This work examines the influences of fault movement, induced fluid flow, and thermal mudstone evolution on the secondary porosity generation and diagenesis in Eocene–Miocene sandstones from the Western Bozhong depression, offshore Bohai Bay Basin, using quantitative petrographic analyses, stable isotopes, quantitative fluorescence, and quantitative evaluations of fault movement, seal, and fluid flow. Mudstones at shallow, moderate, and deep depths have different capacities in providing organic acids/CO₂ for mineral dissolution and ions for the precipitation of carbonate cements in adjacent sandstones. Moderate and deep mudstones are mature and able to provide organic acids/CO₂ and other ions to adjacent sandstones. However, shallow mudstones are not mature and have limited mass supply capacity. Subsequent fault movement activates fluid migration, resulting in the redistribution of organic acids/CO₂ (including naphthenic acid/CO₂ from hydrocarbon biodegradation), the acceleration of secondary porosity generation, and the export of byproducts. Under the joint influences of thermal mudstone maturity and fault movement-induced fluid flow, organic acids/CO₂ could have been sourced from adjacent mudstone (internal source, deep model, >2500 m), transported from deep formations through faults (external source, shallow model,