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

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

Weilin Zhu is chief geologist at China National Offshore Oil Corporation. He received his Ph.D. in 2001. His main research interests include petroleum systems, basin analysis, and risk evaluation of petroleum exploration.

Baojia Huang is a senior geologist at the Research Institute of China National Offshore Oil Corporation Ltd., Zhanjiang, and a guest professor in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. He received his Ph.D. in petroleum geology from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences in 2002. His main research interests include petroleum geochemistry, reservoir geochemistry, and petroleum systems.

Lijun Mi is a project director in the Exploration Department, China National Offshore Oil Corporation Ltd. He received his Ph.D. from China University of Petroleum in 2007. His current research interests include petroleum system analysis and risk evaluation of petroleum exploration.

Wilkins was formerly a chief research scientist at Commonwealth Scientific and Industrial Research Organization Petroleum, where he worked on fluid inclusions, laser instrumentation, and novel methods for maturity determination based on the fluorescence of organic matter.

Ning Fu is a senior geologist at the Research Center of China National Offshore Oil Corporation Ltd. His main research interests are petroleum geochemistry and geology.

Xianming Xiao is a professor in Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. He received his Ph.D. in geology from Beijing University of Mining and Technology in 1989. His current research interests include the dynamics system of petroleum accumulation, with particular attention to hydrocarbon generation and cracking kinetics.

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 ¹³C₂ values (–25 to –27). 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 ¹³C₂ values (–24 to –34). Widely variable concentrations of bicadinane and oleanane were identified 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.