NOVELTY - Stimulating microbial hydrogen production in a petroleum-bearing subterranean formation involves, detecting microbial consortium including fermentative syntrophic microorganism(s), in the formation; assessing activeness of the microorganisms; and characterizing the syntrophic microorganism(s) and hydrogen-consuming microorganism(s) (e.g. methanogens, sulfate reducers) of the consortium, for comparison with known characterized microorganisms.    USE - For stimulating microbial hydrogen production in a petroleum-bearing subterranean formation such as oil shale deposits, coal seams, tar sand and other fossil deposits.    ADVANTAGE - The process produces commercial quantities of hydrogen due to microbial degradation of petroleum, by accelerating the fermentative syntrophic microorganisms while reducing the activity of methanogens, sulfate reducers and hydrogen-consuming microorganisms; prevents consumption of hydrogen by methanogenic or sulfate-reducing organisms; and removes hydrogen from site of reaction for hydrogen generation, preventing a thermodynamically unfavorable reaction. Direct generation of hydrogen in petroleum reservoirs, produce an environmentally neutral fuel and chemical feedstock with reduced carbon emission.    DETAILED DESCRIPTION - The hydrogen production stimulation process further involves determining an ecological environment, that promotes in situ microbial degradation of hydrogen by the fermentative syntrophic microorganism(s) of the consortium, or that inhibits in situ microbial degradation of hydrogen by hydrogen-oxidizing microorganism(s), based on the presence of methanogens, sulfate reducers and other hydrogen-consuming microorganisms, obtained from previously executed steps; and modifying the formation environment based on the ecological environment by introducing in the formation, an additive selected from major nutrients, vitamins, trace elements, buffers or waters with different salinities, pH values or complexing agents, preferably phosphorus or ammonium ion. The removal of hydrogen is carried out using purge or sparge gas injected in the petroleum reservoir. The known characterized microorganisms have known physiological and ecological characteristics.