A multi-stage process for transforming a high sulfur ISO 8217 compliant Feedstock Heavy Marine Fuel Oil involving a core desulfurizing process that produces a Product Heavy Marine Fuel Oil that can be used as a feedstock for subsequent refinery process such as anode grade coking, needle coking and fluid catalytic cracking. The Product Heavy Marine Fuel Oil exhibits multiple properties desirable as a feedstock for those processes including a sulfur level has a maximum sulfur content (ISO 14596 or ISO 8754) between the range of 0.05 mass % to 1.0 mass. A process plant for conducting the process is also disclosed.