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AAPG Bulletin, V.
Overpressuring mechanisms in the Yinggehai Basin, South China Sea
1Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China; email: [email protected]
2China Offshore Oil Nanhai West Company, Zhanjiang, Guangdong, 524057, China
3China Offshore Oil Nanhai West Company, Zhanjiang, Guangdong, 524057, China
4Department of Geology, Wichita State University, Wichita, Kansas, 67260; email: [email protected]
Xiaorong Luo obtained his B.S. and M.S. degrees in geology from Northwestern University, China, where he taught petroleum geology for 5 years. He obtained his Ph.D. in geophysics from the University of Montpellier, France. His research interests of last 20 years have been in petroleum geology, currently focusing on numerical modeling, geopressuring, and hydrocarbon migration and accumulation.
Weiliang Dong has a B.S. degree in geology from Southwestern Institute of Petroleum, China. He works on geological studies and management of hydrocarbon exploration for the China National Offshore Oil Company, with research interests on geological conditions of oil and gas accumulation and exploration of natural-gas plays. He conducted research on hydrocarbon systems in petroliferous basins and has over 20 years of exploration experience in Chinese offshore basins.
Jihai Yang obtained a B.S. degree in geology from Zhongshan University of China in 1985. He has worked on petroleum geology and natural-gas exploration, principally in Yinggehai and Qiongdongnan basins, for the China National Offshore Oil Company since 1985. His research interests are in hydrocarbon systems, especially hydrocarbon migration and accumulation in high-temperature and -pressure basins.
Wan Yang is an assistant professor researching in outcrop and subsurface sedimentology and stratigraphy of marine and nonmarine mixed-carbonate and siliciclastic rocks, paleoclimatology, and petroleum geology. He previously worked with Phillips Petroleum Company and the Bureau of Economic Geology, University of Texas, on hydrocarbon exploration and exploitation. He holds a B.S. degree from Northwestern University, China, an M.S. degree from California State University, Fresno, and a Ph.D. from the University of Texas at Austin.
The authors benefited much from the discussions with G. Vasseur, J. Y. Wang, Z. S. Gong, Q. M. Zhang, S. T. Li, F. Hao, H. H. Chen, and X. N. Xie. We thank K. Magara, J. G. Gong, R. Erickson, and an anonymous reviewer for their suggestions. This study was supported partly by the Chinese National Major Fundamental Research Developing Project (G1999043310) and by the Chinese National Natural Science Foundation (49732005), both to X. R. Luo.
Yinggehai Basin is an elongate Cenozoic rift basin on the northwestern margin of the South China Sea continental shelf. Its thick (17 km) basin fill is characterized by high geothermal gradient and high overpressure. Overpressure associated with nonequilibrium compaction mainly occurs at depths more than 2800 m at the basin center and more than 4000 m at the basin margin because the shallow-buried Neogene and Quaternary strata lack effective seals. This regional overpressure distribution, however, is disrupted at basin center where high overpressure occurs in permeable formations at a depth as shallow as 1400 m on top of a series of deep-seated faults and fractures. We studied the processes and mechanisms of overpressuring via numerical modeling that couples basin filling, sediment compaction, and thermal and pressure fields to approach the origin of the shallow high overpressure. Model results indicated that an increase of fluid volume due to natural-gas generation by organic cracking is not large enough to generate the overpressure because of the limited amount of organic matter. The shallow overpressure has probably been generated allogenically. Deep open faults have served as vertical hydraulic conduits and channeled the deep high pressure into shallow permeable formations.
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