Orign and Characteristics of Natural Gas in Taibei Seg of Turpan - Kami Basin

Abstract: Origin of gaseous hydrocarbon is discussed on the basis of geochemical characteristics of natural gas in Taibei depression of the Turpan -Hami Basin, Northwestern China. More than 10 gas samples were collected from the production wells. Molecular composition and carbon, hydrogen and helium isotope composition of natural gas were analyzed. The gases contain 60. 85 - 84. 40% mathane and 13. 04-38. 76% heavy hydrocarbons (Cu₂₊). Values of wetness (C₁/C₂₊) are between 1. 57 to 6. 37. The gases are isotopically characterized by δ¹⁸C₁ = -43. 0～-49. 4‰, δDc₁ = - 220～-271‰, δ¹⁸C₂ = -20.1～-34. 0‰. δDc₂= -259～-257‰. δ¹⁸C₃= -21.3～-26. 7‰, δDc₃= -114～-203‰, δ¹⁸C₄= -22. 2～-28. 2‰, δDc₄ =-93～-116‰, and ³He/⁴He = (3. 17-7. 01)×10^-6. Estimated maturity of natural gas is different from that of Jurassic sediment organic matter. Based on the carbon isotope composition of mathane, ethane and propane, values of vit-rinite reflectance (R.) are calculated. The data indicate that natural gas in Taibei depression of the Tuipan -Hami Basin is wet gas, and belongs to thermocatalytical natural gas, that the gases may be generated in different stages, which causes the gases have larger △¹³C_2-1 values, and that the gases may be partially derived from Jurassic System, and partially from Pre - jurassic strata.