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.
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Data Availability Statement
The seismic data that support the findings of this study are available from the corresponding author upon reasonable request.
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Acknowledgment
We thank the crew and scientists of R/Vs Tanbao and Dongfangkantan No. 1 for seismic data collection. Dr. Pengchun LI form South China Sea Institute of Oceanology, Chinese Academy of Sciences is acknowledged for insightful discussions. We also thank two anonymous reviewers for their crucial and constructive comments. Some of the figures in this study are generated with Generic Map** Tools (GMT) (Wessel and Smith, 1995).
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Supported by the Guangdong Basic and Applied Basic Research Foundation (Nos. 2022A1515011836, 2021A1515110851), the Science and Technology Planning Project of Guangzhou (No. 202201010230), the Special Support Program for Cultivating High-Level Talents in Guangdong Province (No. 2019BT02H594), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (No. GML2019ZD0104), the Open Fund of Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences (CAS) (No. MGE2020KG01), and the National Natural Science Foundation of China (Nos. U1901217, 91855101, 41876052, 42076218). Dr. Junhui YU is funded by the Special Research Assistant Funding Program of CAS
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Origin of a giant fuzzy reflection zone and its implication for natural gas exploration in the southwestern Qiongdongnan Basin of the South China Sea
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Yu, J., Yan, P., Wang, Y. et al. 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. Ocean. Limnol. 41, 710–728 (2023). https://doi.org/10.1007/s00343-022-1452-3
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DOI: https://doi.org/10.1007/s00343-022-1452-3