Study on mechanical properties and control mechanism of fine-grained sedimentary rocks of the Permian Longtan Formation in western Guizhou, SW China

Rock mechanical properties of the Permian Longtan Formation in western Guizhou control sweet spot identification and fracturing optimization. Using integrated organic geochemical, petrological, petrophysical, and mechanical analyses, this study investigated the mechanical properties and their controlling mechanisms in fine-grained sedimentary rocks. Key findings include: (1) The fine-grained sedimentary rocks are characterized by rich organic matter, over-maturity, high clay content, low brittle mineral, and low porosity and ultra-low permeability. Although their present-day hydrocarbon generation potential is limited, the conditions are favorable for effective shale gas preservation. (2) Decreasing grain size correlates with lower total organic carbon (TOC), quartz, porosity, and water saturation, but higher vitrinite reflectance ( R o ), clay content, pore sorting, and mesopore proportion, identifying mudstone and carbonaceous mudstone as hydrocarbon “sweet spots”. (3) During uniaxial compression, siltstone and silty mudstone exhibit high strength/brittleness with tensile failure, whereas mudstone and carbonaceous mudstone show low strength/brittleness with shear failure. (4) Quartz is the primary positive control on strength and brittleness, while TOC and clay content are the main negative controls. (5) The tidal flat siltstone and silty mudstone represent the typical high compressive strength and high brittleness model, characterized by low TOC, clay content, porosity, macropore proportion, and water saturation, coupled with high brittle mineral content, pore sorting, and permeability. Their dominant failure mode under uniaxial compression is tensile splitting. This study provides a theoretical basis for predicting the sweet spot and selecting the fracturing interval.