Application of carbon and hydrogen isotopes in the study of natural gas origins

Different types of natural gas exhibit distinct carbon and hydrogen isotopic compositions, making these isotopic compositions crucial indicators for identifying gas origins. With ongoing advancements in natural gas exploration technology and the increasing volume of exploration data, our understanding of natural gas origins and sources continues to deepen, and how to update and verify the existing data to ensure the applicability of gas genetic diagrams has become crucial. This study comprehensively analyzes the stable carbon and hydrogen isotope characteristics of different genetic types of natural gases in Sichuan, Tarim, Ordos, Turpan-Hami, Songliao, Northern Jiangsu, Sanshui, Qaidam, and Bohai Bay basins in China, together with abiotic gases from the Lost City of the Middle Atlantic Ridge, and the genetic diagrams related to commonly used carbon and hydrogen isotopes are evaluated. The study yields the following four conclusions: (1) The carbon isotopic values of methane (δ¹³C₁), ethane (δ¹³C₂), propane (δ¹³C₃) and butane (δ¹³C₄) of natural gases from China are from −89.4‰ to −11.4‰ (average of −36.6‰), −66.0‰ to −17.5‰ (average of −29.4‰), −49.5‰ to −13.2‰ (average of −27.3‰), −38.5‰ to −16.0‰ (average of −25.6‰), respectively. (2) The hydrogen isotopic values of methane (δD₁), ethane (δD₂) and propane (δD₃) of natural gases from China range from −287‰ to −111‰ (average of −177‰), −249‰ to −94‰ (average of −158‰), and −237‰ to −75‰ (average of −146‰), respectively. (3) The carbon and hydrogen isotopic distribution patterns among methane and its homologues of natural gases in China are mainly in positive order (δ¹³C₁<δ¹³C₂<δ¹³C₃<δ¹³C₄, δD₁<δD₂<δD₃). In most natural gas samples, the fractionation amplitude between methane and ethane is greater than that between ethane and propane (Δ(δ¹³C₂−δ¹³C₁) > Δ(δ¹³C₃−δ¹³C₂), Δ(δD₂−δD₁) > Δ(δD₃−δD₂)). (4) The δ¹³C₁–δ¹³C₂–δ¹³C₃, the δ¹³C₁–δD₁, δ¹³C₁–C₁/C₂₊₃, Δ(δ¹³C₂−δ¹³C₁)–Δ(δ¹³C₃−δ¹³C₂) and Δ(δD₂−δD₁)–Δ(δD₃−δD₂) diagrams, can be used to identify the gas origin in many different cases, and the combined application between different charts can enhance the identification effect.