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 and adjacent areas contains widely developed organic-rich source rocks, which represent a crucial interval for hydrocarbon exploration in the region. Based on 402 core samples from 15 wells, 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 TOC content of the T3x5 source rock ranges from 0.50% to 71.73%, with an arithmetic mean of 2.09% (n = 402). Lacustrine mudstones, which dominate the source rock volume (n = 359), have a mean TOC of 1.25%. Organic matter is predominantly of mixed sapropelic-humic type (Type II2). The TOC content generally increases from uplift areas to depressions, reflecting the control of the depositional environment on organic matter preservation. Thermal maturity exhibits a westward increase and eastward decrease, with the western Sichuan depression reaching high to over-mature stages (Ro mostly 1.3–2.0%, locally > 2.0%), while the eastern and southern areas are mainly mature to low-mature (Ro 0.7–1.3%), locally within the condensate–wet gas window. The thickness of the upper submember reaches >320 m, with depocenter 120–280 m in the central–southern part of western Sichuan; the lower submember reaches >240 m, centered in the Chengdu–Deyang area with central thickness 160–220 m. Basin modeling estimates a cumulative gas generation of approximately 1.92 × 1014 m3 (192 trillion cubic meters), with the highest generation intensity coinciding with the western Sichuan depression. It should be emphasized that this value represents the total generated gas over geological history and does not equate to accumulated or recoverable resources; further evaluation of the migration, expulsion, and accumulation efficiencies is required. Overall, the T3x5 source rocks possess a favorable material base and significant gas generation potential. Tight sandstone reservoirs adjacent to the gas kitchen should be prioritized in future explorations, with differentiated strategies adapted to various maturity zones.