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 (δ 13 C 1 ), ethane (δ 13 C 2 ), propane (δ 13 C 3 ) and butane (δ 13 C 4 ) 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 (δD 1 ), ethane (δD 2 ) and propane (δD 3 ) 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 (δ 13 C 1 <δ 13 C 2 <δ 13 C 3 <δ 13 C 4 , δD 1 <δD 2 <δD 3 ). In most natural gas samples, the fractionation amplitude between methane and ethane is greater than that between ethane and propane (Δ(δ 13 C 2 −δ 13 C 1 ) > Δ(δ 13 C 3 −δ 13 C 2 ), Δ(δD 2 −δD 1 ) > Δ(δD 3 −δD 2 )). (4) The δ 13 C 1 –δ 13 C 2 –δ 13 C 3 , the δ 13 C 1 –δD 1 , δ 13 C 1 –C 1 /C 2+3 , Δ(δ 13 C 2 −δ 13 C 1 )–Δ(δ 13 C 3 −δ 13 C 2 ) and Δ(δD 2 −δD 1 )–Δ(δD 3 −δD 2 ) 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.