بررسي منشاء گازهاي مخازن پرمين - ترياس جنوب ايران

A Study of Natural Gas Origins in Permian-Triassic Gas Reservoirs in Southern Iran

The Upper Permian Dalan and Lower Triassic Kangan Formations contain extensive gas reservoirs in the south Iran. Gas samples from Upper Permian and Lower Triassic gas reservoirs are composed of 87% methane-dominated light hydrocarbon, C02, N2 with minor quantities of noble gases. The analytical results demonstrated fairly uniform chemical compositions for gases and isotopic compositions of methane and its homologues in the section overlying the anhydrite zone (Nat- member). For instance, the carbon isotope composition of methane in samples from this part of the section (Late Permian Dalan Formation, zone D,C,E and Early Triassic Kangan Formation) varied from -39.95 to - 41.28%. This allows us to conclude that gas accumulations in the carbonate collectors of the Kangan and upper part of Dalan.formations represent a single gas reservoir. Quite different characteristics are displayed by gases from the lower zone (below the anhydrite) of the Dalan Formation (zone G). These gases are characterized by considerable depletion in the light carbon isotope. For instance, methane from the lower part of Dalan formation have -26.22%. They show also a number of other distinctive feature significant enrichment in nitrogen, occurrence of isotopically light C02; (-21.87%), and an inversion in the isotope relationships of ethane and propane. These peculiarities suggest that the composition of gases in this zone was modified by the process of the thermal chemical reduction of anhydrite. The source of gases could be either the rocks of the Dalan Formation or Ordovician-Silurian shales.