Abstract: In this study, by analyzing natural gas components, hydrocarbon isotopic compositions, flakes, inclusions, and other geochemical features, we investigated the genesis of hydrogen sulfide (H₂S) in the gas reservoirs of the Changxing, Feixianguang, and Maokou Formations of the Permian and the Triassic in the southeast of the Sichuan Basin. The results indicated that the natural gas in these formations is mainly constituted by alkane gas, with an average ratio of non-hydrocarbon of 24.68%. The alkane gas of the analyzed samples was consistently dry. The ethane content of some samples was too low to be detected. Among the samples with a detectable content of ethane, the concentration of ethane ranged from 0.03% to 0.39% (0.15% in average). In contrast, the content of δ¹³C₁ was high across the samples, ranged from -28.3‰ to -35.2‰ (-31.1‰ in average); some samples showed a δ¹³C₁ > δ¹³C₂ inversion. The non-hydrocarbon component was mainly constituted by CO₂ and H₂S, with the concentration of H₂S ranging from 1.0% to 21.7% (5.3% in average). Bitumen of various types, which would suggest histories of oil filling, and cracking of paleo-reservoirs was uncommon in the reservoirs. The hydrocarbon inclusions in calcite veins were predominantly in gaseous state, whereas oil inclusions were rare. By combining our results with published ones, we concluded that the fractured vuggy gas reservoirs of the Maokou Formation in the southeast of the Sichuan Basin (occurring in north Fulin and Qijiang), which barely contained any H₂S, showed no signs of thermal sulfate reduction (TSR) alteration. The geneses of the H₂S contained in the reservoirs of the Feixianguang Formation (in north Fulin), the reservoirs of the Changxing Formation (mainly in north and central Fulin), and the hydrothermal dolomite reservoirs of the Maokou Formation (mainly in central and south Fulin) all showed TSR. Some of the gas reservoirs altered by TSR showed a low H₂S content, which is likely due to recent reduction.In the reservoirs of the Feixianguan Formation, the hydrocarbons participating in the TSR reaction were predominantly in liquid state. However, in reservoirs at other horizons, hydrocarbons were mainly gaseous (methane). This probably depends on the amount of liquid hydrocarbons present in the reservoir when the TSR occurred. In addition, it was found that the TSR reactions took place independently in each reservoir other than being affected by those occurring in the Feixianguan Formation. The source of sulfur for the TSR reaction in the Changxing Formation could have come from the lateral brine discharge of the Feixianguang Formation during the deposition period, while that required in the Maokou Formation could have come from the fluid of the adjacent horizon rich in SO₄^2-.

Key words: southeastern Sichuan Basin, hydrogen sulfide, TSR alteration, geochemical characteristics, carbon isotopes