湘西北早寒武世热水活动特征及其对有机质富集的影响

The black rock series with hydrothermal deposited genesis in the Lower Cambrian Niutitang Formation in Northwestern Hunan show relatively high abundance of organic matter, and are one of the most important Lower Paleozoic source rocks strata in Southern China . However, no study has provided clear insights into the primary characteristics of this hydrothermal activity in the sediments as well as the mechanism and degree of this hydrothermal activity’s effect on the organic matter enrichment. In this study, the Sancha, Siduping, and Sanjiaoping sections located in Zhangjiajie and Yuanling in Northwestern Hunan were chosen as the type sections for the Niutitang Formation. The mineralogical, petrological, and geochemical properties of the black rock series and their relationship there were systematically analyzed through a detailed section observation and mass of sample analyses, and the comprehensive geological sections were then established based on the results. Then in hydrothermal sedimentary rocks, through element, isotopes, cathodoluminescence and energy spectrum, fluid inclusion analysis and so on, we were performed to investigate the properties of the hydrothermal fluids, and the intensity and evolution of the hydrothermal activity, divide the cycles of sedimentation and reveal the type and characteristic of hydrothermal sedimentation. In the hydrothermal activity sedimentary setting, the relationship of the abundance of organic matter with the amounts and types of the organic matter source, and preservation conditions was established based on the results of the element, biomarker, organic macerals analyses et al. The main factors that affected the organic matter enrichment and the corresponding mode of enrichment were finally determined. Therefore, the explanations and solutions presented in this paper can act as a complement to the existing of hydrothermal sedimentation, and this study has both theoretical and practical significance to future research on the origins of high-quality source rocks found in Lower Paleozoic of Southern China.

湘西北下寒武统牛蹄塘组热水成因的黑色岩系均具有较高的有机质丰度，是南方下古生界重要的烃源岩层位之一，但该区热水活动的主要特征及其对沉积有机质富集的作用机理和影响程度尚不清楚。本项目拟分别选取湘西北张家界和沅陵地区三岔、四都坪和三角坪等三条典型沉积剖面，旨在通过详细的剖面观测和大量的采样分析，系统揭示牛蹄塘组黑色岩系矿物岩石学、地球化学特征及组合关系，建立综合地质剖面。通过热水沉积岩元素、同位素、阴极发光及能谱、流体包裹体等分析，阐明热水活动的流体性质、活动强度和演化过程，划分热水沉积旋回并揭示热水沉积作用类型和特征。结合元素、生物标志化合物和显微组分等分析，揭示热水活动与烃源岩有机质来源、丰度、类型及保存条件之间的内在联系，阐明影响有机质富集的主要因素并最终建立富集模式。上述问题的研究和解决将进一步认识热水活动，并对湘西北牛蹄塘组优质烃源岩的成因机理研究有重要的理论和现实意义。

湘西北下寒武统牛蹄塘组热水成因的黑色岩系均具有较高的有机质丰度，是南方下古生界重要的烃源岩层位之一，但该区热水活动的主要特征及其对沉积有机质富集的作用机理和影响程度尚不清楚。本项目对该地区典型的露头剖面和井剖面对比分析，研究表明黑色岩系具有有机质丰度高、多元素富集、生物化石发育和强还原环境下形成的等特点；其属于混合硅质或含粘土硅质页岩相，压裂指数较高，并处于晚成岩阶段B-C亚期，有机质演化处于高-过成熟阶段；TOC含量普遍较高，变化大最高可近30%，TOC分布剖面上一般向上逐渐减小，平面上位于保靖-慈利断裂带上的斜坡区相对较高；热水活动局限于热液或火山喷口附近，但其产物可广泛沉积于整个地区，张家界和贡溪可能是平面上两个热水活动比较集中的区域；花页1井元素地球化学特征表明远离当时的热水活动及沉积中心，受热水沉积的影响非常有限，整体地球化学指标趋势表明热水活动强度向上逐渐减弱至正常沉积，但有部分微量元素ppm级含量可能变化很大；热水活动带来的影响使测井曲线与TOC值响应变得更为复杂，通过大量、多相关变量数据学习和基于神经网络方法建立了预测模型，其效果显著优于传统测井预测方法；页岩的吸附性能对于湘西北牛蹄塘的页岩气富集非常重要，TOC是湘西北牛蹄塘组黑色页岩纳米孔隙发育的主要影响因素，深度则对致密页岩纳米级孔隙性能的影响较小，保存条件应是研究区页岩气富集的关键因素之一；平面上有机质丰度较高的范围在吉首、张家界一带的斜坡区，与热水物质沉积的主要区域大致一致，剖面上TOC分布均具有底部值高，向上逐渐减小的特征，也与热水活动向上逐渐减弱的趋势一致，热水活动与有机质富集作用良好的时空耦合关系，反映了热水活动对有机质富集正面的促进作用，但可能只是有机质富集的一个充分非必要条件；高生产力和强还原的保存条件应是黑色岩系有机质富集的两个最重要条件，而黑色页岩的沉积及有机质富集是多方面、多因素条件综合形成的。