Non-fossil gaseous fuel, evolved in underwater carbon arcing, and characterized by significant heat content and substantial freedom of its combustion effluents from noxious gases and/or particulates, is similarly useful in whole or part as an additive to predominantly hydrocarbon fuels—whether in bulk storage or transport, flowing in a pipeline, fueling a cutting/welding torch, or fueling an internal-combustion engine. Dosing a predominantly hydrocarbon fuel with all or a selected part of such gaseous fuel mixture inhibits leakage and substantially diminishes noxious effluent gases and particulates as characteristic of the combustion of predominantly hydrocarbon fuels.