Logging Evaluation of Shale Porosity with Variable Matrix Parameters in Continental Facies: A Case Study of the Lianggaoshan Formation, Sichuan Basin

Accurate porosity evaluation is critical for the assessment of continental shale oil reservoirs, yet remains challenging due to complex lithology and significant burial depth variations, as exemplified by the Lianggaoshan Formation in the Sichuan Basin. Conventional fixed-matrix-density models often yield unsatisfactory accuracy in porosity estimation from density logs. This study proposes a variable matrix-density logging method to improve porosity calculation. The approach integrates core X-ray diffraction and lithology scanning logs to convert mineral mass fractions into volumes, constructing a petrophysical model that accounts for crystalline minerals, clay minerals, kerogen, and fluids. A depth-dependent dynamic matrix density model was established by analyzing compaction effects across varying depths. By incorporating this model into the density-log response equation, shale porosity was quantitatively derived. Application to the Lianggaoshan Formation demonstrates that the method reduces the absolute error in porosity estimation by 2.55 porosity units compared to conventional approaches, while also addressing the limitations of NMR-based porosity evaluation in shales. The proposed method provides a reliable, applicable technique for porosity assessment in continental shale reservoirs with similar geological conditions.