The development of faults within cap rocks commonly affects the sealing capacity and preservation conditions of oil and gas reservoirs. A systematic study on this issue is of great significance for hydrocarbon exploration. This study focuses on faults within effective mudstone cap rocks to elucidate their impact on hydrocarbon preservation capacity by systematically characterizing the lithology and stress state of fault planes, in conjunction with the distribution of gas reservoirs. Taking the mudstone cap rock of the Guantao Formation in an oilfield in the Bohai Bay Basin as an example, the lithology and stress state of fault planes are characterized using the shale gouge ratio (SGR), slip tendency, and dilation tendency based on geo-physical interpretation, formation pressure, and in-situ stress data. By integrating the morphology and trapping characteristics of gas reservoirs, the impact of faults on gas reservoir preservation is comprehensively analyzed. Fault lithology and dilation tendency are the main factors affecting fault plane sealing. Sealing faults are characterized by low dilation tendencies (<0.6) and high SGR (>0.3). For footwall gas reservoirs, their up-dip direction is often obscured by faults, and reservoir development is influenced by fault plane properties. In the hanging wall, two scenarios exist: drag folds and thrust-related folds, causing reservoir development to be affected by different factors. When drag folds develop in the hanging wall, reservoir development is influenced by the sealing capacity of the fault. When thrust-related folds develop in the hanging wall, the cap rock is the primary factor for natural gas preservation, and the fault sealing capacity mainly affects the reservoir scale. This comprehensive analysis of fault plane parameters and reservoir development characteristics provides new insights into how fault development within mudstone cap rocks affects their preservation ability. The findings provide important insights for evaluating the preservation conditions and guiding exploration deployment in fault-developed areas.