Abstract: An exploration breakthrough has been made in the overpressure tight sandstone gas reservoir in the fifth member of the Upper Triassic Xujiahe Formation (Xu 5 member) in Well LG 163, drilled in Yilong–Pingchang area, Sichuan Basin, marking an important field for increasing reserves of continental facies tight oil and gas with a large scale and benefits. Based on the discovery of tight sandstone gas in Well LG 163 and the previous exploration results in Xujiahe Formation, and combined with the experimental and lab test data, a systematic study of reservoir characteristics and gas accumulation and evolution in Xujiahe Formation in Yilong–Pingchang area has been conducted focusing on the hydrocarbon generation center in Xu 5 member. The study results suggest that thick dark shale was developed in Xu 5 member in Yilong–Pingchang area, with an average TOC of greater than 1%, Type Ⅱ₂-Ⅲ organic matter, and high–over maturity, showing a good source rock as a whole. The meandering river delta front facies sandstone reservoir was developed in the southeastern part, with consistent distribution in lateral, porosity range of 1.5%–7.7% and an average of 4.83%, well-developed microfractures, and good reservoir physical properties. The tight sandstone reservoir in the second sub-member (Xu 5₂ sub-member) was wrapped in source rock in Xu 5 member, forming a high-quality hydrocarbon accumulation combination of “sand wrapped by source rock” in vertical. The formation overpressure is generally observed in Xu 5 member, and the pressure coefficient gradually increases from the slope to the center of the lake basin, resulting in tight gas charging and preservation under overpressure conditions. The comprehensive study of source rock conditions and reservoir physical properties indicates that the high-quality source rock in Xu 5 member has an area of 5600 km², the sand enriched zone has an area of 3553 km², and the predicted tight gas resources are 5600×10⁸ m³, showing good exploration prospects, which is expected to be a major replacement field for obtaining largescale gas discoveries.

Key words: Sichuan Basin, Yilong-Pingchang area, Xu 5 member, tight sandstone gas, accumulation condition