Geochemistry International, Vol. 39, No. 8. 2001, pp. 780-792. Translated from Geokhimiya. No. 8. 2001. pp. 862-874. Original Russian Text Copyright © 2001 by Galimov, Rahhani. English Translation Copyright © 2007 by MAIK "Nauka/Interperiodica" (Russia). ============================================================================================================================================ Geochemical Characteristics and Origin of Natural Gas in Southern Iran E. M. Galimov and A. R. Rabbani Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow: 117975 Russia Received February 26, 2001 Abstract—Isotopic geochemical characteristics were determined in gases and condensates from southern Iran, which is one of the world's richest gas-bearing territories. The isotopic composition of methane and its homologues, the carbon isotope composition of CO 2, the chemical composition of gases, the isotopic composition of condensates, and the proportions of individual hydrocarbons in the condensates were analyzed. 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. For instance, the values δ 13 C 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 δ 13 C = -26.22‰. They show also a number of other distinctive features: significant enrichment in nitrogen, occurrence of isotopically light CO 2 ; (δ 13 C = -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 isotopic compositions of condensates from the overlying and underlying the anhydrites zone are similar. The source of gases could be either the rocks of the Dalan Formation or Ordovician-Silurian shales. The formation of gas accompanied by condensate began when the Ordovician-Silurian rocks entered the stage of catagenesis corresponding to R O = 1.0-1.2%, which probably took place at the end of the Jurassic and beginning of the Cretaceous. The gas formation continued simultaneously with the burial of the sedimentary prism. Younger deposits including Permian ones entered the stage of gas generation. The Permian-Triassic collector was filled with gas. The subsequent portions of gas and condensate were accumulated in the lower part of the Paleozoic section. At that time, both the graptolite shales and anhydrites occurred at depths where the process of the thermal chemical reduction of sulfates exerted considerable influence on the composition of gases and organic matter. This resulted in the formation of the observed geochemical characteristics of gases in the lower zone, which are significantly different from the geochemical characteristics of gases from the upper zone. INTRODUCTION The Upper Permian Dalan and Lower Triassic Kangan Formations contain extensive gas reservoirs in the south Iran and Persian Gulf area. In such gas fields as Kangan, Aghar, Nar,Parss and some others in Southern Iran more than 18 % of the proved gas reserves of the world are concentrated[1]. The Southern Parss field, which is one of the largest gas accumulations in the world, contains about 8.5 × 10 12 m 3 of gas in the Dalan and Kangan gas reservoirs. In addition to the Southern Parss field, considerable volumes of Permian gas in Iran have been recovered in the Fars and Boshehr coastal provinces from the Kangan ,Aghar, Nar, Varavi, Mond, Dalan, and Parss structures (Fig. 1). In the Persian Gulf shelf, the Permian section is penetrated by several boreholes drilled into the Parss (Iran), Idd al Shargi, Shaijah (United Arab Emirates), and Iminoko (Bahrain) structures. Gas pools have long been exploited in Bahrain and Saudi Arabia (Damman field), where they are confined to the Upper Permian section. Gas accumulations related to the Permian sequences were first developed in Bahrain. Large gas pools are also known to exist in Saudi Arabia, Kuwait, and the United Arab Emirates. Thus, the zone of the Persian Gulf including southern Iran is one of the world's largest gas-bearing provinces. However., the problems of gas origin in these territories are still open, and the geochemical characteristics of gases are poorly known. The objectives of this paper are to: (1) the characterization of the chemical and isotopic compositions of gases from southern Iran, (2) compare the composition of gases with source rock type and source rock thermal maturity, and (3) interpret the origin of gases. GEOLOGICAL DESCRIPTION In the Persian Gulf zone, the Permian gas basin is known as the Khuff Formation. This sequence is composed mainly of carbonate rocks and exists in Bahrain Qatar, Abu Dhabi, 780