In recent years, Ordos Basin has been the largest petroleum field in China and represents significant potential for gas exploration in the Lower Paleozoic carbonate reservoirs. Taking the central-eastern Ordos Basin as an example, this work studies the reservoir characteristics, origin, and controlling factors of the crystalline dolomites of the Sanshanzi Formation by means of petrological and geochemical studies. The Sanshanzi Formation dolostones comprise three kinds: crystalline dolostone with grain-shadow (Rd1), very fine–fine crystalline dolostone (Rd2), and medium-coarse crystalline dolostone (Rd3). The pore spaces include intercrystalline pores and intercrystalline solution pores, residual intergranular pores, vugs, and fractures. The medium-coarse crystalline dolostone is the best reservoir. The average porosity is 4.61%, and the average permeability is 0.91 mD. The sedimentary environment in the upper Cambrian was a large area of tidal flats. Under the influence of penecontemporaneous meteoric water leaching, dissolution developed at the top of the dolomite flat–lagoon depositional cycle, with vugs developing in the very fine–fine crystalline dolostone. Penecontemporaneous dolomitization enhanced the compaction resistance and effectively preserved the pore space of the reservoir. In addition, recrystallization improved the crystalline texture, turning the intergranular pores into residual intergranular pores and intercrystalline micropores into intercrystalline pores. In this case, the very fine–fine crystalline dolostone was transformed into medium-coarse crystalline dolostone. The results of this study provide a case for the in-depth understanding of the development and preservation mechanism of ancient dolomite reservoirs and also point to the research pathways for oil and gas exploration in the Cambrian in the Ordos Basin in the future.

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.