Geochemical characteristics of natural gas in saline basin of western Qaidam Basin:A case study of Paleogene-Neogen strata

In this paper, the authors investigated the geochemical characteristics of natural gas in Western Qaidam Basin with the ba-sic theory of natural gas geochemistry, analyzed components and carbon isotopes data based on testing and collected 83 natural gas samples from 18 oil and gas fields and oil-bearing structures. The result shows that natural gas is main made up of hydrocarbon, with the hydrocarbon content between 79.24% and 99.81%, the natural gas proportion is 0.72~1.36, the dryness coefficient(C₁/C₂⁺) is 1.04~617.8, the methane carbon isotope value is-51.4‰~-24.2‰and the ethane carbon isotope value is-34.2‰~-17.4‰. On such a basis, the authors studied the natural gas genetic types of different structural belts and established the identification indices of natural gas genetic types in this area. The research demonstrates that the genetic types of natural gas can be divided into three kinds in Western Qaidam Basin, namely sapropel-type gas, humic-type gas and mixed gas, that the northwestern Qaidam Basin has mainly sapropel-type gas, mixed gas and a small amount of humic-type gas, and that the southwestern Qaidam Basin mainly has sapropeltype gas and a small amount of mixed gas.