Origin and geological significance of the first discovered seafloor oil seepage in the Beikang Basin, southern South China Sea

Seafloor oil seepage, resulting from fluids migration from depth, provides valuable insights into the characteristics of potential source rocks and hydrocarbons. For the first time, seafloor seepage oils were obtained by piston coring in the Beikang Basin, southern South China Sea (SSCS). Based on integrated analysis of the seismic data, core sediments, porewater, headspace gas and heat flow, the geochemical characteristics of seafloor seepage oils and their geological implications were elucidated. Results indicate that deep-sourced hydrocarbons intruded into subsurface sediments via paleo-uplift-related faults, fluid seepage pipes and mixed with in -situ modern marine sediments. Biomarkers suggest the presence of both marine and terrestrial organic matter, as well as a diverse range of thermal maturity levels from immature to mature. The seafloor seepage oil was most likely originated from coal-related source rocks in the Oligocene-Lower Miocene. Deep hydrocarbons were expelled through fractures induced by hydrocarbon-generating overpressure, as well as source rock-connecting faults and finally accumulated atop the paleo-uplift beneath the T3 unconformity. The gradually increased fluid pressure ultimately leads to the fracturing of overlying caprocks, resulting in the formation of fluid seepage pipe and the occurrence of oil seepage on the seafloor. Seafloor oil seepage and the accompanied high methane flux caused biogeochemical process, elements cycling and shoaled the depth of sulfate-methane transition zone (SMTZ), which provides valuable information in indicating gas hydrates occurrence. This study offers significant geological and geochemical information to reveal potential origin of source rocks and hydrocarbons, particularly in the case of limited drilling wells in the SSCS. In addition, this study also advances the understanding of gasbearing fluids activity and the coupling relationship between deep and shallow strata under a focused fluid seepage background.