Fractal Characteristics and Controlling Factors of Pore–Throat Structure in Tight Feixianguan Oolite Reservoirs, SW China

ABSTRACT

Oolites serve as a crucial foundation for the formation of reservoirs within marine carbonate strata. Due to their chemically unstable components, oolitic grains are highly susceptible to diagenetic alteration, resulting in multiple diagenetic stage. On the basis of thin‐section and scanning electron microscopy (SEM) identification, six distinct diagenetic phases (DPs) have been identified within these reservoirs on the basis of their lithological and pore characteristics. To quantitatively characterize the pore–throat structure, nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and x‐ray diffraction (XRD) analyses were applied to determine fractal properties in pore–throat distributions. The pore–throat distribution in oolite reservoir is categorized into large, medium, and small pores. In various DPs, the fractal dimension (FD) of large pores is the highest. Large pores exhibit diverse shapes and complex distributions. The FD of small pores is significantly higher than that of medium pores, indicating greater structural complexity and heterogeneity, and thus contributing substantially to the overall heterogeneity of the reservoir. In contrast, medium pores show considerable variability in FD. On the basis of cathodoluminescence (CL) and elemental elements analysis, the controlling effect of diagenesis on pores and throats has been revealed. Fabric‐selective dissolution enhances porosity but does not effectively generate pore throats. Intense recrystallization after dolomitization transform the particle‐supported framework into a crystalline structure, which not only increases porosity and permeability but also enhances heterogeneity in the pore–throat structure.