Classification and Microscopic Pore Characterization of Volcanic Rock Reservoirs in the Southern Songliao Basin Based on Fractal Theory

With unconventional oil and gas reservoirs emerging as critical exploration targets, systematic classification and quantitative evaluation of reservoir pore structures have gained paramount significance. This study resolves critical classification challenges in tight volcanic reservoirs from the southern Songliao Basin through the integration of fractal theory with high-pressure mercury intrusion (HPMI) techniques, thereby developing an innovative quantitative characterization methodology for pore structure analysis. Experimental results demonstrate that capillary pressure curves effectively categorize pore sizes into three distinct classes: micropores (< 0.1 μm), mesopores (0.1–1.0 μm), and macropores (1.0–10 μm). Fractal dimension analysis confirms the geological rationality of this classification framework. Through systematic coupling of petrophysical parameters with pore structure characteristics, a novel tripartite classification standard is proposed: Class I reservoirs (micropore-dominated) exhibit high storage capacity but low permeability; Class II reservoirs demonstrate favorable flow capacity yet limited storage potential; Class III reservoirs, featuring optimized micropore-macropore distributions, present balanced permeability and superior hydrocarbon productivity. The study reveals that extreme values (either excessively high or low) of pore-throat sorting coefficients impair fluid migration, while increasing sorting coefficients promote hybrid pore-type configurations. Moreover, elevated micro-mesopore distribution frequencies correlate with enhanced mercury ejection efficiency and structural homogeneity, whereas predominant macropore distributions significantly improve permeability and hydrocarbon mobility. Consequently, Class III reservoirs, characterized by synergistic storage-seepage properties, constitute the most favorable targets for prioritized exploration and sustainable extraction. This investigation establishes a fractal-HPMI integrated characterization methodology that advances both theoretical understanding and engineering practices for optimizing tight volcanic reservoir exploitation.