Abstract: Paleogene hydrothermal activity in Bohai Bay Basin has a significant influence on the organic matter (OM)enrichment and formation of source rocks. Based on the analytical and testing data of rock pyrolysis, major and trace elements, rare earth elements and biomarkers, the paper explores the mechanism of OM enrichment under the background of hydrothermal activity in the Paleogene Shahejie Formation in the southeastern margin of Bohai Bay Basin, and then establishes the OM enrichment mode under different intensity of hydrothermal activity. The results show that using the threshold value of δEu=2, total organic carbon (TOC)content tends to increase first and then decrease with an increase in hydrothermal activity, and the water reducibility follows a pattern of first strengthening and then weakening. This suggests that hydrothermal activity plays a role in controlling the formation of OM-rich shale. Specifically, under the appropriate intensity of hydrothermal activity (δEu<2), the upwelling of deep hydrothermal fluid brings a large amount of nutrients, which can promote algal blooms and rapidly increase paleo-productivity. Additionally, a large number of reductive gas from hydrothermal fluids, such as H₂S and SO₂, and can also facilitate the formation of anoxic water body, which is favorable for the sedimentary OM preservation. Therefore, the hydrothermal activity with appropriate intensity is conducive to large-scale OM enrichment. Under the strong hydrothermal activity (δEu>2), the intensified upwelling destroys the OM preservation conditions, and the amount of dissolved oxygen in bottom waters increases. Moreover, a large amount of OM will be oxidized and decomposed, leading to the gradually decreasing TOC content. Using δEu=2 as the threshold value, this paper sets up the OM enrichment mode dominated by paleo-productivity and preservation conditions under the moderate hydrothermal activity, and the OM differential distribution pattern under intense hydrothermal activity. Future research should focus on investigating the coupling relationships between hydrothermal activity, water conditions (involving salinity, temperature, and non-nutrients), hydrocarbon-forming organism species, and OM enrichment, so as to better explain the mechanisms by which hydrothermal activities influence the OM formation and accumulation. The research results are of significant guidance and reference value for identifying the distribution of high-quality source rocks and then locating the oil and gas reservoirs.

Key words: faulted lacustrine basin, organic matter enrichment mode, hydrothermal activity, main controlling factors, Eu anomaly

摘要： 渤海湾盆地古近纪热液活动对沉积物中的有机质富集和烃源岩形成具有重要影响。综合岩石热解、常量元素、微量元素、稀土元素以及生物标志化合物等分析测试资料,探究了热液活动背景下渤海湾盆地东南缘古近系沙河街组—东营组烃源岩的有机质富集机理,建立了不同热液作用强度下的有机质富集模式。研究结果表明,随着热液活动强度不断增大,以δEu=2为界,烃源岩的总有机碳(TOC)含量具有先增大后减小的变化趋势,水体还原性具有先增强后减弱的变化规律,指示热液作用对研究区富有机质泥页岩的形成具有控制作用。当热液作用强度适宜(δEu＜2)时,深部热液上涌带来的大量营养物质会促进藻类勃发,古生产力迅速增大,且热液流体中携带的大量H₂S、SO₂等还原性气体将促进缺氧性水体的形成,有利于沉积有机质保存,因此强度适宜的热液作用有利于有机质大量富集。当热液活动强烈(δEu＞2)时,强烈的喷涌作用破坏有机质的保存条件,底层水体中氧气的溶解量增加,有机质被大量氧化分解,导致TOC含量逐渐降低。以δEu=2为界,建立了热液强度适宜条件下古生产力-保存条件主导的有机质富集模式,以及热液活动强烈条件下的有机质差异分布模式。未来,应重点探究热液活动-水介质条件(包括盐度、温度和非营养物质)-成烃生物种属-有机质富集之间的耦合关系,以更好地解释热液作用对沉积中有机质形成与富集的影响。研究认识对于明确优质烃源岩的分布进而寻找油气藏具有重要指导与借鉴意义。

关键词: 断陷湖盆, 有机质富集模式, 热液活动, 主控因素, 铕异常