This article explores the emerging concept of in situ hydrogen generation (ISHG) within oil reservoirs as a transformative pathway toward clean energy and hydrocarbon upgrading. With the global energy sector facing decarbonization pressures and limitations of conventional hydrogen production, ISHG offers a promising alternative by turning mature or depleted oil fields into subsurface reactors for hydrogen and upgraded oil co-production. Through thermochemical processes such as in situ combustion (ISC), steam methane reforming (SMR), coke gasification and water-gas shift reactions (WGSR), hydrogen is produced underground while carbon dioxide and by-products remain trapped, minimizing surface... More >