Bentonite Characterization and Its Specific Significance of Silurian-Ordovician Shale Gas Exploration and Development in the Southern Sichuan Basin, China

As a product of ancient volcanic ash deposition, Bentonite exhibits unique lithological, mineralogical, and mechanical characteristics in shale formations, yet its impact on shale gas reservoir exploration and development remains inadequately understood. Taking the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in the southern Sichuan Basin as an example, this study investigates the paleogeographic environment of Bentonite development. By integrating lithostratigraphy, paleobiological stratigraphy, chemicostratigraphy, shale reservoir sensitivity test and core-based Frac test, it proposes that Bentonite layer could be a critical factor influencing shale exploration and development effectiveness in southern Sichuan. Research reveals that bentonite layers are characterized by high clay mineral content, pronounced natural gamma ray response, and stable lateral distribution, with mechanical properties differing from the surrounding shales, potentially affecting hydraulic fracture vertical propagation and fracture network morphology. Through innovative construction of the Bentonite Index (BI Index), which comprehensively evaluates its development frequency, cumulative thickness ratio, elastic parameters, stress difference, and varying combinations with adjacent shales, three distinct distribution patterns are identified to establish a quantitative hierarchical assessment of bentonite layers' impact on shale gas development effectiveness. This index provides geological basis for sweet spot and fracturing interval selection, fracture height control, and stimulation effectiveness prediction, promoting integrated “geology-engineering” optimization design and holding significant implications for enhancing shale reservoir exploration and development.