The Upper Permian succession of the northern Sichuan Basin records plume-related extension within a carbonate-platform system characterized by pronounced lateral facies differentiation. However, basin-scale sequence correlation, the evolutionary history of the Kaijiang–Liangping extensional trough, the stratigraphic affinity of fine-grained siliceous deposits developed in the upper Changxing Formation, and the distribution of organic-rich mudstone/shale remain poorly constrained. In this study, outcrop sections, well logs, core observations, and 2D seismic profiles were integrated to establish a high-resolution sequence-stratigraphic framework for the Wujiaping and Changxing formations, reconstruct the growth–partial infill–reactivation history of the Kaijiang–Liangping extensional trough, and evaluate the spatial distribution of fine-grained sediments. Six third-order sequences (SQ1–SQ6) were identified and correlated across the platform–trough system. The results demonstrate that the fine-grained siliceous deposits developed within the upper Changxing Formation represent a local facies variant rather than an independent lithostratigraphic unit. Facies patterns show marked differentiation between platform and trough-related domains, with deep-water shelf environments restricted to the trough axis during specific depositional stages. Organic-rich mudstone/shale intervals occur mainly near the base of transgressive systems tracts, and their thickest and most laterally continuous successions are developed along the trough axis. These results indicate that the distribution of fine-grained sediments was primarily controlled by accommodation partitioning, palaeogeographic differentiation, and variations in carbonate productivity, providing a framework for predicting organic-rich fine-grained intervals in extension-modified carbonate-platform systems.