Quantitative Characterization and Differential Evolution of Pore–Fracture Systems in Volcanic Reservoirs of the Changling Fault Depression, Songliao Basin

Abstract

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.