Abstract: In recent years, significant breakthroughs have been made in the exploration of the Upper Permian Changxing Formation in Mianyang–Guang’an shallow continental shelf and its periphery. However, the gas geochemical characteristics and gas-bearing properties in different reef beach bodies vary greatly, and the genesis/origin and main source rocks of different gas reservoirs remain unclear, as well as the gas enrichment law and accumulation pattern, restricting the evaluation of favorable traps and exploration deployment. Based on laboratory analysis and experimental data such as natural gas composition, carbon isotopes, hydrogen isotopes, nitrogen isotopes, reservoir bitumen and source rocks, a systematic study has been conducted on the genesis of natural gas and the mixing ratio of mixed gases. Combined with geological study results, a gas accumulation model for Changxing Formation gas reservoir has been established. The study results show that: (1) The natural gas in Changxing Formation is mainly composed of hydrocarbon gases with a dryness coefficient of greater than 0.98, generally showing crude oil cracking gas, with medium–high H₂S content in local areas. (2) Three sets of source rocks were deposited in the study area, namely the Upper Permian Longtan Formation/Wujiaping Formation TypeⅡ1–Ⅲ, the Lower Paleozoic Qiongzhusi Formation TypeⅠ, and Longmaxi Formation TypeⅠ–Ⅱ1 source rocks. The geochemical differences in Changxing Formation natural gas are related to the development degree of main source rocks and the stratigraphic horizon cut by source-connecting faults. Natural gas generated by the Upper Permian source rock has heavy carbon isotopes, with δ¹³C₂ values generally heavier than -28.0‰. When mixed with gas generated by the Lower Paleozoic source rocks, carbon isotopes of the mixed gas become lighter, with a contribution ratio of 53.9%–77.0% of the Lower Paleozoic gas estimated by the end-member gas δ¹³C₂ values. (3) Three types of gas accumulation patterns have been identified for Changxing Formation gas reservoirs, i.e., single-source, dual-source, and triple-source hydrocarbon supply patterns, and the main source rocks for various accumulation patterns have been clarified. The results indicate that in the area where the Upper Permian Longtan Formation/Wujiaping Formation served as source rocks, the superposition of large-scale beach facies reservoirs and current structures shows favorable exploration zone. In the area where there is a mixture of the Lower Paleozoic hydrocarbon source, the effective matching of source-connecting faults and large-scale beach facies reservoirs leads to an overall higher degree of oil and gas enrichment. This research provides geological basis for further exploration deployment decisions in Sichuan Basin.

Key words: Changxing Formation, natural gas, geochemical characteristics, accumulation pattern, shallow continental shelf, Mianyang–Guang’an, Sichuan Basin