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

Abstract

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 2 H 6 and C 3 H 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‰—63‰) 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‰), consistent with highly mature natural gases. Carbon isotopic compositions of CO 2 in the gas inclusions of deep carbonate rocks are similar (from -4‰ to -13‰) to those of deep natural gases, indicating predominantly an inorganic origin.