Characterization and classification of pore structures in deeply buried carbonate rocks based on mono- and multifractal methods

The heterogeneity of deeply buried carbonates (burial depth >4500 m) is directly related to the morphology of the micropores and connectivity of the pore network. It is important to know the fractal characteristics of the microstructure to understand the heterogeneity and fluid flow properties. In this study, N2 gas adsorption measurements, mercury intrusion porosimetry, and nuclear magnetic resonance were conducted. Based on the pore size distribution (PSD), we compared the mono- and multifractal characteristics and established the relationship between the fractal parameters and petrophysical properties (porosity, permeability, median capillary pressure, and the cutoff value of transversal relaxation time (T2cutoff)). Finally, two multifractal parameters were introduced to characterize the pore structure. A cross-plot of D0 (capacity dimension) vs f0 (fractal spectrum) was established for the pore structure classification with predefined parameters such as the porosity and permeability. The results show that the PSD can be obtained by integrating the three above-mentioned measurement methods. The mono fractal dimensions are weakly correlated with the petrophysical parameters, whereas D0 obtained from multifractal analysis correlates with the porosity, permeability, median capillary pressure, and T2cutoff. The unique multifractal characteristics of D0 and f0 can be used as indicators of pore structure types in case studies. This study introduces an effective method that can be used for the pore structure characterization and classification of carbonates.