四川盆地奥陶纪-志留纪之交火山沉积的页岩有机质富集与生烃响应

Volcanic deposition develop in many shale gas section in China and abroad, and are correlated with the development of organic-rich shale and the accumulation of shale gas. However, the internal coupling mechanism is indeterminate, and the organic matter accumulation and hydrocarbon generation response of volcanic deposition are unclear. Deepen the study of previous work, the volcanic deposition and organic-rich shale through the Ordovician and Silurian transition in Sichuan Basin are chosen as the research object. Sedimentary characteristics of them are carried out in "Centimeter-millimeter scale" and "Micron-nanometer scale" to clear the spatio-temporal coupling relationship. On the basis of paleoenvironment multi-scale restoration (sedimentary environment evolution stage-lithofacies combination-lithofacies), taking the sedimentary environment response of volcanic events as the medium, the influence of volcanic events on organic matter generation - aggregation - preservation is clarified. Through static analysis of hydrocarbon generation effect of volcanic deposition and dynamic simulation experiment, hydrocarbon generation mechanism and dynamic process of shale organic matter under the influence of volcanic deposition were defined. The influence of volcanic deposition to shale gas formation is quantitatively evaluated. Finally, the accumulation process and hydrocarbon generation model of shale organic matter under the influence of volcanic deposition are established. This project explains the genetic mechanism of organic-rich shale and shale gas enrichment mechanism from a new perspective, and the research results can play an important role in shale gas geological evaluation and "sweet spot" prediction.

火山沉积广泛发育于国内外多套页岩气产层，且与富有机质页岩发育及页岩气富集密切相关，但其内在耦合机制并不明确，具体表现在火山沉积的有机质富集与生烃响应不清。基于前期工作，该项目选取四川盆地奥陶纪-志留纪之交的火山沉积与富有机质页岩为研究对象，开展“厘米-毫米级”、“微米-纳米级”宏微观沉积特征解析，明确二者的时空耦合关系；以古环境多尺度恢复（沉积环境演化阶段-岩相组合-岩相）为媒介，厘清火山沉积对有机质生成-聚集-保存的影响；通过火山沉积的生烃效应静态分析与生烃过程动态模拟实验，明确火山沉积影响下的页岩有机质生烃机理与动力学过程，定量评价其对页岩气富集的影响；最终建立火山沉积不同发育程度影响下的有机质富集过程与生烃模式。该项目从全新视角阐释富有机质页岩的成因机制与页岩气富集机理，研究成果将为页岩气地质评价与“甜点”预测提供理论支撑。

世界范围内的多处勘探实践表明，页岩气的富集高产与火山沉积存在关联。然而火山沉积的有机质富集与生烃响应尚不清晰。该项目以四川盆地奥陶系五峰组-志留系龙马溪组火山沉积与富有机质页岩为研究对象，分析了火山沉积与页岩的矿物学、岩石学及沉积学特征，明确了火山沉积对有机质富集与生烃的影响。研究认为：1、火山活动构造背景为岛弧环境，原始岩浆主要来自粗面英安岩及安山岩等，呈中酸性特征。火山沉积以黏土矿物为主，均值52%；非黏土矿物为石英、黄铁矿、长石等。页岩矿物主要为石英（均值35%）、黏土矿物（均值41%）及碳酸盐矿物（均值15%）等。研究区内识别出硅质页岩、凝灰岩等六种岩相，以及悬浮沉积、上升流等六种沉积作用类型。2、五峰组-龙马溪组一段自下而上识别出五个沉积环境阶段（Ⅰ-Ⅴ）。阶段Ⅰ以沉积环境的骤变为特征，沉积速率在全井段最低（介于2.04-4.08m/Ma）。阶段Ⅱ中古生产力高，水体呈缺氧-还原条件。阶段Ⅲ中沉积环境小幅变化。阶段Ⅳ-Ⅴ中古生产力降到最低，沉积速率达到最高（介于10-100.74m/Ma）。3、对于有机质的生成，尽管可以提供营养元素，但火山沉积对古生产力米级-厘米级的变化影响较小，这可能是因为：1）高频低强度的火山活动提升古生产力的持续时间较短；2）本区火山沉积富集Pb、Hg 等有毒元素，以及硫化物等有毒气体，对成烃生物有毒害作用。在研究区自身高生产力背景下，火山沉积对古生产力的提升被弱化。对于有机质的保存，火山沉积后的页岩层段并未延续火山沉积时的缺氧环境，研究区的强还原环境主要受冰期后的气候回暖及海侵作用影响。火山活动带来的地球内部高温气液对页岩生烃的影响相对较小，但火山沉积中的无机物质，主要是黏土矿物、放射性元素（U和Th）及过渡金属元素（Zr和Ni）长期与有机质相互接触和作用，对有机质的生烃起到催化作用，提高了生烃量及生烃效率。