavy metal complexes after recovery can be recycled by injection into a reservoir.