The origin of low molecular weight hydrocarbons associated with biogenic gas from the Eastern Depression in Qaidam Basin, China

Abstract

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.