Spatial distribution and inventory of natural gas hydrate in the Qiongdongnan Basin, northern South China Sea

Natural gas hydrate is a potential clean energy source and is related to submarine geohazard, climate change, and global carbon cycle. Multidisciplinary investigations have revealed the occurrence of hydrate in the Qiongdongnan Basin, northern South China Sea. However, the spatial distribution, controlling factors, and favorable areas are not well defined. Here we use the available high-resolution seismic lines, well logging, and heat flow data to explore the issues by calculating the thickness of gas hydrate stability zone (GHSZ) and estimating the inventory. Results show that the GHSZ thickness ranges between mostly ∼200 and 400 m at water depths >500 m. The gas hydrate inventory is ∼6.5×10 9 -t carbon over an area of ∼6×10 4 km 2 . Three areas including the lower uplift to the south of the Lingshui sub-basin, the Songnan and Baodao sub-basins, and the Changchang sub-basin have a thick GHSZ of ∼250–310 m, 250–330 m, and 350–400 m, respectively, where water depths are ∼1 000–1 600 m, 1 000–2 000 m, and 2 400–3 000 m, respectively. In these deep waters, bottom water temperatures vary slightly from ∼4 to 2 °C. However, heat flow increases significantly with water depth and reaches the highest value of ∼80–100 mW/m 2 in the deepest water area of Changchang sub-basin. High heat flow tends to reduce GHSZ thickness, but the thickest GHSZ still occurs in the Changchang sub-basin, highlighting the role of water depth in controlling GHSZ. The lower uplift to the south of the Lingshui sub-basin has high deposition rate (∼270–830 m/Ma in 1.8–0 Ma); the thick Cenozoic sediment, rich biogenic and thermogenic gas supplies, and excellent transport systems (faults, diapirs, and gas chimneys) enables it a promising area of hydrate accumulation, from which hydrate-related bottom simulating reflectors, gas chimneys, and active cold seeps were widely revealed.