Cite this paper:
Junhui Yu, Pin Yan, Yanlin Wang, Yan Qiu, Guanghong Tu, Changliang Chen. Origin of a giant fuzzy reflection zone and its implication for natural gas exploration in the southwestern Qiongdongnan Basin of the South China Sea[J]. Journal of Oceanology and Limnology, 2023, 41(2): 710-728

Origin of a giant fuzzy reflection zone and its implication for natural gas exploration in the southwestern Qiongdongnan Basin of the South China Sea

Junhui Yu1,2,3, Pin Yan1,2, Yanlin Wang1,2, Yan Qiu4, Guanghong Tu4, Changliang Chen1,5
1. Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou, 510301, China;
2. Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou), Guangzhou, 511458, China;
3. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, 266071, China;
4. Key Laboratory of Marine Mineral Resources, Guangzhou Marine Geological Survey, Ministry of Natural Resources, Guangzhou, 511458, China;
5. University of Chinese Academy of Sciences, Beijing, 100049, China
Abstract:
The southwestern depression of the Qiongdongnan Basin (QDNB) hosts thick Cenozoic sediments and awaits major hydrocarbon discovery. Multichannel seismic (MCS) profile CFT2011 across the southwestern QDNB reveals a ~60-km-wide fuzzy reflection zone (FRZ) within the sediments, but its origin and distribution remain unclear. Here ocean bottom seismometer (OBS) data of Line CFT2011 are processed with focus on the velocity structures by traveltime tomography inversion and analyzed together with the coincident and adjacent MCS profiles. The OBS velocity results show that the giant FRZ features lower velocity with difference up to 1.5 km/s and smaller vertical velocity gradient than the surrounding sedimentary sequences at the same depth, likely resulting from enhanced fluid infilling. The MCS profile exhibits that the giant FRZ is about 3-9-km thick and extends from the Paleogene strata rich in organic matters upward to the lower Pleistocene sediments. Within the shallow overlying sediments, multiple bright spots with reverse polarity are imaged and their reflection amplitudes increase with offset, consistent with the features of gas-charged sediments. They are probably shallow gas reservoirs with gases sourced from the deep FRZ. Therefore, the FRZ is proposed to be a giant gas-charged zone, which probably contains lots of hydrocarbon gases migrated vertically from the deep Paleogene source rocks through the boundary faults of the depressions and the minor fractures generated under overpressure. This FRZ is also imaged on the adjacent MCS profiles MCS-L1 and MCS-L2 with the width of about 40 km and 68 km, respectively. It is roughly estimated to cover an area of ~1 900 km2 and host a volume of ~11 400 km3 assuming an average thickness of 6 km, implying huge natural gas potential in the sedimentary depression of the southwestern QDNB of the South China Sea.
Key words:    southwestern Qiongdongnan Basin|fuzzy reflection zone|low velocity|gas charging|natural gas potential   
Received: 2021-12-31   Revised:
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Articles by Junhui Yu
Articles by Pin Yan
Articles by Yanlin Wang
Articles by Yan Qiu
Articles by Guanghong Tu
Articles by Changliang Chen
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