Chemical and isotopic compositions of natural gases from the Japanese major oil and gas fields: Origin and compositional change due to migration

Abstract

Carbon isotopic ratios (l3C/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. 5,3C(C!) 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/rc-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 (C,/C2j C2/C3, C2/i-C4, C2 /h-c4i C3/i-C4t C3/i-c4and /i-C4/h-c4) of the UHR and normal gases, it was found that the natural gases tend to lose their hydrocarbons during migration in the order: /j-C4=C3>C2=f-C4>C|. © 1988, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.