扬子地区奥陶纪-志留纪转折期火山灰沉积与有机质富集关系探讨

The Ordovician-Silurian transition is a critical turning interval in geological history, which was characterized by dramatic turnovers of sea-level, palaeoclimate, biotic mass, and so on, and extensive studies have been carried on. Organic-rich black shale series intercalated by volcanic ash layers were extensively deposited in the Yangtze area, which has been the most important target interval of shale gas exploration and development in China. However, the present studies less involved in relationship between these volcanic ash layers and organic enrichment. This project will focus on the Wufeng Formation and Lungmachi Formation, and aims to characterize the high-resolution primary bio-productivity and further to discuss on the relationship between volcanic ash layers and organic enrichment, based on integrated studies on sedimentology, TOC, element geochemistry and isotope geochemistry of typical section, which maybe unravel the distribution of organic-rich shale. Therefore, this study not only will have a significant implication for better understanding of the correlation between organic enrichment and volcanism, but also will provide an useful guide to exploration and development of shale gas in Sichuan Basin and its adjacent areas.

奥陶纪-志留纪转折期是地质历史上的关键时期，全球海平面、气候、生物面貌等均发生了重大转变，以此开展的相关研究较多。该时期我国扬子地区广泛沉积了一套夹大量火山灰层的黑色富有机质页岩，是我国页岩气勘探开发的重点层系。但目前对这些火山灰沉积与有机质富集关系方面的研究涉及较少。本项目以扬子地区五峰组-龙马溪组为主要研究对象，选择火山灰层发育的典型剖面，通过开展沉积学、TOC、元素地球化学、同位素地球化学等综合研究，建立（火山灰层）高分辨率的地层综合剖面，重点开展五峰组-龙马溪组底部高分辨率古生产力综合表征，探讨火山灰沉积对有机质富集的影响程度，进一步明确有机质富集层段及分布规律。因此，本项研究不仅对探索奥陶纪-志留纪转折期火山活动与有机质富集的关系具有重要的科学意义，也可为我国四川盆地及周缘页岩气勘探开发提供一些理论指导。

现代火山活动喷发形成的火山灰沉降到海水中能够提高海洋表层生产力、促进浮游生物繁盛，有利于有机质形成与富集沉积。而地质历史时期火山灰（斑脱岩）沉积与有机质富集关系研究相对较少，且一直存在着争议。我国华南地区奥陶纪-志留纪转折期五峰组-龙马溪组含气页岩中发育大量火山灰沉积，目前为我国页岩气勘探开发的重点层系，是开展这一研究的典型层系。通过开展沉积学、TOC、元素地球化学、同位素地球化学等综合研究，建立（火山灰层）高分辨率的地层综合剖面，重点开展五峰组-龙马溪组底部高分辨率古生产力综合表征，探讨火山灰沉积对有机质富集的影响程度。研究结果初步表明，奥陶纪-志留纪转折期，火山灰沉积对海洋生产力提高相对较弱，并未明显促进该时期的有机质富集；而沉积水体氧化还原条件（富氧—贫氧—缺氧变化）与有机质含量关系密切，是这一时期有机质富集的主控因素。具体来说，五峰组—龙马溪组页岩沉积时期的海洋表层水体总体高生产力，是有机质大量生成的重要前提条件；硫化缺氧的水体条件是控制页岩有机质纵向上及区域上富集沉积的关键因素。这也是我国南方五峰组-龙马溪组页岩气差异富集的关键因素。