Origin of Oils in “Subtle pools” in the Dongying Depression, Bohai Bay Basin, China
Origin of Oils in “Subtle pools” in the Dongying Depression, Bohai Bay Basin, China
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DOI：
10.1111/j.1755-6724.2008.tb00337.x
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发表时间：
2008-02
期刊：
影响因子：
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通讯作者：
Li Sumei;Zhengxue Jiang;L. Keyu;Guiqiang Qiu;Yongjin Gao
中科院分区：
文献类型：
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作者：
Li Sumei;Zhengxue Jiang;L. Keyu;Guiqiang Qiu;Yongjin Gao
摘要：隐蔽圈闭或油藏已成为渤海湾盆地东营凹陷重要的勘探方向。尽管近年来在勘探方面取得了一些成功，但隐蔽圈闭的形成机制仍然没有得到很好的理解。东营凹陷隐蔽油藏大多发育在古近系沙河街组沙三段中段，隐蔽圈闭主要为泥岩包裹的透镜状盆底浊积砂体。隐蔽圈闭中的石油以前被认为是直接从同一地层（Es 3）的周围烃源岩运移而来的。目前，对该凹陷41种原油和41种岩石样品的详细地球化学调查表明，隐蔽圈闭中的原油与周围的沙三段烃源岩不能很好地对比，其特征是Pr/Ph值高（>1），γ蜡烷/C30 Hopane值低，代表淡水湖相环境。与此相反，原油具有低Pr/Ph（<1）和相对高的伽马蜡烷含量，显示与下伏沙四段烃源岩的成因相似，其也具有相同的性质，指示半咸水湖环境。沙三段隐蔽圈闭中的原油可能为混源油，沙四段上部烃源岩的贡献占主导地位。因此，非常规的石油运移和聚集机制需要被援引来解释来自沙四段烃源岩的石油聚集，这些石油可能通过沙三段烃源岩的厚的低层段而没有明显的构造或地层通道。我们认为，隐蔽油气运移通道可能起着重要的作用。这一发现对东营凹陷今后的油气勘探和剩余资源评价具有重要意义。
Abstract: Subtle traps or oil pools have become an important exploration play in the Dongying Depression, Bohai Bay Basin, east China. Despite recent successes in exploration, the formation mechanisms of subtle traps are still not well understood. The majority of subtle oil pools in the Dongying Depression are developed in the middle interval of the Es3 Member of the Paleogene Shahejie Formation with the subtle traps being primarily of lenticular basin‐floor turbidite sands encompassed in mudstones. Oil in the subtle traps was previously thought to have migrated directly from the surrounding source rocks of the same formation (Es3). Detailed geochemical investigation of 41 oils and 41 rock samples from the depression now indicates that the oils from the subtle traps cannot be correlated well with the surrounding Es3 source rocks, which are characterized by high Pr/Ph (>1), low Gammacerane/C30hopane, representing a freshwater lacustrine setting. In contrast the oils features low Pr/Ph (<1) and relatively high Gammacerane content, showing a genetic affinity with the underlying Es4 source rocks, which also have the same qualities, indicating a brackish lacustrine setting. Oils in the Es3 subtle traps are probably derived from mixed sources with the contribution from the upper Es4 source rocks predominating. Therefore unconventional oil migration and accumulation mechanisms need to be invoked to explain the pooling of oils from the Es4 source rocks, which probably came through a thick low interval of the Es3 source rocks with no apparent structural or stratigraphic pathways. We suggest that the subtle oil migration pathway probably plays an important role here. This finding may have significant implications for future exploration and the remaining resource evaluation in the Dongying Depression.