Gas hydrates associated with gas chimneys in fine-grained sediments of the northern South China Sea

Abstract

Drilling for gas hydrates at the northern continental margin of the South China Sea (SCS) provides a unique insight into the formation of the gas hydrate system in fine-grained sediments, even though most high concentrations of hydrates are found in coarse sediments. Detailed studies of 3D seismic data from a deepwater basin (Baiyun Depression) of this region reveal the presence of overpressured fluid flow, mostly manifested in gas chimneys. Gas chimneys are commonly characterized by disrupted reflections (DR), dim amplitude anomalies (DA), and enhanced reflections (ER) on conventional seismic profiles. Gas chimneys are also characterized by chaotic, low-continuity and low-frequency on instantaneous frequency and instantaneous amplitude profiles. Gas hydrates and a prominent BSR were discovered in the region of gas chimney occurrence. Logging data and sample analyses from drill holes over the chimney structures indicate close relationships between active fluid flow and the formation and accumulation of gas hydrates. Our results document that gas chimneys consisting of a connected network of fractures provide a passageway along which fluids ascend beneath the gas hydrate stability zone (GHSZ) for hydrate formation in fine-grained sediments of the northern South China Sea. Fluids accumulate in uppermost part of the slope where they are trapped beneath the prograding submarine delta sequence, which is not permeable enough for fluids to migrate through. Gas-charged fluids may originate from deep-seated hydrocarbon reservoirs, which are indicated by the molecular and isotopic signatures of gases in gas hydrates occurrence zone. We suppose that the fractures extend into the GHSZ, creating a space for hydrate condensation in the fine-grained sediments of the slope. Gas chimneys may account for the overpressured fluid activity and, most likely, the gas hydrate enrichment in the northern SCS.