Geochemical characteristics and origin of natural gas in southern Iran

Isotopic geochemical characteristics were determined in gases and condensat es from southern Iran, which is one of the world's richest gas-bearing terr itories. The isotopic composition of methane and its homologues, the carbon isotope composition of CO2, the chemical composition of gases, the isotopi c composition of condensates, and the proportions of individual hydrocarbon s in the condensates were analyzed. The analytical results demonstrated fai rly uniform chemical compositions for gases and isotopic compositions of me thane and its homologues in deposits overlying the anhydrite zone. For inst ance, the values delta C-13 of methane in samples from this part of the sec tion (Late Permian Dalan Formation, zone D and Early Triassic Kan gan Forma tion) varied from -39.95 to -41.28 parts per thousand. This allows us to co nclude that gas accumulations in the carbonate collectors of the Kangan and Dalan formations (zone D) represent a single gas reservoir. Quite differen t characteristics are displayed by gases from the lower zone (below the anh ydrite) of the Dalan Formation (zone G). These gases are characterized by c onsiderable depletion in the light carbon isotope. For instance, methane fr om a depth of 3600-3655 m in the Homa field yields delta C-13 = -26.22 part s per thousand. They show also a number of other distinctive features: sign ificant enrichment in nitrogen, occurrence of isotopically light CO2 (delta C-13 = -21.87 parts per thousand), and an inversion in the isotope relatio nships of ethane and propane. These peculiarities suggest that the composit ion of gases in this zone was modified by the process of the thermal chemic al reduction of anhydrite. The isotopic compositions of condensates from th e deposits overlying and underlying the anhydrites are indistinguishable. T he source of gases could be either the rocks of the Dalan Formation or Ordo vician-Silurian shales. The formation of gas accompanied by condensate bega n when the Ordovician-Silurian rocks entered the stage of catagenesis corre sponding to R-0 = 1.0-1.2%, which probably took place at the end of the Jur assic and beginning of the Cretaceous. The gas formation continued simultan eously with the burial of the sedimentary prism. Younger deposits including Permian ones entered the stage of gas generation. The Permian-Triassic col lector was filled with gas. The subsequent portions of gas and condensate w ere accumulated in the lower part of the Paleozoic section. At that time, b oth the graptolite shales and anhydrites occurred at depths where the proce ss of the thermal chemical reduction of sulfates exerted considerable influ ence on the composition of gases and organic matter. This resulted in the f ormation of the observed geochemical characteristics of gases in the lower zone, which are significantly different from the geochemical characteristic s of gases from the upper zone.