Classification and exploration practices of shale gas reservoirs in the Wufeng-Longmaxi formations in the Sichuan Basin and its periphery

The Sichuan Basin and its periphery exhibit multiple shale sequences, various types of shale gas reservoirs, and considerable resource potential. Continuous advancements in shale gas exploration and exploitation have revealed that the geological conditions for shale gas accumulation vary significantly across different zones and sequences in this region, accompanied by complex shale gas enrichment patterns. This necessitates systematic classification of shale gas reservoirs to guide exploratory drilling for new types of shale gas reservoirs in underexplored zones and sequences. In this study, we investigate the conditions favorable for shale gas enrichment and high productivity, as well as the gas reservoir characteristics, of the Wufeng-Longmaxi formations. The shale gas reservoirs are systematically classified based on 12 factors and 21 specific indicators. Focusing on six individual key factors (i. e., sedimentation, reservoir property, formation pressure coefficient, burial depth, in-situ stress, and structural style), a set of classification schemes and criteria is proposed. This approach enables the establishment of a comprehensive classification and assessment system for shale gas reservoirs based on the dynamic combinations of multiple factors. Accordingly, shale gas reservoirs or gas-bearing structures within the Sichuan Basin and its periphery are classified. The classification and assessment system is applied to the classification of shale gas reservoirs in underexplored areas along the basin margin and beyond. The results reveal that the Nanchuan shale gas field, located along the basin margin, is characterized as moderately deep to deep, marine, fractured-porous, and dry shale gas reservoirs with medium to high in-situ stress and normal pressure conditions. In contrast, the Wulong gas-bearing structure, positioned outside the basin, is classified as shallow to moderately deep, marine, fractured-porous, and dry shale gas reservoirs with medium to low in-situ stress and normal pressure conditions. These findings provide a solid foundation for reserves reporting and the emplacement of exploration and appraisal wells in the Sichuan Basin and its periphery.