Geological conditions and favorable zones for shale gas in the Silurian Longmaxi formation of the Qianbei Slope

To address the lack of integrated research on depositional environment, gas-bearing properties, and reservoir characteristics in the early-stage exploration of the Longmaxi Formation shale gas in northern Guizhou, this study integrates field outcrops, drilling cores, measured sections, and multiple experimental methods (porosity/permeability testing, X-ray diffraction, TOC analysis, Ro measurement, kerogen type identification, SEM observation, and gas content analysis). Favorable exploration zones were delineated using the comprehensive information overlay method, while burial history modeling and fluid inclusion analysis were applied to reconstruct the shale gas accumulation process. Results indicate that the Longmaxi Formation shale in northern Guizhou has a residual area of approximately 14,600 km² and a thickness of 50–200 m. It is characterized by Type I kerogen (δ¹³C: − 28.5‰ to − 30.8‰), residual TOC contents of 1.0%–4.0%, and high thermal maturity (Ro: 1.40%–2.45%). The shale exhibits an average brittle mineral content of 43.2%, porosity of 4%–10%, and average permeability of 0.088 × 10⁻³ µm², with nanopores accounting for over 40% of the pore space, indicating favorable fracturability but low-porosity, ultra-low-permeability reservoir characteristics. The accumulation process involved two phases of charging and one phase of dissipation. The current formation pressure coefficient ranges from 0.82 to 0.90, reflecting a normal to slightly negative pressure state. The probability-weighted average geological resource volume is estimated at 4.35 × 10¹² m³ by the probabilistic volumetric method, consistent with the geological analogy estimate of (3.52–4.69) × 10¹² m³. Comprehensive evaluation identifies the Xishui–Renhuai and Daozhen–Wuchuan–Yanhe areas as the most favorable exploration zones. The Lower Silurian Longmaxi Formation in northern Guizhou exhibits favorable geological conditions for shale gas accumulation and represents a key target for future exploration, providing important geological insights for shale gas exploration in structurally complex areas outside the Sichuan Basin in southern China.