Quantitative Characterization and Differential Evolution of Pore-Fracture Systems in Volcanic Reservoirs of the...

The extreme heterogeneity of pore-fracture systems in deep volcanic reservoirs fundamentally limits the reliable prediction of high-quality productive intervals. Focusing on the Changling Fault Depression, southern Songliao Basin, this study elucidates the composition, origin, and differential evolution of these complex networks. Initially, ten distinct pore-fracture types were identified and genetically classified into two categories: primary pore-fracture types, including vesicles, inherited intragranular pores, phenocryst-hosted resorption pores, and shrinkage fractures; and secondary pore-fracture types, including moldic pores, sieve pores, cavernous pores, intragranular micropores, matrix spongy pores, and tectonic fractures. A comprehensive quantitative evaluation was then conducted based on petrographic observations of 162 core samples and 75 cast thin sections, integrated with lithological characteristics, petrophysical properties, and quantitative areal-porosity analyses (via JMicroVision). This evaluation reveals a pronounced lithological control on pore-fracture assemblage evolution. Specifically, lava reservoirs are dominated by preserved primary porosity (primary-to-secondary ratio of ∼2.2:1), whereas volcaniclastic lavas exhibit a transitional mixed assemblage (∼1.0:1). Conversely, volcaniclastic reservoirs are pervasively modified by diagenesis, dominated by secondary dissolution porosity (primary-to-secondary ratio of ∼1:2.0). These architectural differences directly dictate reservoir quality, enabling a tripartite quantitative classification: Class I reservoirs (porosity >7.0%, permeability >1.0 × 10^-3 μm²) are driven by well-connected vesicles and fractures; Class II reservoirs (5.0% < porosity ≤ 7.0% and 0.1 × 10^-3 μm² < permeability ≤ 1.0 × 10^-3 μm²) are characterized by primary-dissolution superimposed networks; and Class III reservoirs (porosity ≤ 5.0% and permeability ≤ 0.1 × 10^-3 μm²) are constrained by isolated secondary pores or late-stage cementation. Spatially, reservoir evolution is highly compartmentalized. Rhyolites in the Darhan and Jubaoshan areas rely predominantly on primary preservation coupled with overprinted dissolution or fracture-assisted pore-fracture connectivity. Conversely, tuffaceous reservoirs record progressive dissolution enhancement in the Chaganhua subdepression but experience early cementation followed by late-stage dissolution reactivation in the Longfengshan area. Ultimately, effective volcanic reservoirs are governed by the synergistic coupling of primary pore preservation, dissolution-driven porosity enhancement, and structural fracturing. This study provides a quantitative geological basis for evaluating favorable volcanic reservoir intervals, offering a useful reference for analogous complex volcanic plays worldwide.