Natural gas characteristics and gas-source comparisons of the lower triassic Jialingjiang formation, Eastern Sichuan basin

Abstract

There are abundant natural gas resources in the Lower Triassic Jialingjiang Formation in the Eastern Sichuan Basin (ESB). Important progress has been made in the study of reservoir characteristics and sedimentary facies of marine reef-beach facies in the Jialingjiang Formation. However, there are still differences in the understanding of its natural gas origin and gas source rocks, which greatly restricts the understanding of the reservoir forming process and formation mechanism of the gas reservoirs, so further study is urgently needed. Based on experimental tests of natural gas composition, stable isotopes, and noble gas isotopes with gas chromatography (GC) and mass spectrometry (MS) studies of source rocks and reservoir asphalt saturated hydrocarbons, the natural gas geochemical characteristics, genetic identification and gas-source comparison of the Jialingjiang Formation were studied. The results showed that (1) the natural gas of the Jialingjiang Formation was mainly hydrocarbon gas, and methane was dominant. The average drying coefficient was 99.266%, which is typical of high maturity dry gas reservoirs. The δ13C1, δ13C2 and δ13C3 values ranged from −36.00‰ to 24.80‰, −37.80‰–27.60‰ and −39.20‰–23.20‰, respectively. The δD1, δD2 and δD3 values were between −164.00‰ and −124.00‰, −162.00‰ and −111.00‰ and −173.21‰ and −71.00‰, respectively. (2) The natural gas has a crustal source origin, which was mainly oil-type thermogenic gas generated by secondary cracking of crude oil. The gas-generating parent material is mainly partial sapropelic kerogen and was in the sedimentary environment of salt to brackish water. (3) The natural gas was mainly generated by argillaceous source rocks of the Upper Permian Longtan Formation. (4) The natural gas accumulation in the Jialingjiang Formation has experienced three stages, including paleo-oil reservoir, paleo-gas reservoir and current gas reservoir. The results provide a basis for the exploration decision-making of natural gas in the Jialingjiang Formation in the ESB.