Abstract: A multi-stage process for reducing the environmental contaminants in a ISO 8217 compliant Feedstock Heavy Marine Fuel Oil involving a core desulfurizing process and an ionic liquid extraction desulfurizing process as either a pre-treating step or post-treating step to the core process. 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 0.5% wt. A process plant for conducting the process is also disclosed.