The Pen-1 West Sag is an important area for future hydrocarbon exploration in the hinterland of the Junggar Basin. However, systematic studies on the characteristics and development mechanisms of its deep to ultra-deep Permian–Triassic reservoirs remain limited.

Core, well-log, analytical, and seismic data were integrated to systematically investigate the petrological characteristics, reservoir properties, pore structures, and diagenetic evolution of the Permian–Triassic reservoirs in the Pen-1 West Sag and to identify the main factors controlling differences in reservoir quality.

The reservoirs are predominantly lithic sandstones, with volcanic lithic fragments accounting for more than 80% on average; intermediate to mafic volcanic rock fragments dominate. Reservoir properties show significant vertical variations. The Karamay and Baikouquan Formations exhibit the best reservoir properties, with porosities mainly ranging from 5% to 13% and permeabilities mostly in the range of 0.1×10⁻³–10.0×10⁻³ mD, indicating relatively high-quality reservoirs. In contrast, the Upper and Lower Urho Formations have poorer reservoir properties. Secondary pores dominate the reservoir pore system, with dissolution pores within zeolite cements and intragranular dissolution pores within volcanic lithic fragments being the most abundant secondary pore types. Secondary pores account for more than 70% of the total pore space in the Baikouquan and Upper Urho Formations, whereas they account for 40%–60% in the Karamay Formation, where residual primary intergranular pores remain important.

Reservoir property differences are jointly controlled by multiple factors, including provenance characteristics, lithology and grain size, diagenetic alteration (grain-coating development, zeolite cementation and dissolution, and clay-mineral filling), the composition of volcanic lithic fragments, and temperature and pressure conditions. Based on the diagenetic evolution sequence and variations in the proportions of pore types, two reservoir property evolution models are proposed: a primary-pore-preservation type, represented by the Karamay Formation, and a secondary-pore-dominated type, represented by the Baikouquan and Upper Urho Formations.

These findings provide an important basis for predicting and evaluating deep clastic reservoirs in the Pen-1 West Sag and in other areas with similar geological conditions.