Chemical and Isotopic Compositions of Gaseous Hydrocarbons in Liaohe Basin

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 C_n is 0.99, the component of ethane heavier hydrocarbons (C₂₊) is Carbon isotopic composition of the methane (δ¹³C_CH4 ) is -60.7‰(PDB) , hydrogen isotopic composition of the methane (δC_CH4 ) is-204‰ (SMOW) . Type B: Gas of transitional zone, C₁ / Σ C_n is 0.95, C₂₊ 5% , δ¹³C_CH4 ranges from - 50‰ to -55‰, δ¹³C_CH4 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 C_n ranges from 0.45 to 0.95, δ¹³C_CH4 ranges from - 40‰ to -50‰ , δ¹³C_CH4 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 C_n ranges from 0.45 to 0.95、 δ¹³C_CH4 ranges from -185‰ to -235‰, δ¹³C₃-δ₁₃C₂2>‰ and δ¹³C₂-δ¹³C₁ 14‰. Type E: Gas produced from kerogen of type Ⅲ, C₁ /ΣC_n 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.