Geological conditions for shale gas accumulation and favorable zone evaluation in the Longmaxi formation of the Yanyuan Basin

The Yanyuan Basin, situated along the southern margin of the Songpan-Ganzi fold belt and outside the Sichuan Basin, is a Meso-Cenozoic fault-depression superimposed basin. The Lower Silurian Longmaxi Formation within this basin exhibits promising shale gas potential; however, its accumulation conditions remain poorly constrained due to complex structural settings. In this study, we integrate core observations, thin-section analysis, well-log interpretation, and laboratory data to comprehensively investigate the lithology, stratigraphic distribution, depositional environment, reservoir characteristics, and preservation conditions of the Longmaxi shale. On this basis, favorable and prospective areas for shale gas exploration are delineated. Results demonstrate that organic-rich shale is mainly developed in the lower part of the first member of the Longmaxi Formation, characterized by thick intervals of deep-water carbonaceous siliceous mudstone with high total organic carbon content and type I-II kerogen. The formation is generally deeply buried, with high overpressure coefficients and limited tectonic disturbance, which has facilitated the preservation of pore-fracture systems. Furthermore, the central and southern parts of the basin are distinguished by thick sealing strata, relatively stable structural frameworks, and weak deformation, providing optimal conditions for shale gas enrichment and defining these regions as priority targets for exploration.