Abstract: Implementations relate to treatment of a petroleum reservoir to produce a hydrogen-bearing gas. Implementations may include methods and systems for processing a reservoir to generate hydrogen in-situ and produce a large fraction of the generated hydrogen to the surface. Processing the reservoir may include injecting oxygen into the reservoir to enable combustion within the reservoir to create heat and carbon monoxide in the reservoir. The heat may generate steam from both boiling of in-situ formation water and steam generated from the oxidation reactions. The generated heat may enable gasification, steam reforming, and aquathermolysis reactions and both the heat and carbon monoxide enables the water-gas shift reaction. Each such reaction may generate hydrogen within the reservoir.