Gas-Source Comparison and Hydrocarbon Generation and Expulsion Histories of the Main Source Rocks of the Middle Jurassic Shaximiao Formation, Western Sichuan Basin

With the discovery and exploration of tight sandstone gas reservoirs in the Sichuan Basin, the Middle Jurassic Shaximiao Formation has gradually become the focus of exploration. However, the gas source of the Shaximiao Formation in the Western Sichuan Basin is still controversial, and research on the hydrocarbon generation and expulsion histories of the primary source rocks lags behind, which restricts the study of tight sandstone gas reservoir dynamics within the Shaximiao Formation in the Western Sichuan Basin and further affects the oil and gas exploration process. This paper, utilizing geochemical parameters, including 82 natural gas components and 68 carbon isotopes, conducted an investigation into the origin identification of natural gas and gas-source comparisons within the Shaximiao Formation in the Western Sichuan Basin. By utilizing basin modeling technology, we reconstructed the histories of hydrocarbon generation and expulsion of the primary source rocks and identified the potential exploration areas. The findings indicate that the natural gas within the Shaximiao Formation in the Western Sichuan Basin is characterized by a typical high-maturity wet gas-dry gas reservoir. It is a thermogenic coal-type gas produced by kerogen cracking, and the gas source is primarily type-III kerogen. The main source of natural gas is the coal-measure source rock of the Upper Triassic Xujiahe 5 Formation, which has mainly experienced two stages of increasing maturity, characterized by "two stages of gas generation and one stage of gas expulsion". The hydrocarbon generation and expulsion histories for the source rocks within the Xujiahe 5 Formation is shown as follows: the source rocks of the Xujiahe 5 Formation reached the hydrocarbon generation threshold by the end of the Early Jurassic and peaked in hydrocarbon generation by the end of the Late Jurassic, and then the natural gas migrated to the paleostructural highs of the Shaximiao Formation to form the paleo-gas reservoir. From the end of the Early Cretaceous, the paleo-gas reservoir underwent adjustments and transformations to evolve into the present-day gas reservoir. The source rocks exhibit substantial potential for the forming of large-medium and extralarge gas fields, with favorable exploration zones concentrated in the southern and central portions of the study area. The research findings can provide a scientific foundation for the next exploration and deployment of natural gas resources within the Shaximiao Formation in the Western Sichuan Basin.