Abstract <p>The presence of isotopically heavy carbon dioxide (CO₂) in gases of hydrocarbon (HC) accumulations and mud volcanoes (MVs) of the South Caspian Basin (SCB) served as a starting point for studying the distribution of a similar phenomenon in deposits of other sedimentary basins. The paper summarizes a large set of data from long-term geochemical studies in different regions of the world. The global distribution of isotopically heavy CO₂ in gases of basins of various tectonic types, both in traditional and unconventional hydrocarbon systems, has been established. The identity of processes involved in its formation has been revealed. Secondary methanogenesis is the most common phenomenon leading to enrichment of CO₂ with the heavy carbon isotope ¹³C with possible subsequent transformation. The formation of isotopically heavy CO₂ is assumed in the depth interval of up to 2000 m. However, under certain geological and tectonic conditions, its appearance at greater depths is possible. In the South Caspian Basin, isotopically heavy CO₂ in gases of HC accumulations and MVs is predominantly formed in closed/partially closed systems under conditions of unbalanced methanogenic biodegradation.</p>