Assessment of Shale Gas Enrichment Factors and Delineation of Exploration Potential Zones in theLower Member of the LongmaxiFormation, Tiangongtang Region, Southwest Sichuan

The shale gas reservoir in the first member of the Longmaxi Formation in the Tiangongtang area of southwestern Sichuan exhibits significant heterogeneity. [Objective] To investigate the factors influencing shale gas enrichment in southwestern Sichuan, this study systematically analyzed the enrichment conditions and exploration potential of the region. [Methods] This was achieved through an interdisciplinary approach incorporating core analysis, geochemical testing, mineralogical characterization, and well-logging interpretation. [Results] Results indicate that the Long-1₁ sub-member contains continuous organic-rich shale with a thickness of 25~60m, featuring TOC content of 1.0%~4.0% (average 3.19%). The lower intervals (Long-1₁¹ to Long-1₁³) show significantly higher TOC values than upper intervals. With Ro values ranging from 2.58% to 3.16%, the shale is in an overmature stage. Mineral composition is dominated by quartz (35%~45%), clay minerals (25%~35%), and carbonate minerals (15%~25%), with brittle mineral content exceeding 59% and brittleness indices of 60.3%~71%, indicating favorable fracability. The reservoir space comprises organic pores (0.02~0.9 μm in diameter), inorganic pores (including intragranular dissolution pores and intergranular pores), and a multi-scale fracture system (including structural fractures, non-structural fractures, and microfractures). Porosity ranges from 3.0% to 6.0% (average 4.2%), while permeability varies between 0.0003 and 0.2352mD, exhibiting a vertical permeability profile of low values at the top and bottom, and high values in the middle. Analysis of structural preservation conditions reveals that gentle anticlines and monoclinal structures provide better preservation conditions, whereas areas near steep fault zones experience significant gas dissipation. Formation pressure coefficients show a positive correlation with production, with wells exceeding a pressure coefficient of 1.4 typically achieving daily production rates above 2.0×10⁴ m³/d (e.g., Well Y203 at 3.626×10⁴ m³/d). [Conclusion] Through establishing a comprehensive evaluation index system, the study area was classified into Class I (TOC>2.4%, brittleness index>65%, porosity>4.5%, high-quality shale thickness>35m, burial depth 3000~4000m, pressure coefficient>1.4) and Class II favorable zones, providing a scientific basis for shale gas exploration and development in the Tiangongtang area. [Significance] The research findings elucidate the key controlling factors of shale gas enrichment in complex structural settings and offer guidance for efficient shale gas development in southwestern Sichuan.