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AAPG Bulletin

Abstract

AAPG Bulletin, V. 93, No. 6 (June 2009), P. 741-761.

Copyright copy2009. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/02170908099

Geochemistry, origin, and deep-water exploration potential of natural gases in the Pearl River Mouth and Qiongdongnan basins, South China Sea

Weilin Zhu,1 Baojia Huang,2 Lijun Mi,3 Ronald W. T. Wilkins,4 Ning Fu,5 Xianming Xiao6

1China National Offshore Oil Corporation Ltd., Beijing 100010, China
2Research Institute of China National Offshore Oil Corporation Ltd.-Zhanjiang, Zhanjiang 524057 China: present address: State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China
3Exploration Department, China National Offshore Oil Corporation Ltd., Beijing 100010, China
4CSIRO Petroleum, P.O. Box 136, North Ryde, New South Wales, Australia
5Research Center, China National Offshore Oil Corporation Ltd., Beijing 100010, China
6State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China; xmxiao@gig.ac.cn

ABSTRACT

More than 10 gas pools have been discovered since 1983 in the shallow-water region of the Pearl River Mouth (PRM) Basin and the Qiongdongnan (QDN) Basin, offshore South China Sea. Gases produced from the QDN basin are characterized by high contents of benzene and toluene and relatively heavy delta13C2 values (minus25 to minus27permil). The associated condensates have a high abundance of bicadinanes and oleanane, providing a good correlation with the coal-bearing sequence of the Oligocene Yacheng Formation in the basin. In contrast, the gases from the PRM basin contain lower amounts of benzene and toluene and lighter delta13C2 values (minus24 to minus34permil). Widely variable concentrations of bicadinane and oleanane were identifiied from the associated condensates, which can be mostly correlated with the lower Oligocene Enping Formation source rocks formed in a swamp to shallow lake environment. Oil-cracked gases sourced from the Eocene oil-prone source rock may also provide some contribution to the PRM basin gases. The available geochemical data indicate that both the Yacheng and Enping formations contain mainly type III and II2 kerogens with dominant gas potential. Regional geological information indicates that the deep-water regions of the two basins share the same hydrocarbon source sags with the shallow-water areas, and they developed massive sandstone reservoirs during the Oligocene and Miocene. Fluid-flow modeling results show that the deep-water regions were on the pathway of lateral migrating gases, and the interpreted reservoirs in these zones have developed abundant seismic bright spots, which may reflect the presence of gas. The deep-water regions of the offshore South China Sea are believed to have great gas exploration potential.

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