A multi-stage process for the production of an ISO 8217 Table 2 residual marine fuel Product Heavy Marine Fuel Oil from a Feedstock Heavy Marine Fuel Oil that is ISO 8217:2017 Table 2 compliant except for the Environmental Contaminants involves a Reaction System composed of one or more reactor vessels selected from a group reactor wherein said one or more reactor vessels contains one or more reaction sections configured to promote the transformation of the Feedstock Heavy Marine Fuel Oil to the Product Heavy Marine Fuel Oil. The Product Heavy Marine Fuel Oil has an Environmental Contaminant level less than 0.5 wt % and preferably a maximum sulfur content (ISO 14596 or ISO 8754) between the range of 0.05 mass % to 0.5 mass %. A process plant for conducting the process for conducting the process is also disclosed.