Shale reservoir characteristics and exploration potential of the Permian Wujiaoping Formation in the Nanya Syncline, Sichuan Basin, China

The Permian Wujiaping Formation (WJP FM) in the Sichuan Basin is a significant successor shale gas play to the Silurian Longmaxi Formation. To systematically evaluate the reservoir characteristics of the Permian WJP FM shale, this study integrates multi-scale experimental analyses, including geochemistry, X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM) on core samples from the Nanya Syncline, Sichuan Basin. Results show that the third member (Wu-3 Member), deposited in deep-water shelf facies, is the core reservoir interval. Within it, siliceous shale concentrated in Sublayers 3–6 represents the most favourable lithofacies. This siliceous shale is rich in biogenic silica, with reservoir space dominated by organic-matter-hosted pores and enhanced by lamellation fractures. It exhibits moderate porosity (avg. 5.6%), high gas content (avg. 5.2 m ³ /t), and high brittleness (brittle minerals 83.95%), with Sublayer 4 identified as the optimal fracturing interval. Organic matter is predominantly Type II ₁ –III kerogen, over-mature (avg. Ro 2.8%), providing abundant gas and organic pores. Reservoir development is governed by three synergistic factors: an anoxic deep-water shelf environment that enabled organic enrichment and biogenic silica formation; over-mature thermal evolution that generated abundant organic pores; and Yanshanian–Himalayan tectonic compression that induced microfracture networks, while the broad, gentle synclinal geometry favoured pressure maintenance and gas preservation. Based on these findings, an evaluation system integrating sedimentary environment, reservoir quality, and preservation conditions is established, and three categories of favourable zones are delineated. The Class I favourable zone in the syncline core is the primary exploration target. This study provides a systematic framework for shale gas potential evaluation in structurally complex areas and direct guidance for well placement and stimulation design.