Determinants of high gas content and restoration of in situ gas content in Permian Wujiaping shale of the eastern Sichuan Basin, China
摘要
The Permian Wujiaping Formation (P3w) shale in eastern Sichuan Basin exhibits high gas content. Through total organic carbon (TOC) content determination, mineral composition analysis, N2/CO2 adsorption, high-pressure methane adsorption, and scanning electron microscopy experiments, we identified determinants of the high gas content and restored in-situ gas content. Findings demonstrate that the supercritical Dubinin–Astakhov (S-DA) model (based on micropore-filling theory) represents the optimal approach for accurately restoring the absolute adsorption amount of the third member of P3w (P3w3) shale within the study region compared to the supercritical Langmuir (S-L) model (based on monolayer adsorption theory) widely used in the assessment of gas content of the Wufeng–Longmaxi shale. The P3w3 and Wufeng–Longmaxi shales have similar adsorption capacities at equivalent TOC levels, with the high adsorbed gas content of P3w3 attributed to the development of rich organic shale (with a TOC content of 7.68%) in the anoxic reduction environment of the deep marine shelf facies. The high free gas content is due to the abundant siliceous minerals that play an anti-compaction and pore-preservation effect during the diagenesis, forming rich pore spaces (porosity of 6.05%). Under the influence of formation temperature and pressure, the in-situ gas content of P3w3 shale reservoir gradually increases with burial depth. The P3w3 shale gas reservoir with a burial depth of 4300–4400 m mainly exists in the free state, with the ratio of free gas to adsorbed gas approximately 7:3.