High-temperature thermal simulation experiments (500–1000 °C) were conducted with samples from the Lower Cambrian Niutitang Formation shale to investigate its mineral evolution and pore development. Integrated analyses, including total organic carbon (TOC) content determination, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and pore structure characterization, reveal that clay minerals progressively decompose and transform into quartz during heating, with a new mineral phase mullite produced at a temperature above 800 °C. Concurrently, organic matter undergoes thermal evolution and shrinkage, creating distinctive shrinkage-induced fractures that enhance pore development. Clay mineral decomposition produces a more complex internal pore structure and fragmented reservoir matrix, generating pores at various scales. This process increases the number and size of micropores, mesopores, and macropores, creating intricate pore networks favorable for shale reservoir development. The study illuminates the intrinsic relationship between mineral transformation and pore development in highly mature marine shales exposed to elevated temperatures.