This is a partial oxidation process in which heavy hydrocarbonaceous fractions from a petroleum refinery and light liquid paraffinic hydrocarbon extractants are used in the production of synthesis gas, reducing gas, or fuel gas i.e. gaseous mixtures comprising H.sub.2 and CO. In the process, substantially all of the particulate carbon and soot that are simultaneously produced are recovered and recycled to the reaction zone. A dispersion of carbon-soot-light paraffinic hydrocarbon extractant from the decanting zone is combined with a heavy refinery fraction in admixture with a liquid aromatic-rich hydrocarbon to produce a pumpable single liquid phase mixture which is introduced into a fractionation zone where the following streams are separated from each other: (a) a stream of light paraffinic hydrocarbon extractant which is recycled to the decanting zone, (b) a stream comprising at least a portion of the aromatic-rich hydrocarbon which is recycled for mixing with the heavy refinery fraction, and (c) a stream of pumpable dispersion of carbon-soot heavy refinery fraction and any remaining aromatic-rich hydrocarbon which may be used as fuel in the system or exported. Troublesome asphaltene precipitation that causes operational problems and increased viscosity is avoided by the subject process. Comparatively low cost heavy refinery fractions which ordinarily are difficult to handle may be now used along with light liquid paraffinic hydrocarbon extractants.