As hydrocarbon exploration in the Songliao Basin progressively shifts from conventional to unconventional targets and from shallow to deep horizons, substantial breakthroughs and significant advancements have been achieved in the emerging area of tight gas exploration in the basin. However, tight gas exploration and development in the Yingcheng Formation of the Fulongquan fault depression remain restricted due to the limited previous studies on tight-sand reservoirs in this formation. In this study, integrated analytical techniques, including high-pressure mercury injection (HPMI), thin-section petrography, scanning electron microscopy (SEM), and X-ray diffraction (XRD), are employed to systematically investigate the petrophysical characteristics of the reservoirs and the primary factors controlling high-quality reservoirs. The results indicate that the reservoir spaces within the Yingcheng Formation in the Fulongquan fault depression are dominated by secondary pores formed by acid dissolution. The physical properties of the reservoirs are jointly governed by mechanical compaction and post-depositional diagenetic processes. Among these, acid dissolution is identified as the most favorable diagenetic process. Effective secondary pores can be generated through acid dissolution only when three conditions are met: an abundant supply of organic acids, favorable dissolution sites characterized by relatively high feldspar content, and an open fluid-flow system with substantial reservoir thickness and well-developed microfractures. Reservoir classification is conducted using HPMI data and fractal dimension analysis, with three classes of reservoirs identified. Accordingly, a comprehensive reservoir evaluation parameter (A) is established to delineate different classes of reservoirs. Based on the comprehensive classification criteria, play fairways are predicted by integrating single-well analysis with sedimentary facies distribution using a superposition method.

Volcanic reservoirs demonstrate strong heterogeneity and substantial variations in productivity due to the complexity of volcanic eruption and lithology. The main types of reservoir space are not clear, and the dominant lithofacies distribution, particularly the favorable areas for high-quality reservoirs, remains to be determined. In this paper, the Huoshiling Formation in the Dehui faulted depression, Songliao Basin is taken as an example to carry out the multi-scale joint characterization of its pore throat structure, establish a reservoir evaluation standard that considers both the gas content and seepage capacity, and perform reservoir evaluation and play fairway mapping under facies control. The results show that the storage space types of the gas-bearing reservoirs in the faulted depression can be ascribed into three categories and six subcategories according to the pore throat and pore characteristics. In terms of pore sizes, volcaniclastic lava rank the first, followed by volcaniclastic rocks, sedimentary volcaniclastic rocks and volcanic lava. The comprehensive evaluation parameter (Φ·K·S_g, where Φ is porosity, K permeability, and S_g gas saturation) of high-quality reservoirs are all greater than 0.1. The volcanic reservoirs in the Stage-Ⅲ strata are the highest in quality and largest in area of play fairways. The thermal debris flow sub-facies developed at Stage Ⅲ are mainly seen along the western strike-slip fault zone in the Debei sub-sag and the southwest Nong'an tectonic belt, while those developed at Stage Ⅰ are distributed along the central and eastern fault zones in the southeastern Baojia sub-sag. The favorable layer evaluation and favorable area delineation under facies control will be of certain reference significance for subsequent exploration and development of volcanic gas reservoirs.