页岩气吸附/解吸过程中的气体地球化学研究

The geochemical characters of shale gas have certain particularity, for example, δ13C of CH4 in shale gas is obviously lighter than that of conventional gas, and carbon isotope composition mostly shows 'rollover' trend in over mature stage. In the general, the generating mechanism of shale gas is similar with that of conventional gas, which means the origins of shale gas is also of biogenic, thermogenic or the mixture of both. Therefore, the causes resulting in the geochemical particularity may be mainly related to the storage of shale gas, and geochemical parameters fractionation during adsorption/desorption process is a very important one of the reasons. The geochemical particularity of shale gas limits the application of geochemical discriminating methods which is built on the base of conventional gas research, however, it could provide significant information about the forming mechanism, storing state and accumulating process of shale gas if treated from another perspective. This program will investigate the gas composition and carbon/hydrogen isotopic fractionation of shale gas, and their influencing factors during adsorption/desorption process through experimental measurement, theoretical calculation and case analysis, but also the causes of lighter isotope composition and isotopic reversal in over mature stage as well. Make the following three aspects of the research content. First, the relationship between the absorption ability of the shale and fractionation of gas geochemical parameters during adsorption/desorption process. Scond, the study on the influence of adsorption/desorption effect on the geochemical parameters for different maturity of shale gas. finally, the essence and geological significance of the gas geochemical parameters of shale gas. Moreover, combining with the geological background of shale gas in Ordos basin and Sichuan basin, it is expected that the essence and geological significance of the gas geochemical parameters of shale gas could be revealed.

页岩气的气体地球化学特征存在一定的特殊性，例如甲烷碳同位素组成明显偏轻，高过成熟阶段出现碳同位素组成序列的倒转等。页岩气的生成机理与常规天然气基本一致，不外乎生物成因、热成因或者这两种的混合。因此，造成页岩气地球化学特殊性的原因可能主要与其赋存状态有关，其中页岩气吸附/解吸过程的地球化学分馏是一个不容忽视的原因之一。页岩气地球化学特征的特殊性制约了我们利用常规天然气建立的地球化学判识体系在页岩气成因研究中的应用，但是可能从另一个方面为我们提供页岩气形成机制、赋存状态以及成藏过程等有意义的信息。本项目拟以实验测试、理论计算和实例解剖三个方面研究页岩气吸附/解吸过程中组份及碳氢同位素组成分馏的大小及其与影响因素，探讨页岩气甲烷碳同位素组成偏轻以及高过成熟阶段出现同位素组成序列倒转现象的原因，结合我国鄂尔多斯盆地和四川盆地页岩气的地质背景，揭示页岩气气体地球化学参数的内涵和地质意义。

页岩气的气体地球化学特征存在一定的特殊性，这些特殊性是研究页岩气形成过程与赋存状态的理想参数。页岩气吸附/解吸过程中的气体地球化学分馏是造成页岩气地球化学特征特殊性的重要原因之一。因此，本项目开展了3方面的研究工作：①页岩气自然解析过程中气体地球化学参数分馏研究；②不同介质吸附/解析过程中气体地球化学参数分馏实验研究；③鄂尔多斯盆地延长组页岩气厌氧微生物氧化作用研究。研究结果表明，页岩气在解析过程中气体干燥系数逐渐降低，甲、乙烷碳同位素组成逐渐变重，氢同位素组成变化较小，实验温度和气体解析率均是碳同位素组成分馏的影响因素，造成这种现象的原因主要与页岩气的吸附/解吸作用和同位素的扩散运移分馏作用有关。通过穿透曲线法系统地研究了混合气体在不同介质吸附过程中组分及同位素的地球化学分馏，结果表明不同吸附剂吸附过程中的烷烃气体碳同位素分馏特征存在较大差异，出现三种不同的模式，通过理论计算及空白实验表明这些碳同位素分馏不是由扩散造成的，而是由孔径分布特征控制着气体传输过程中的同位素分馏。此外，我们通过气体地球化学特征、轻烃分布特征等证据充分表明了鄂尔多斯盆地延长组页岩气遭受了一定程度的微生物降解作用。研究了乙烷等重烃气体的生物降解顺序及各组分碳、氢同位素组成变化规律，同时表明甲烷也遭受了微生物氧化作用，降低了页岩气资源量。这个研究明确了微生物氧化烃类气体的顺序C3 =nC4 =nC5 > iC5 =iC4 > C2 > C1，提出了轻微-中等微生物氧化作用的敏感识别指标。同时研究认为C6轻烃馏分的生物降解顺序为nC6＞2-MP≈3-MP＞MCP＞2,3-DMB＞2,2-DMB＞CH。明确了轻烃被生物降解的敏感性受三个因素控制：碳骨架、烷基化程度和烷基化位置。