Controlling Factors of Vertical Geochemical Variations in Hydrate-Rich Sediments at the Site GMGS5-W08 in the Qiongdongnan Basin, Northern South China Sea

Huaxin Liu, Meijun Li (), Hongfei Lai (), Ying Fu, Zenggui Kuang and Yunxin Fang
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Huaxin Liu: National Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China
Meijun Li: National Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China
Hongfei Lai: National Engineering Research Center of Gas Hydrate Exploration and Development, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou 511458, China
Ying Fu: National Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China
Zenggui Kuang: National Engineering Research Center of Gas Hydrate Exploration and Development, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou 511458, China
Yunxin Fang: National Engineering Research Center of Gas Hydrate Exploration and Development, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou 511458, China

Energies, 2024, vol. 17, issue 2, 1-21

Abstract: Large amounts of natural gas hydrates have been discovered in the Qiongdongnan Basin (QDNB), South China Sea. The chemical and stable carbon isotopic composition shows that the hydrate-bound gas was a mixture of thermogenic and microbial gases. It is estimated that microbial gas accounts for 40.96% to 60.58%, showing a trend of decrease with the increase in burial depth. A significant amount of gas hydrates is thought to be stored in the mass transport deposits (MTDs), exhibiting vertical superposition characteristics. The stable carbon isotopic values of methane (δ 13 C 1 ) in the MTD1, located near the seabed, are less than −55‰, while those of the methane below the bottom boundary of MTD3 are all higher than −55‰. The pure structure I (sI) and structure II (sII) gas hydrates were discovered at the depths of 8 mbsf and 145.65 mbsf, respectively, with mixed sI and sII gas hydrates occurring in the depth range 58–144 mbsf. In addition, a series of indigenous organic matters and allochthonous hydrocarbons were extracted from the hydrate-bearing sediments, which were characterized by the origin of immature terrigenous organic matter and low-moderate mature marine algal/bacterial materials, respectively. More allochthonous (migrated) hydrocarbons were also discovered in the sediments below the bottom boundary of MTD3. The gas hydrated is “wet gas” characterized by a low C 1 /(C 2 + C 3 ) ratio, from 2.55 to 43.33, which was mainly derived from a deeply buried source kitchen at a mature stage. There is change in the heterogeneity between the compositions of gas and biomarkers at the site GMGS5-W08 along the depth and there is generally a higher proportion of thermogenic hydrocarbons at the bottom boundary of each MTDs, which indicates a varying contribution of deeply buried thermogenic hydrocarbons. Our results indicate that the MTDs played a blocking role in regulating the vertical transportation of hydrate-related gases and affect the distribution of gas hydrate accumulation in the QDNB.

Keywords: gas hydrate; biomarkers; extractable organic matter; sediment; mass transport deposits; Qiongdongnan Basin (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2024
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