Light hydrocarbons composition and carbon isotopes unravel the origin of natural gas in Triassic marine strata of Moxi gas field, central Sichuan Basin

The Triassic marine gas reservoir in the Moxi Gas Field is one of the large gas fields initially discovered in the Sichuan Basin. However, the origin of its natural gas has long been debated. To study the genetic types and origin of natural gas, the light hydrocarbons (LHs) composition, carbon isotopes, chemical compositions, and stable carbon and hydrogen isotopic compositions were analyzed in the Triassic strata of Moxi gas field in the Sichuan Basin. The natural gases from the second member of the Jialingjiang Formation (T 2 j 2 ) and the first member of the Leikoupo Formation (T 2 l 1 ) have dryness coefficients higher than 0.997 and contain low content non−alkane gases such as CO 2 and N 2. The δ 13 C 1 values of natural gas in the Moxi (MX) gas field range from −35.3‰ to −32.9‰, the δ 13 C 2 values range from − 34.0‰ to − 30.3‰, and the δD 1 values range from − 144‰ to − 120‰. As for light hydrocarbons composition, the C 7 LHs are dominated by the methylcyclohexane (MCH), and the C 6 −C 7 LHs have low normal alkane contents. The individual carbon isotopes of light hydrocarbons have the characteristics of δ 13 C > −28‰. The origin identification indicates that the natural gas in this area is oil−type gas that results from the secondary cracking of crude oil. Our results show that the natural gas in T 2 j 2 of the MX gas field is sourced from the shale gas of the Є 1 q , the T 2 l 1 gas is a mixed−source gas from different thermal evolution stages of the Є 1 q source rocks, shale gas and oil−cracking gas. The mixing effect is the main reason for the reversed phenomenon (δ 13 C 1 > δ 13 C 2 ).