A recovery method of porosity evolution based on forward and inverse analyses: a case study of the tight sandstone of Xujiahe Formation, Northeast Sichuan Basin

Quantitative recovery of porosity is one of the core issues of quantitative reservoir research. The conventional semi-quantitative and quantitative recovery methods do not consider chemical diagenesis and changes in the apparent volume of rock, the application conditions are harsh, and the accuracy of recovery results is low. Based on the analysises of rock fabric and diagenetic evolution sequence as well as the accurate restoration of original porosity, while comprehensively considering the influence of changes in the apparent volume of rock on pore evolution, this paper establishes two evolution models of diagenesis and porosity for the compaction- and cementation-controlling sandstone reservoirs of Xujiahe Formation in the Northeast Sichuan Basin. Based on the principle of constant intergranular volume, this paper deduces the computational formula of porosity variation in compaction- and cemented-controlling reservoirs, obtains parameters such as ancient burial depth and paleogeotemperature using the simulation results of burial history and thermal history, and establishes a single quantitative model of diagenesis. According to the original porosity, diagenesis time and temperature range, the authors regard pore evolution in the geological history as the superposition of multiple porosity enhancements or reductions, and construct a new method for quantitative porosity recovery (FIM) based on forward and inverse analyses. Compared with the improved Scherer model, mechanical-chemical compaction model, and the inversion and back-stripping method, the FIM method has reliable porosity recovery results and wider application ranges. Affected by the strong mechanical compaction in the early diagenetic stage, the (feldspar) lithic sandstone reservoirs of Xujiahe Formation in the northeast Sichuan Basin were densified in the early Late Jurassic, and the gas reservoirs have the characteristics of "densifying while accumulating". Due to the strong carbonate cementation, the quartz-bearing calcarenite reservoirs were densified in the Middle Jurassic, and the gas reservoirs have the characteristics of "densification first and accumulation later".