Geological, geophysical, and geochemical characteristics of deep-routed fluid seepage and its indication of gas hydrate occurrence in the Beikang Basin, Southern South China Sea

Abstract

Deep-routed fluid seepage conduits are widely distributed in the Southern South China Sea (SSCS), and gas hydrates have been discovered inside these conduits through drilling and logging. Due to a lack of integrated geological, geophysical, and geochemical data, deep-routed fluid seepage conduits and their relationship to gas hydrate occurrence are still unclear. In this study, a comprehensive investigation of fluid seepage was conducted through systematic analysis of high-resolution 2-D seismic data, multibeam bathymetry data, headspace gas and carbon isotope components, authigenic carbonates, pore water, biomarkers, and hydrate phase equilibrium simulation. Multiple types of deep-routed conduits—which typically manifested as mud volcanoes, diapirs, fluid-escape pipes, and paleo-uplift associated faults—were identified. These manifestations were generally accompanied by Bottom Simulating Reflectors (BSRs), blanking zones, locally enhanced reflections, "pulled-up" reflections, pockmarks, mounds, and gas plumes, which indicated fluid seepage and possible gas hydrate accumulation. The Sulfate Methane Transition Zone (SMTZ) in the fluid seepage sites occurred at relatively shallow depths of 325–660 cmbsf (centimeter below the seafloor), suggesting a high methane flux and sufficient gas supply. The carbon isotope composition of the methane (−92.6‰ < δ13C < −50.2‰) and the detection of ethane, propane, and n-butane, as well as multiple geological configuration biomarkers, all indicate that deep-sourced hydrocarbons and thermogenic gas have migrated into the shallow layer through deep-routed conduits. Several geological and geochemical anomalies, including gas expansion, gas bubbling in piston core sediments, negative pore water chloride anomalies, and δ18O enrichment (3.8‰ < δ18O < 4.1‰) in the cold seep carbonates, provide evidence of hydrate dissociation during core recovery. The deep-routed fluid seepage and the accompanying hydrocarbon gas supply have a significant influence on the potential thermogenic gas-sourced hydrate accumulation and evolution. Overall, the co-existence of fracture-filling hydrates and pore-filling hydrates formed from the same supply of thermogenic gas and biogenic gas was inferred through comprehensive study and comparison with adjacent hydrate drilling areas. This article is the first systematic study of the characteristics of hydrate accumulation under the fluid seepage background in the Beikang Basin, and it provides a favorable reference for revealing the relationship between deep-sourced fluid seepage and hydrate formation. However, more exploration work needs to be conducted to confirm the above assumptions.