We used new ocean-bottom-node seismic data to obtain high-resolution images of a gas-bearing structure that holds one of China’s largest offshore gas reservoirs in the South China Sea. This structure, Dongfang 1-1, comprises clustered high-angle fractures in Miocene source rock intervals, with minimal disruption to stratigraphic layering. Seismic reflections within the structure are continuous without indicators of massive mud intrusion into the overlying sediments; therefore, a mud diapir model is unlikely to be applicable. We interpret that this structure formed from coalescing hydraulic fractures during upward propagation, driven by fluid overpressure from natural gas generation. These fracture networks serve as highly efficient pathways for gas-bearing fluids to bypass the 660-m-thick sealing sequences and accumulate in the Pliocene structural trap. Our results provide new insights into the formation mechanism of the Dongfang 1-1 structure and an exciting opportunity for further exploration of subsurface fluid migration in deep overpressured strata.