ABSTRACT

Carbon isotopic ratios (¹³C/¹²C) and chemical compositions of methane (C₁), ethane (C₂), propane (C₃), i-butane (i-C₄) and n-butane (n-C₄) were measured for natural gases from the Japanese major oil and gas fields. The C₁/(C₂+C₃) vs. δ¹³C(C₁) plot suggests that most samples analyzed in the present study are of thermogenic origin with a minor contribution of biogenic gases. Some gases show unusually high ratios of C₂/C₃ and i-C₄/n-C₄, and low ratios of C₃/i-C₄ (Unusual-Hydrocarbon-Ratio gases: UHR gases). The carbon isotopic ratios and hydrocarbon compositions strongly suggest that these unusual ratios were caused by the chromatographic effect of sediments during migration of the gases. By comparing hydrocarbon ratios (C₁/C₂, C₂/C₃, C₂/i-C₄, C₂/n-C₄, C₃/i-C₄, C₃/n-C₄ and i-C₄/n-C₄) of the UHR and normal gases, it was found that the natural gases tend to lose their hydrocarbons during migration in the order: n-C₄ ≅ C₃ > C₂ = i-C₄ > C₁.