Evaluation of geological sweet spots in fluvial tight sandstone gas: A case study of the first submember of the second member of the Jurassic Shaximiao Formation, central Sichuan Basin

Reservoirs of the Jurassic Shaximiao Formation in the Sichuan Basin feature low porosity and permeability, strong heterogeneity, and complex sand body superposition. These characteristics complicate the identification and accurate prediction of geological sweet spots. To achieve efficient exploitation of the gas reservoirs, we establish the criteria for evaluating geological sweet spots in tight sandstone gas in the 1^st submember of the 2^nd member of the Shaximiao Formation (the Sha 2¹ submember) in the study area by delving into the effects of the sedimentation, diagenesis, and source rock-reservoir configuration on the formation and hydrocarbon accumulation of these gas reservoirs. Key findings are as follows. (1)The sand body scale and reservoir physical properties are governed by sedimentary microfacies and channeltypes. Notably, sand bodies located within the point bars of straight or low-sinuosity meandering rivers exhibit large thicknesses and favorable physical properties, standing as high-quality reservoirs. (2)Reservoir quality discrepancy primarily results from differences in lithic content in magmatic rocks and diagenetic facies. Specifically, the chloritecemented facies exhibits the most favorable physical properties. (3)Natural gas charging in the reservoirs is dictated by the source rock-reservoir configuration. Most especially, sand bodies in contact with or in proximity to source rock-rooted faults boast the most favorable gas-bearing properties. (4)The single-well natural gas production is jointly determined by reservoir quality and gas-bearing properties. Based on the evaluation criteria established in this study, the geological sweet spots in the study area are categorized into three types:types Ⅰ, Ⅱ, and Ⅲ. Type Ⅰ sweet spots primarily encompass sand bodies in the point bars of straight or low-sinuosity meandering rivers, with porosity exceeding 12 % and permeability surpassing 0.30×10^-3 μm², and chlorite-cemented facies dominating.They are situated near source rock-rooted faults, exhibiting favorable gas-bearing properties with water saturation below 30 %, Poisson’s ratio less than 0.24, and singlewell gas production rate over 0.10×10⁴ m³/(d·m). Type Ⅱ sweet spots are dominated by sand bodies in middle-sinuosity meandering rivers, with porosity ranging from 10 % to 12 % and permeability from 0.15×10^-3 to 0.30×10^-3 μm². Their diagenetic facies are dominated by illite-or mixed-layer montmorillonite-illite-cemented facies.They are positioned comparatively far from source rock-rooted faults, with water saturation ranging from 30 % to 40 %, Poisson’s ratio from 0.24 to 0.25, and single-well gas production rate from 0.05×10⁴ to 0.10×10⁴ m³/(d·m). Type Ⅲ sweet spots primarily comprise sand bodies in the point bars of small-scale, medium-sinuosity meandering rivers, characterized by low porosity and permeability. Their diagenetic facies mainly include illite-or mixed-layer montmorillonite-illite-cemented facies sitting far from source rock-rooted faults and with poor gas-bearing properties.