A process for reducing the environmental contaminants in a ISO8217 compliant Feedstock Heavy Marine Fuel Oil, the process involving: mixing a quantity of the Feedstock Heavy Marine Fuel Oil with a quantity of Activating Gas mixture to give a feedstock mixture; contacting the feedstock mixture with one or more catalysts to form a Process Mixture from the feedstock mixture; separating the Product Heavy Marine Fuel Oil liquid components of the Process Mixture from the gaseous components and by-product hydrocarbon components of the Process Mixture and, discharging the Product Heavy Marine Fuel Oil. The Product Heavy Marine Fuel Oil is compliant with ISO 8217A for residual marine fuel oils and has a sulfur level has a maximum sulfur content (ISO 14596 or ISO 8754) between the range of 0.05% wt. to 1.0 mass. The Product Heavy Marine Fuel Oil can be used as or as a blending stock for an ISO 8217 compliant, IMO MARPOL Annex VI (revised) compliant low sulfur or ultralow sulfur heavy marine fuel oil. A device for conducting the process is also disclosed.