An Experimental Study of Dolostone Recrystallization Under Fluid Overpressure and Its Implication for Reservoir Property Improvement

Recrystallization is an important diagenetic process that widely occurs in carbonate rocks as a result of increasing temperature and pressure during burial. Exploration has revealed that carbonate rocks affected by recrystallization vary in their reservoir properties. To investigate the favorable development conditions of recrystallized carbonate reservoirs, we selected the crystalline dolomite reservoir of the Majiagou Formation in the Ordos Basin as the object for experimental simulation and conducted a series of dolostone recrystallization simulation experiments under various fluid pressures at 120 °C with an axial load of 55 MPa. The fluid overpressure experiments showed extensive recrystallization growth of crystals with uniform grain size and a high degree of euhedral development, and they retained the optimal porosity and pore connectivity. However, the hydrostatic pressure group had a lower degree of recrystallization and inferior physical properties compared to the overpressure group. The results indicate that fluid overpressure is a key preservation condition for deep recrystallized carbonate reservoirs. Fluid overpressure effectively preserves the initial porosity and facilitates the euhedral growth of dolostone crystals, thereby enhancing pore connectivity. Furthermore, the Kozeny–Carman equation was employed to verify that recrystallization of dolostone under fluid overpressure significantly enhances reservoir permeability by adjusting the pore structure, specifically the pore tortuosity and the pore–throat ratio. Our study demonstrates that fluid overpressure is an important prerequisite for the development of deep recrystallized carbonate reservoirs. The search for carbonate formation units with overpressure conditions such as the presence of dense layer capping may be a new direction for deep carbonate oil and gas exploration.