Quantifying Pore-Throat Heterogeneity in Bioclastic Limestones Using Multifractal Analysis: Insights from the Upper Cretaceous Khasib Formation

The heterogeneity of carbonate reservoirs constitutes a critical control on fluid flow. Nevertheless, most previous studies rely on qualitative characterization and single-scale statistical methods, which hinder multiscale characterization of pore heterogeneity and localized preferential flow. Furthermore, few quantitative studies have systematically clarified the coupled mechanism of deposition, diagenesis, and reservoir heterogeneity. To tackle this problem, this study targets bioclastic limestones of the Upper Cretaceous Khasib Member from Oilfield B in the Middle East. Combining high-pressure mercury intrusion (HPMI), cast thin-section observation, and multifractal analysis, this study systematically clarifies the interactions among depositional environments, pore structure evolution, and diagenetic sequences. The results demonstrate that permeability exhibits significant positive correlations with the multifractal spectrum width (Δα) and asymmetry parameter (Rd); these two parameters can quantitatively characterize the complexity and asymmetric distribution of pore throats. Sedimentary microfacies analysis reveals that Δα and Rd gradually decrease as the depositional setting evolves from high- to moderate-energy bioclastic shoals to low-energy back-shoal environments. This trend intuitively records the transition of pore systems from multimodal to unimodal distributions under weakened hydrodynamic conditions, accompanied by reduced reservoir heterogeneity. Bioturbation remodels the porosity-permeability framework. During shallow-to-moderate burial, organic acid fluids migrate along early preferential flow paths and trigger differential dissolution, facilitating the generation of secondary dissolution pores and raising Δα and Rd values. This observation explains the coordinated increase in Δα and Rd. Furthermore, overcoming the limitations of conventional single-scale evaluation, a coupled permeability-Δα-Rd relationship is established, and a four-tier classification scheme for flow capacity is defined (high permeability: K > 5.83 mD, Δα > 1.41, Rd > 0.43). This framework enables multiscale quantitative assessment of reservoir quality, pore-throat complexity, and preferential flow pathways. The outcomes offer an effective tool for predicting favorable reservoir intervals in strongly heterogeneous bioclastic limestones characterized by moderate-to-high porosity and low permeability.