Depositional and Diagenetic Controls on Multiscale Pore-Throat Fractal Characteristics of Cambrian Dolomite Reservoirs

The Lower Cambrian Longwangmiao Formation, in the Sichuan Basin (China) represents an important target for natural-gas exploration and development. However, the variations in pore-throat structures among different dolomite lithofacies and their controlling mechanisms remain poorly understood. In this study, dolomite reservoirs from the Longwangmiao Formation were investigated using petrography, mineralogical and geochemical analyses, mercury-intrusion capillary pressure (MICP), petrophysical measurements, fractal analysis, and exploratory principal component analysis (PCA) to characterize lithofacies-dependent differences in mineral composition, reservoir properties, pore-throat structures, and fractal characteristics. The results show that three major lithofacies, including grain dolomite (GD), fine-crystalline dolomite (FCD), and very fine-crystalline dolomite (VFCD), developed in the Longwangmiao Formation. These lithofacies exhibit distinct reservoir quality characteristics. GD displays relatively well-developed pore-throat systems and favorable storage capacity. FCD shows the highest porosity (average 12.12%), the lowest displacement pressure, and the largest median pore-throat radius, representing the most favorable reservoir lithofacies among the investigated samples. In contrast, VFCD is characterized by a tight pore-throat system and relatively poor reservoir quality. MICP results further reveal systematic lithofacies-dependent differences in pore-throat size, capillary behavior, and connectivity. Fractal analysis shows that the overall fractal dimension DT ranges from 2.35 to 2.65, with an average value of 2.52. The fractal dimension follows the order of VFCD > GD > FCD, indicating increasing pore-throat complexity and heterogeneity. Multiscale fractal analysis reveals that among the segmented fractal parameters, D2 (0.15–2 μm) shows the strongest statistical associations with porosity and several MICP-derived pore-throat parameters in the investigated samples, indicating its usefulness as a scale-specific descriptor of effective pore-throat quality. PCA further distinguishes lithofacies associated with relatively favorable reservoir properties from those characterized by higher capillary resistance and pore-throat dispersion, although these relationships represent multivariate covariation. This study highlights the coupled relationships among depositional inheritance, diagenetic modification, and multiscale pore-throat responses. The integration of fractal parameters with conventional petrophysical and pore-throat parameters provides a useful approach for characterizing pore-throat heterogeneity in the investigated Longwangmiao dolostones.