Architecture of a mud diapir-like structure: insights from ocean-bottom-node seismic data

We present high-resolution ocean bottom node (OBN) seismic data of the Dongfang 1-1 structure in the Yinggehai Basin of the South China Sea, which hosts China's largest offshore gas reservoir. The OBN seismic data reveals more continuous and detailed reflections compared to conventional seismic data, highlighting the internal structure and formation mechanism of a diapir-like structure. The seismic images show a tapered conical structure characterized by a concentric distribution of fractures, with a significant increase in fracture intensity and connectivity towards the center. These fractures, particularly the sub-vertical ones, are interpreted as natural hydraulic fractures formed due to intense overpressurization in the Lower Miocene strata, with formation pressure coefficients up to 2.2. The fractures are believed to have originated from thermogenic hydrocarbon gas generation and inorganic CO₂ production. The throughgoing fractures that traverse the entire Neogene succession, including the thick Upper Miocene sealing mudrocks, provide crucial pathways for deep gas-bearing fluids to charge the Pliocene sandstone reservoir. The study underscores the importance of natural hydraulic fractures in bypassing thick sealing sequences and conduiting fluids in deep overpressured environments. Moreover, our results may provide guidance for accurate geological interpretations of mud diapir-like structures in conventional seismic images in many other sedimentary basins.