The origin of natural gas in the Hutubi gas field, Southern Junggar Foreland Sub-basin, NW China

Abstract

The Hutubi gas field is the largest field discovered in the Southern Junggar foreland sub-basin at present. The origin and generation of gas in the field have been discussed based on the geochemical data. The carbon isotopic composition of methane and ethane in the Hutubi gas field is relatively enriched in 13 C with δ 13 C 1 values ranging from −32.8‰ to −30.5‰ (average value = −31.2‰) and δ 13 C 2 values ranging between −23.0‰ and −21.6‰ (average value = −22.0‰). The hydrogen isotope is lighter with δD 1 from −200‰ to −196‰. Methylcyclohexane is abundant and the content is the highest among C 7 light hydrocarbons, ranging from 49.6% to 59.4%, with an average value of 55.5%. The δ 13 C values of individual C 6 –C 7 light hydrocarbons are heavier with the δ 13 C values from −25.2‰ to −17.9‰. The results show that gas in the Hutubi field is coal-derived gas and sourced from the Middle and Lower Jurassic coal measures. The result of gas generation kinetics from Jurassic coal measures show that the gas generation began 10 Ma and the methane generation transformation ratio is very high (up to 87%) at present. The time of gas generation is consistent with that of the Hutubi structural trap and shows that the formation time of the Hutubi gas field is very late. It is concluded that gas preservation in the Southern Junggar foreland sub-basin is excellent and gas exploration potential is favorable. ► The isotope shows that gas in Hutubi gas field is coal- derived and sourced from Jurassic rock. ► Secondary cracking existed in Hutubi gas field. ► The filling history was determined according the gas generation kinectics.