A review of gas leakage from the subsurface into the environment

Subsurface gas leakage represents a complex and multifaceted issue in geosciences and energy sectors. It encompasses the unintentional release of gases such as methane, hydrogen, carbon dioxide, and hydrogen sulfide from (deep) subsurface into the environment. These leakages originate from both natural processes and anthropogenic activities, including hydrocarbon extraction, geothermal energy production, and the operation or abandonment of storage sites. This article provides a comprehensive review of main gases of concern, namely methane (CH₄), hydrogen (H₂), hydrogen sulfide (H₂S), and carbon dioxide (CO₂). It explains why seepage of these gases to the environment is important. Also, the sources of gas leakage are discussed, and categorized into natural, anthropogenic, and combined origins. Finally, major cases of gas leakage around the world, from natural or anthropogenic sources, are presented and their main features and effects are discussed. The paper provides a comprehensively synthesized description of leakage of multiple gases from both natural and anthropogenic subsurface systems within a unified framework.