Comparison of geological conditions and prospects of exploration potential of shale gas in upper permian Wujiaping formation and Longtan Formation, Sichuan Basin

Shale gas is a vital transitional energy source during the energy transformation period. With the ongoing development of the Longmaxi Formation shale gas field in the Sichuan Basin, identifying new alternative stratigraphic sequences has become a key focus of current shale gas exploration. The Upper Permian Longtan and Wujiaping Formations in the Sichuan Basin, characterized by contemporaneous heterotopic facies, have demonstrated favorable hydrocarbon shows during previous drilling operations. To evaluate the resource potential and exploration prospects of these two formations, this study utilized various experimental methods, including shale gas content measurement, field emission scanning electron microscopy (FE-SEM), total organic carbon (TOC) analysis, vitrinite reflectance (Ro), and X-ray diffraction (XRD). These techniques were utilized to systematically investigate gas-bearing characteristics, organic geochemical properties, reservoir spaces, and mineral compositions of both formations, thereby establishing comprehensive evaluation criteria for favorable exploration zones and geological resource calculation. Results indicate that the Longtan Formation, of sea-land transitional facies, consists of multiple thin shale layers, with a single-layer thickness of 0.5–20.0 m. Its average TOC content is 2.7%, its kerogen types are II 2 and III, and its Ro ranges from 1.4% to 2.8%. The reservoir space is mainly composed of interlayer pores of clay minerals, with an average porosity of 4.5%, an average brittle mineral content of 41.7%, and an average gas content of 5.0 m 3 /t. The favorable area is in the Guangan-Zigang-Shibao region, with a resource abundance of about 1.60 × 10 8 m 3 /km 2 . The Wujiaping Formation belongs to the deep-water shelf facies, with a single layer thickness of 6–12.0 m. Its average TOC content is 3.5%, its kerogen types are I and II1, its Ro ranges from 2.0% to 2.8%. The reservoir space is mainly composed of organic matter pores, with an average porosity of 5.5% and an average brittle mineral content of 76.6%. The average gas content of the shale section at the top of Wujiaping Formation can reach 8 m 3 /t, and the favorable area is in the Dazhou Liangping block, with a resource abundance of 3.30 × 10 8 m 3 /km 2 . Both of these formations have broad exploration prospects and can provide crucial support for shale gas succession in the Sichuan Basin.