Organic Geochemical Characteristics and Gas Generation Potential of the Upper Triassic T3x5 Source Rocks in Southwestern Sichuan Basin and Adjacent Areas

The fifth member of the Upper Triassic Xujiahe Formation (T₃x⁵) in the southwestern Sichuan Basin contains widely developed organic-rich source rocks, which represent a crucial interval for hydrocarbon exploration in the region. This study systematically evaluates the organic geochemical characteristics, spatial distribution, and hydrocarbon generation potential of these source rocks using total organic carbon (TOC) analysis, organic matter typing, vitrinite reflectance (Ro) measurements, and basin modeling. The results show that the upper and lower submembers have average TOC values of 3.19% and 3.41%, respectively, with organic matter predominantly of mixed sapropelic-humic type (Type II₂). TOC content generally increases from uplift areas to depressions, reflecting the control of depositional environment on organic matter preservation. Thermal maturity exhibits a westward increase and eastward decrease, with the southwestern part reaching high to over-mature stages (Ro generally >1.3%), while the eastern part remains in the condensate-wet gas window. Thickness of the upper submember ranges from 60 to 320 m, with depocenter in the central-southern part of western Sichuan; the lower submember is 60-200 m thick, centered in the Chengdu-Deyang area. Basin modeling, which integrates burial and thermal histories, estimates a cumulative gas generation of approximately 192×10¹² m³ (192 trillion cubic meters), with the highest generation intensity coinciding with the western Sichuan depression. It is concluded that the T₃x⁵ source rocks possess excellent material bases and huge resource potential. Tight sandstone reservoirs adjacent to the gas kitchen should be prioritized in future exploration, with differentiated strategies adapted to various maturity zones.