Abstract

The Eastern Depression of Qaidam Basin in China is the largest Quaternary gas producing area in the world. The chemical compositions, C₁–C₂ carbon and hydrogen isotopic compositions, low molecular weight hydrocarbons (C₆–C₇LMWHs) in 10 natural gas samples from three large gas fields (Tainan, Sebei 1 and Sebei 2) in Eastern Depression of Qaidam Basin were analyzed. The carbon and hydrogen isotopic composition of methane in the three gas fields is relatively enriched in ¹²C and ¹H with δ¹³C₁ values ranging from −69.6 ‰ to −65.5 ‰ (average −67.4 ‰) and δ¹H from −251 ‰ to −231 ‰ (average −244 ‰). The data shows that the gases are defined as microbial in origin and generated primarily by bacterial CO₂ reduction. The composition distribution of C₆–C₇LMWHs differs between the fields. LMWHs associated with the gas in the Sebei 1 gas field are different from those in the Tainan and Sebei 2 gas fields. The relative content of cycloalkane among C₆–C₇LMWHs in the Tainan and Sebei 2 gas fields is very high, ranging from 37.02% to 43.36% and 33.99% to 43.71% respectively. However, in the Sebei 1 gas field, of the total C₆–C₇LMWHs, the relative content of isoalkane is the highest, ranging from 37.17% to 45.92% with an average of 42.48. These traits indicate that the origins of C₆–C₇LMWHs are probably different from the bacterial action that is predominant in the Sebei 1 gas field. We infer that catalytic reaction affects the C₆–C₇LMWHs compositions in the Tainan and Sebei 2 gas fields.