Characteristics and distribution of gas hydrate bearing sediments with high saturation in the Shenhu area, South China Sea

In 2017, the China Geological Survey conducted a gas hydrate exploration test in the Shenhu area of the northern South China Sea. The test achieved a cumulative gas production of 30.9×104 m3, with an average daily output exceeding 5000 m3 and a methane content of up to 99.5%. This successful test marked a historic breakthrough in the field of gas hydrate exploration. Prior to the gas hydrate exploitation test, detailed analyses were carried out on well logging data, seismic data, and drilling core samples from the study area. This analysis focused on characterizing the depositional environment, faults, gas composition, carbon isotopes, and gas hydrate saturation within the formation. The result revealed the following: ① Middle-high saturation gas hydrate is mainly concentrated along the ridges of the migrating canyon groups, while low saturation gas hydrate is mainly found in the valley bottoms. ② Submarine canyons serve as crucial pathways for transporting terrigenous materials to the deep sea. The development of turbidity channels and natural levees within these canyons is closely related to gas hydrate accumulation. ③ The resistivity and acoustic velocity of highly saturated gas hydrate layers increase significantly, whereas neutron porosity, density, and natural gamma values decrease. ④ Highly saturated gas hydrate displays characteristics of seismic reflections, including bottom simulating reflections (BSR), dome-like reflections, and chaotic reflections. Gas chimneys and fault pathways are often observed beneath the BSR, indicating the upward migration of hydrocarbon gas to shallower, stable temperature and pressure conditions favorable for formation. ⑤ Gas hydrate is mainly uniformly diffused in fine-grained sediments. ⑥ The gas source for hydrate formation is mainly a mixture of thermogenic and biogenic gas. Thermogenic gas migrates from deeper layers through fractures, fissures, and gas chimneys, where it degrades and mixes with in-situ biogas, which together provide a gas source for gas hydrate formation. © 2024 Geological Society of China. All rights reserved.