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AAPG Bulletin

Abstract

AAPG Bulletin, V. 93, No. 8 (August 2009), P. 1015-1037.

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

DOI:10.1306/04140908156

Mechanisms for oil depletion and enrichment on the Shijiutuo uplift, Bohai Bay Basin, China

Fang Hao,1 Xinhuai Zhou,2 Yangming Zhu,3 Yuanyuan Yang4

1State 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; [email protected]
2Tianjin Branch of China National Offshore Oil Company Ltd., Tianjin 300452, China
3Department of Earth Sciences, Zhejiang University, Hangzhou, Zhejiang 310027, China
4State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Fuxue Road No. 18, Changping, Beijing 102249, China

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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