--> Abstract: Origin of Crude Oils in the Subtle Petroleum Traps in the Dongying Depression and Implications for Exploration in Deep Horizons of the Bohai Bay Basin, by Sumei Li, Maowen Li, Xiongqi Pang, Keyu Liu, Zhengxue Jiang, Guiqiang Qiu, and Yongjin Gao; #90078 (2008)

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Origin of Crude Oils in the Subtle Petroleum Traps in the Dongying Depression and Implications for Exploration in Deep Horizons of the Bohai Bay Basin

Sumei Li1, Maowen Li2, Xiongqi Pang1, Keyu Liu3, Zhengxue Jiang1, Guiqiang Qiu4, and Yongjin Gao4
1Key Laboratories for Petroleum Formation Mechanisms of Chinese Ministry of Education, Basin & Reservoir Research Center, China University of Petroleum, Beijing, China
2Geological Survey of Canada, Calgary, AB, Canada
3CSIRO, Perth, WA, Australia
4Sinopec Shengli Oilfield Company, Dongying, China

Subtle petroleum traps have become an important exploration play in the Bohai Bay Basin, east China. Although significant discoveries have been made from such traps, the mechanisms of petroleum accumulation in these traps remain to be resolved. Most subtle traps in the Dongying Depression occur in the middle section of the Es3 member of the Eocene-Oligocene Shahejie Formation, where lenticular turbidite sands are encompassed in deep-water lacustrine mudstones. Oils trapped in these turbidite sands were thought to have been derived from the surrounding lacustrine mudstones within the Es3 strata, but this presumed oil-source relationship is not supported by the results of our latest study. Detailed analysis of 41 produced oils and a large number of potential source rock samples indicates that the Es3 mudstones enclosing the turbidite sands are characteristic of freshwater lacustrine sediments. However, oils trapped in these reservoirs correlate well with the underlying Es4 source rocks deposited in a brackish lacustrine setting. Mass balance calculations using biomarker concentrations suggests that oils in the Es3 subtle traps were most likely derived from mixed sources, with the contribution from the upper Es4 source rocks being dominant. In the absence of obvious structural or stratigraphic pathways, oil migration from the Es4 source rocks through a thick interval of relatively fine-grained rocks in the lower section of the Es3 member needs to be invoked in order to explain the observed oil-source relationship. This finding has important implications for future exploration in the deep horizons of the Bohai Bay Basin, as the petroleum resource potential of the Es4 and older source rocks formed at an early stage of Cenozoic rift development may be significantly larger than that predicted by current petroleum system models.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas