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1 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249; present address: State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; haofang{at}cup.edu.cn
2 Tianjin Branch of China National Offshore Oil Company Ltd., Tianjin 300452, China
3 Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
4 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249, China
Fang Hao received his Ph.D. from China University of Geosciences in 1995. He is now the director of the State Key Laboratory of Petroleum Resources and Prospecting and the chair of the Academic Committee of the China University of Petroleum. He has conducted petroleum geology and geochemistry studies in several Chinese basins. His interest includes petroleum generation, migration, and accumulation in the Bohai Bay Basin.
Xinhuai Zhou received his Ph.D. in geology from the China University of Geosciences. He is now the chief geologist of the Technology Department of the Tianjin Branch of China National Offshore Oil Company Ltd. He has conducted petroleum geology studies in the Bohai Bay Basin for more than 10 years. His publications include studies of petroleum generation, migration, and accumulation in the offshore area of the Bohai Bay Basin.
Yangming Zhu received his Ph.D. from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences in 1993. He is now a professor of geochemistry at Zhejiang University. He has conducted petroleum geochemistry studies in several Chinese basins. His interest is now in the study of deposition and evolution of lacustrine source rocks.
Yuanyuan Yang graduated in 2007 with a degree in geochemistry from the Yangtze University and is now a graduate student at the China University of Petroleum. Her interest is in the study of biomarker compositions of lacustrine source rocks and crude oils in the Bohai Bay Basin.
ABSTRACT
The Shijiutuo uplift is a major uplift to the north of the Bozhong depression, the largest generative kitchen in the Bozhong subbasin, Bohai Bay Basin. Although the N35-2 trap on this uplift contains a medium-size oil accumulation and the Q32-6 trap contains China's third largest offshore oil accumulation, the Q31-1 trap between the N35-2 and Q32-6 traps with very similar evolution history was confirmed to be dry. Biomarker associations of crude oil and source rock samples were analyzed, and three-dimensional migration pathway modeling was conducted to investigate the origin of oils and mechanisms for oil enrichment and depletion on the uplift. Multiple-parameter oil-source correlation and hierarchical cluster analysis using 10 selected biomarker parameters allowed the identification of four source-related oil classes. Almost all oils from the Shijiutuo uplift are derived from the Eocene Shahejie Formation, whereas oils found between the Shijiutuo uplift and the Bozhong depression either are derived from or have important contributions from the Oligocene Dongying Formation. Variations in oil classes and biomarker parameters suggest sequential migration of oil generated from the Shahejie and then Dongying formations in the Bozhong depression, which is reasonably supported by petroleum migration pathway modeling. Oil charge from two oil-prone source rock intervals and, more importantly, focusing of oil originating from a large area of the Bozhong generative kitchen into the same trap accounted for oil enrichment and formation of China's third largest offshore oil field in the Q32-6 structure. The complexity and primary control of the sealing surface (top surface of the carrier bed) morphology on the positions of migration pathways caused the Q31-1 trap to be shielded from migration of oil originating from the Bozhong depression, resulting in oil depletion in this trap. Shadows to petroleum migration may occur because of the three-dimensional behavior of petroleum migration, and two-dimensional migration modeling may be misleading in predicting petroleum occurrences.
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