Three petroleum extraction technologies offer potential for large- scale, low-cost, and long-term sequestration of carbon dioxide. Enhanced oil recovery (EOR) using CO₂ injection is a commercially proven process with more than 120 Gt of “value added” CO₂ sequestration potential worldwide. Enhanced gas recovery (EGR) using CO₂ injection is conceptually feasible but has not yet undergone testing. Depleted natural gas fields offer more than 750 Gt of moderate-cost CO₂ sequestration potential, not including EGR. Enhanced coal-bed methane (ECBM) recovery using CO₂ injection is undergoing pilot testing in the United States, with favorable early results. ECBM could be used to sequester more than 150 Gt of CO₂ in coal basins worldwide. Challenges facing large-scale application of geologic sequestration in hydrocarbon fields include: (1) high capture and processing costs of anthropogenic CO₂; (2) inadequate understanding of many petroleum and coal reservoirs, particularly in frontier areas; (3) rigorous monitoring and verification to convince regulators and the public at large that sequestration is secure and long term; (4) achieving recognition and certification from emissions trading systems; and (5) resolving operational conflicts between sequestration and enhanced recovery. These challenges could be overcome by building on existing technologies from the EOR, underground gas storage, and natural CO₂ production and transportation industries and by targeted basic and applied R&D.