火山灰对湖相沉积有机质富集的影响机制及其页岩油地质意义

Volcanic ash sediments have always been discovered in high-quality source rock layers from prolific shale oil plays all over the world, so it can be deduced that the presence of volcanic ash may be significant to organic matter accumulation. Previous studies have controversial views on the relationship between volcanic ash and lacustrine productivity and the influence mechanism of volcanic ash on preservation and dilution of organic matter is still limited and unknown. In this study, the influence of volcanic ash on organic matter accumulation and its significance for shale oil formation will be researched systematically in lacustrine Lucaogou Formation of Jimsar Sag in Junngar Basin which is one of main shale oil plays in China. First of all, different core samples will be optimized based on content of volcanic ash, and then petrological and geochemical analysis of these samples will be investigated to define characteristics of volcanic ash sediments and associated source rock by integrating geological and geochemical data. Secondly, through comprehensive qualitative and quantitative analysis of the relationship between volcanic ash and water productivity, depositional environment, sulphate reduction, clay minerals, organic matter dilution respectively, the influence of volcanic ash on organic matter productivity, preservation and dilution will be clarified.. Considering characteristics of volcanic ash eruption, transportation and distribution, the model of organic matter accumulation and origin of tuffaceous source rock will be established. At last, through reservoir anatomization of typical shale oil plays, we will reveal the impacts of volcanic ash on shale generation potential and shale oil enrichment, and the role the volcanic ash play in the shale oil accumulation can be figured out to shed some light on shale oil exploration.

页岩油富集区优质烃源岩层与火山灰沉积具有良好的时空耦合关系，表明火山灰对有机质富集和页岩油勘探具有重要意义。前人研究主要集中于火山灰与生产力的关系且争议较大，而且火山灰对有机质保存和稀释的影响机制等关键问题缺乏系统研究。以准噶尔盆地吉木萨尔凹陷芦草沟组为靶区，开展火山灰对湖相沉积有机质富集的影响机制及其页岩油地质意义研究。针对靶区含火山灰烃源岩、泥岩及页岩等优选岩样，基于烃源岩岩石学和地球化学特征综合分析，阐明火山灰与生产力、沉积环境、硫酸盐还原作用、粘土矿物及有机质稀释的关系，明确火山灰对湖相有机质生产、保存和稀释的影响机制。结合火山灰喷发特征、搬运方式和沉积范围，揭示火山灰对湖相有机质富集的影响机制，构建湖相含火山灰沉积有机质富集模式，丰富有机质富集理论。解剖典型页岩油富集区，研究火山灰对烃源岩生烃潜力和页岩油富集的控制作用，揭示火山灰的页岩油地质意义，为页岩油勘探提供理论指导。

页岩油富集区优质烃源岩层与火山灰沉积具有良好的时空耦合关系，表明火山灰对有机质富集和页岩油勘探具有重要意义。前人研究主要集中于火山灰与生产力的关系且争议较大，而且火山灰对有机质保存和稀释的影响机制等关键问题缺乏系统研究。以准噶尔盆地吉木萨尔凹陷芦草沟组为靶区，开展火山灰对湖相沉积有机质富集的影响机制及其页岩油地质意义研究，揭示了火山灰对生产力和有机质保存条件的控制作用，建立了含火山灰页岩的有机质富集模式。芦草沟组组页岩矿物类型多样，有机质丰度和类型非均质性强。页岩中高峰度β-胡萝卜烷和C29规则甾烷表明蓝藻是有机质的重要来源之一，芦草沟组下段生产力最高、上段次之、中段最低，火山灰输入明显提高了生产力。芦草沟组下段沉积环境以还原环境为主，受火山灰输入影响，硫酸盐含量较高，有机质遭受硫酸盐还原作用降解明显，伴生大量草莓状黄铁矿，火山灰的输入促进了有机质降级，不利于有机质保存；芦草沟组中段和下段，沉积环境以弱氧化环境为主，有机质降解主要为有氧降解，火山灰对有机质降解没有明显的促进作用。综合分析火山灰对有机质生产和有机质保存的影响机制，建立了不同沉积环境的含火山灰页岩有机质富集模式。氧化环境下，火山灰引起藻类勃发，明显提高生产力，火山灰输入促进有机质富集；还原环境中，火山灰输入引起藻类暴发提高生产力的同时，可能引起生物硫酸盐还原作用，有机质富集受生产力和生物硫酸盐还原作用耦合控制，有机质丰度非均质性强。