Abstract: [Objective] The Pen-1 West Sag is an important replacement area for hydrocarbon exploration in the hinterland of the Junggar Basin. However, systematic studies on the characteristics and formation mechanisms of its deep to ultra-deep Permian–Triassic reservoirs remain relatively weak. [Methods] This study comprehensively utilizes core, logging, analytical testing, and seismic data to systematically investigate the petrology, physical properties, pore structure, and diagenetic evolution characteristics of the Permian–Triassic reservoirs in this sag, and reveals the main controlling factors for reservoir quality differences. [Results] The results show that the reservoirs are predominantly lithic sandstones, with volcanic lithic fragments averaging over 80%, among which intermediate–basic extrusive rock fragments dominate. The reservoir physical properties vary significantly vertically: the Karamay Formation and Baikouquan Formation have the best physical properties, with porosity mainly ranging from 5% to 13% and permeability mostly concentrated between 0.1 mD and 10 mD, belonging to relatively high-quality reservoirs, while the Upper Urho Formation and Lower Urho Formation have poorer physical properties. The reservoir pores are dominated by secondary pores, among which dissolution pores of zeolite cements and intragranular dissolution pores of volcanic lithic fragments are the most developed. [Conclusions] Statistics show that secondary pores account for more than 70% of the total pore space in the Baikouquan Formation and Upper Urho Formation, whereas in the Karamay Formation, secondary pores account for about 40%–60%, and residual primary intergranular pores still occupy an important position. The differences in reservoir physical properties are jointly controlled by multiple factors, including provenance characteristics, lithology (grain size), diagenetic alteration (coating development, zeolite cementation and dissolution, clay filling), composition of volcanic lithic fragments, and temperature–pressure field conditions. Based on the analysis of diagenetic evolution sequences and the differences in reservoir pore type proportions, two types of reservoir physical property evolution models are proposed: the primary pore-preserved type (represented by the Karamay Formation) and the secondary pore-dominated type (represented by the Baikouquan Formation and Upper Urho Formation). [Significance] This understanding provides an important basis for the prediction and evaluation of deep clastic reservoirs in the Pen-1 West Sag and similar geological settings.