Solid hydrocarbon materials present in subsurface earth formation such as, for example, the coke like residue remaining in a subterranean tar sand deposit which has previously been exploited by controlled oxidation depletion, is converted to a synthesis gas composition by contacting the solid hydrocarbon material with an oxygen enriched gas or essentially pure oxygen and a moderating fluid such as water, steam or carbon dioxide to control the reaction temperature so as to ensure the generation of carbon monoxide and hydrogen within the formation. The oxygen and steam or carbon dioxide may be injected as a mixture or simultaneously by separate injection means, or oxygen may be injected for intervals of time interrupted by brief periods of carbon dioxide, steam or water injection. The effluent is predominantly gaseous carbon monoxide, hydrogen, and lesser amounts of carbon dioxide and methane and, occasionally liquid hydrocarbons. The mixture of carbon monoxide and hydrogen may be utilized directly as a fuel gas, or may be utilized as feed stock for petro chemical manufacturing processes. Carbon dioxide may be separated from the effluent gaseous mixture and recycled with steam into the formation.