Objective The essence of diagenesis is the transformation of substances between different phases (solid, gas and liquid phases) and the material migration transported by formation water. Diagenesis controls the formation, evolution, and distribution of deep−buried sandstone reservoirs in sedimentary basins. Fluids transport in deep_buried formations is generally slow, which lead the reaction systems remain in equilibrium or near−equilibrium state. This allows for thermodynamic quantitative calculations of various diagenetic processes in sandstone reservoirs to elucidate the formation mechanisms and controlling factors of diagenetic products and secondary porosity.

Methods This study focus on the solubility of minerals in sandstone reservoirs under CO₂−rich acidic fluid conditions, aiming to clarify the genetic mechanisms of secondary porosity. The research methodology includes: reviewing literature to identify the types and occurrences of components in the H₂O−CO₂−calcite−albite−quartz system; calculating the Gibbs free energy of formation for pure substances (e.g., H₂O(l), feldspar, calcite, CO₂(g)) under varying temperature and pressure conditions using a constant−pressure heat capacity model; determining the partial molar Gibbs energy of dissolved ions via the HKF model; computing chemical equilibrium constants for reactions within the system using Gibbs free energy functions; and calculating the solubility of components under different thermal and pressure conditions based on charge balance principles. The calculated results are compared with the content of diagenetic products in sandstone reservoirs to identify the genetic mechanisms and dominant controls of secondary porosity.

Results Studies show that the contents of various particles in the aqueous solution are mainly controlled by temperature and CO₂ partial pressure, while the influence of pressure is relatively small. The variation patterns of different ions with temperature exhibit significant differences. The compositions of calcium−bearing and carbon−bearing particles in the aqueous solution change with temperature, whereas those of aluminum− and silicon−bearing particles remain relatively stable.

Conclusions Solubility of various particles in aqueous solution, the fluid migration rate, and the duration of fluid migration determine the content of secondary pores. The variation patterns of various particles in solution with temperature and pressure determine the distribution pattern and range of secondary pores under different formation fluid migration modes. Controlled by hydrocarbon−generation processes and stratigraphic framework, the Dongying Sag mainly develops three types of fluid seepage styles, each characterized by distinct development patterns of secondary−porosity zones and cementation zones.