While the demand for hydrocarbon resources has been continuously increasing in the past 150 years, the industry is, however, criticized for carbon dioxide (CO2) emissions and concomitant global warming concerns. The oil and gas industry also face growing pressures in the ongoing energy transition. Generating and producing hydrogen (H2) directly from petroleum reservoirs has the potential to mitigate environmental impacts while revolutionizing the traditional petroleum industry and enabling it to become a clean hydrogen industry. This paper proposes a novel approach to generate high-purity, CO2-free hydrogen from the abundant oil and gas resources in petroleum reservoirs using microwave heating. In this work, laboratory experiments were conducted to validate this scientific proof-of-concept and examine the roles of crushed rocks, catalysts, and water/oil ratio in hydrogen generation from crude oils in a reactor. A maximum of 63% ultimate hydrogen content is obtained in the generated gas mixtures, while the original CO2content in all experiments is negligible (<1%). Catalysts can promote hydrogen generation by accelerating rate and locally enhancing microwave (MW) absorption to create 'super-hot spots'. Water also participates in reactions, and additional hydrogen is generated through water-gas shift reactions. The water-oil ratio in porous rocks affects the ultimate hydrogen yield. Overall, this research demonstrates the great potential of using MW heating to generate high-purity, CO2-free hydrogen from in situ petroleum reservoirs. Further research and wide application of this technology would potentially transform petroleum reservoirs to hydrogen generators, thus mitigating the environmental impacts of traditional petroleum industry while meeting the increasing demand for clean hydrogen energy. This technology would also benefit the safe transition towards a decarbonized society.