Geological characteristics and exploration potential of shale oil and gas in Jurassic Lianggaoshan Formation, eastern Sichuan Basin

The Jurassic Lianggaoshan Formation in the Sichuan Basin is characterized by widely distributed, thick organic-rich shale. In recent years, high-yield industrial oil and gas flows have been obtained in shale reservoirs of the first member of the Lianggaoshan Formation (Liang 1) as well as sandstone and mudstone interbedded reservoirs of the second member of the Lianggaoshan Formation (Liang 2), showing good resource potential. The high yield makes the Lianggaoshan Formation an important successor field for shallow oil and gas exploration in the Sichuan Basin. Using whole rock X-ray diffraction, organic geochemical analysis, and scanning electron microscopy (SEM), combined with a comparison of accumulation elements, the main controlling factors of shale oil and gas accumulation in the Lianggaoshan Formation of the eastern Sichuan Basin were discussed. The average porosity of shale in Liang 1 member is 4.45%, with reservoir space mainly consisting of intragranular and intergranular dissolution pores of clay minerals, and locally developed organic pores, bedding fractures, and high-angle fractures. The shale reservoirs have high brittle mineral content, moderate two-way horizontal stress difference, and good compressibility. The total organic carbon (TOC) content isolines exhibit ring-shaped distribution, with TOC content exceeding 1.8% in the Dianjiang, Liangping and Zhongxian hydrocarbon generation center. Organic matter types are mainly type Ⅱ₂ and type Ⅲ, with an average organic matter maturity (R_o) of 1.14%, generally in the mature to highly mature stage. Three main controlling factors were observed for shale oil and gas enrichment and high production in the Lianggaoshan Formation: (1) sedimentation, which controlled source, (2) overpressure, which promoted enrichment, and (3) fracturing, which facilitated migration. Among them, sedimentation-controlled source and abnormal overpressure-driven enrichment are more critical than fracturing in controlling oil and gas migration. Using parameters such as continuous thickness of high-quality shale, formation pressure coefficient, micro-fracture development degree, and brittleness index, the evaluation criterion for favorable areas of shale oil and gas in the Lianggaoshan Formation of the eastern Sichuan Basin was established. Based on exploration experience in adjacent areas, it is recommended to prioritize the three high-quality sublayers of Liang 1 member as the primary targets vertically, while focusing regionally on favorable type Ⅰ areas such as the Zhongxian and Wanxian synclines. An integrated geological and engineering exploration for core areas of the syncline with burial depth greater than 2 500 m should be conducted.