甲烷簇同位素非稳态行为主控因素及预测模型

Multiply isotopically substituted molecules (‘clumped’ isotopologues) can be used as geothermometers because their proportions at isotopic equilibrium relative to a random distribution of isotopes amongst all isotopologues are functions of temperature. This has allowed measurements of clumped-isotope abundances to be used to constrain formation temperature of several natural materials. This is a great technological revolution which can be used in solving many important and fundamental issues in almost all the fields. Presently, measurement of the clumped isotopologues in methane turned to be reality after that of carbonate. It will play a great role in natural gas industry due to methane as the primary component in natural gases and lack of effective measures. However, Kinetic processes during generation, modification, or transport of molecules can affect the clumped-isotope composition of natural materials. Non-equilibrium compositions lead to the measured proportions of clumped species to be lower than expected for a random distribution of isotopes (i.e. negative ∆18 values) in both biogenic methane and thermal methane. Such non-equilibrium clumped isotopic components in methane make it uncertainty to constrain the methane formation temperature. Above all, it needs us to understand the factors controlling the clumped isotopolues distribution and to know how to predict the non-equilibrium of clumped isotopolues. In this project, we design pyrolysis experiments to mimic the distribution of the clumped isotoplogues in thermal methane from two coals and some special materials. According to the difference of ∆18 values among samples at serial pyrolysis temperatures from those different source rocks, the trend of clumped isotopologues in thermal methane (non-equilibrium anti equilibrium) during pyrolysis experiments will be gotten. Compared to the various conditions in pyroysis experiments, the factors will be constrained in controlling the clumped isotopologues distribution. Natural gases will be collected and used to verify the results. Based on the results, we will propose a kinetically model of the evolution of the clumped isotopologues in methane with temperature and time. Also a model will be set up through which we can predict the non-equilibrium. This project will help us to understand the non-equilibrium distribution of clumped isotopologues in other materials. It will help us to clear the mechanisms of natural gases accumulations in the complex geological basins of our country.

簇同位素是继传统单元素同位素之后又一技术革命，作为温度计广泛应用于各领域重大基础问题的解决。甲烷簇同位素技术继碳酸盐之后获得成功开发，可大大突破天然气成分简单、可用信息少、研究手段受限的不足。然而，簇同位素组成非稳态现象时有发生，其偏离热力学平衡状态而无法反应生成温度，在生物成因和热成因甲烷中均有发现，为技术应用和发展带来不确定性。这就亟需我们查明造成簇同位素非稳态的因素并能够预测偏离及程度。本项目拟通过对比不同类型煤及特殊化合物热动力学模拟实验中甲烷簇同位素组成演化规律，通过分析实验过程中各种条件变化，结合地质盆地内甲烷簇同位素组成自然分布规律，查明热成因甲烷簇同位素非稳态形成机理及影响因素；通过数值模拟，建立预测模型；并对我国典型气藏分析，揭示天然气成藏机理；为深地领域资源勘探提供技术支撑，为发展天然气地质理论、簇同位素理论奠定基础。

项目基于物理模拟与数值模拟，通过对典型气藏解剖，通过收集世界相关类型天然气藏数据，详细研究了生物气、深层气、页岩气等类型天然气甲烷簇同位素平衡与分馏机制，建立了不同类型甲烷簇同位素不平衡主要控制因素与地质模式，取得了一系列认识，圆满完成了合同设定的任务。主要创新性成果有：（1） 建立了甲烷生成过程簇同位素动力学模型，成藏后期甲烷扩散同位素动力学分馏模型和多来源混合模型，甲烷聚合反应同位素分馏模型、甲烷氢同位素交换反应同位素分馏模型。查明甲烷簇同位素随着成熟度演化规律性，提出热成因甲烷簇同位素更易受动力学因素(KIE)影响，打破了常规上认为甲烷簇同位素只受热力学平衡（EIE）控制。为簇同位素技术开发和应用奠定了基础；（2）基于生物甲烷簇同位素平衡与不平衡特征及与常规参数之间关系，构建了一种快速准确评价生物甲烷生成机制的模型。该模型对监控生物气产生、成藏，地质微生物圈层变化具有重要意义，可灵敏反映生物甲烷的微小变化，未来检测浅层碳源原地生物气化发挥重要作用；（3）发现四川盆地高磨地区震旦系~寒武系深层天然气明显受TSR改造影响、沥青和天然气之间不具备亲子关系而是姊妹关系；构建超晚期差异压差驱动天然气成藏机制，并建议前寒武系天然气勘探应打破‘古油气区找气’的流行看法，应围绕高效页岩发育区设定勘探目标；（4）揭示松辽深层天然气簇同位素表观温度处于180~220oC，跟天然气干燥系数、CO2含量、甲乙烷分馏系数等等参数之间的关系证明烷烃气主要来自高过成熟煤系。