Chemical and Isotopic Compositions of Gaseous Hydrocarbons in Liaohe Basin
摘要: 辽河盆地是一新生代断陷盆地,盆地内天然气资源丰富,天然气成因复杂.本文通过对该盆地80个天然气样中烃类气体组分组成及同位素组成的分析研究,探讨了该盆地烃类气体的成因,为盆地天然气勘探提供了地球化学方面的信息.Abstract: Liaohe Basin is a fault basin of rift valley type formed in Cenozoic Era. In this basin there are several sets of natural gas source rocks and good geological circumstances for the origination of natural gases. It has been proven by geologic exploration that there are plenty of natural gas resources in this basin. The result of the analysis of the chemical and isotopic compositions of 80 natural gas samples shows that the main components of the natural gases in Liaohe Basin are hydrocarbons. The proportion of hydrocarbons in the total natural gas components is higher than 90%. The carbon isotopic composition of methane ranges mainly from -35‰ to 55‰ (PDB), with the lightest -60.7‰ and heaviest -29‰. The hydrogen isotopic composition of methane ranges from -185‰ to 275‰ (SMOW) . The carbon isotopic compositions of ethane and propane range from -25‰ to -33‰ (PDB) and -24‰ to -30‰ (PDB), respectively. Based on the analysis of the chemical compositions and isotopic compositions of the gaseous hydrocarbons, the geneses of the gaseous hydrocarbons is discussed and five genetic types of gaseous hydrocarbons have been identified as follows. Tyep A: Biogenic gas, C1 / Σ Cn is 0.99, the component of ethane heavier hydrocarbons (C2+) is Carbon isotopic composition of the methane (δ13CCH4 ) is -60.7‰(PDB) , hydrogen isotopic composition of the methane (δCCH4 ) is-204‰ (SMOW) . Type B: Gas of transitional zone, C1 / Σ Cn is 0.95, C2+ 5% , δ13CCH4 ranges from - 50‰ to -55‰, δ13CCH4 ranges from -225‰ to -275‰. Natural gas of these characteristics was considered as a mixture of biogenic and thermogenic gas in the past. In some areas it could be possible, but in Liaohe Basin, this type of gas was original both in generation and in reservation. Type C: Low mature to mature oil-associated gas, C1 / Σ Cn ranges from 0.45 to 0.95, δ13CCH4 ranges from - 40‰ to -50‰ , δ13CCH4 ranges from - 200‰ to-260‰, δ13C3-δ13C22>‰ and δ13C2-δ13C12>‰ value ranges from 10‰ to 20‰. Type D: High mature and condensate generated gas, C1 / Σ Cn ranges from 0.45 to 0.95、 δ13CCH4 ranges from -185‰ to -235‰, δ13C3-δ13C22>‰ and δ13C2-δ13C1 14‰. Type E: Gas produced from kerogen of type Ⅲ, C1 /ΣCn is 0.96, C2+5%, carbon and gydrogen isotopic compositions of the methane are -29‰ and -194‰, respectively. This type of gas was generated from the coal-bearing strata of Mesozoic Erathem.
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