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对世界上天然气最富区之一的伊朗南部天然气和凝析油的同位素地球化学特征进行了探讨,并分析了甲烷及其同系物的同位素组成,CO_2的碳同位素组成,天然气的化学组成和凝析油的同位素组成及其凝析油的轻烃组成。分析结果表明石膏层之上气藏具有相当一致的天然气化学组成和甲烷及其同系物同位素组成。例如,在该气藏剖面(上二叠统Dalan组D段和下三叠统Kangan组)的气样中甲烷的δ~(13)C值分布在—39.95‰～-41.28‰范围。证明Kangan组和Dalan组(D段)碳酸盐岩储层中气体代表同一天然气藏。而Dalan组(G段)的下部(石膏层之下)的天然气的特性具有很大的差异,这些天然气的~(12)C明显亏损,例如Homa油田深度为3 600～3 655 m的甲烷δ~(13)C 为—26.22‰。它们也表现出许多其它显著特征:氮的异常富集、轻CO_2碳同位素(δ~(13)C =21.87‰)以及乙烷和丙烷同位素反序列。这些特性显示了这个地区的天然气组成由于石膏层的热化学还原反应而变化。石膏层下气藏中的凝析油碳同位素组成几乎没有差别。天然气来源于Dalan组源岩或者奥陶系—志留系页岩。当奥陶系—志留系源岩进入成岩阶段,即相应的R_o=1.0％～1.2％,可能发生在侏罗系晚期和白垩系初期,天然气随着凝析油的产生而形成。这些气体的形成同时伴随着沉积层的连续埋藏。
The isotopic geochemical characteristics of natural gas and condensate in southern Iran, one of the most abundant natural gas in the world, are discussed. The isotopic composition of methane and its homologues, the carbon isotope composition of CO 2, the chemical composition of natural gas, and condensate Analysis of oil isotope composition and condensate light hydrocarbon composition. The results of the analysis show that the gas reservoir above the gypsum layer has a fairly consistent chemical composition of natural gas and isotopic compositions of methane and its homologues. For example, the δ 13 C values of methane in the gas reservoir profile (section D of the Upper Permian Dalan Group and Kangan Formation of the Lower Triassic) range from -39.95 ‰ to -41.28 ‰. It is proved that the gas in the carbonate reservoirs of Kangan and Dalan (D) represent the same natural gas reservoir. However, the characteristics of natural gas in the lower part of the Dalan group (below the gypsum layer) are quite different. The ~ (12) C of these natural gas is obviously depleted. For example, the methane δ of the Homa field with a depth of 3 600-3 655 m ~ (13) C is -26.22 ‰. They also show many other notable features: abnormal accumulation of nitrogen, light CO 2 carbon isotope (δ ~ (13) C = 21.87 ‰) and ethane and propane reverse isotope sequences. These characteristics show that the natural gas composition in this area varies due to the thermochemical reduction of the gypsum layer. Gypsum gas reservoirs in the condensate carbon isotope composition almost no difference. Natural gas is sourced from Dalan source rocks or Ordovician-Silurian shales. When the Ordovician-Silurian source rock enters the diagenetic stage, ie, R_o = 1.0% ~ 1.2%, it may occur in the late Jurassic and Early Cretaceous. Natural gas is formed with the formation of condensate. The formation of these gases is accompanied by the continuous burial of sedimentary layers.