Abstract:

Analysis of the geochemical characteristics of natural gas from the main structural units of the Kuqa Depression in Tarim Basin shows that the natural gas from Dabei-Kelasu structural belt is dominated by methane and has a dryness coefficient [C₁/(C₁-C₅)] near 1.00,indicating typical dry gas;while that from the frontal uplift zone has a relatively low content of methane and a dryness coefficient less than 0.95,indicating typical wet gas.In terms of carbon isotope compositions,the gas from the Dabei-Kelasu structural belt contains significantly heavier methane and ethane isotopes(>-31‰ and-20‰,respectively),while that from the frontal uplift zone has obviously lighter methane and ethane isotopes(<-33‰ and-20‰,respectively).Maturity calculation reveals that the gas from the Dabei-Kelasu structural belt belongs to over-mature gases with a vitrinite reflectance ranging between 1.65% and 2.25%,while that from the frontal uplift zone belongs to highly-mature gas with a vitrinite reflectance ranging from 1.00% to 1.35%.All these indicate that the natural gases in different structu-ral units in the Kuqa Depression were generated from the organic matters in the source rocks at different thermal evolution stages.

Key words: carbon isotope, maturity, dryness coefficient, natural gas, Kuqa Depression, Tarim Basin