Molecular and carbon isotopic compositions of gas inclusions of deep carbonate rocks in the Tarim Basin

摘要

Gaseous components of gas inclusions in deep carbonate rocks (>5700 m) from the Tacan 1 well were analyzed by online mass spectrometry by means of either the stepwise heating technique or vacuum electromagnetism crushing. The carbon isotopic compositions of gases released by vacuum electromagnetism crushing were also measured. Although the molecular compositions of gas inclusions show differences between the two methods, the overall characteristics are that gas inclusions mainly contain CO_2, whilst hydrocarbon gases, such as CH_4, C_2H_6 and C_3H_8, are less abundant. The content of CO is higher in the stepwise heating experiment than that in the method of vacuum electromagnetism crushing, and there are only minor amounts of N_2, H_2 and O_2 in gas inclusions. Methane δ_(13)C values of gas inclusions in Lower Ordovician and Upper Cambrian rocks (from 5713.7 to 6422 m; -52 per thousand —63 per thousand) are similar to those of bacterial methane, but their chemical compositions do not exhibit the dry character in comparison with biogenic gases. These characteristics of deep gas inclusions may be related to the migration fractionation. Some deep natural gases with light carbon isotopic characteristics in the Tazhong Uplift may have a similar origin. The δ~(13)C_1 values of gas inclusions in Lower Cambrian rocks (7117—7124 m) are heavier (-39 per thousand), consistent with highly mature natural gases. Carbon isotopic compositions of CO_2 in the gas inclusions of deep carbonate rocks are similar (from -4 per thousand to -13 per thousand ) to those of deep natural gases, indicating predominantly an inorganic origin.