气体吸附过程的碳同位素分馏机理及地质意义研究

Carbon isotope composition is a key parameter for natural gas geochemical research. Compared with conventional natural gas, the geochemical characteristics of shale gas have certain specificities. For example, the methane carbon isotope composition of partial shale gas is obviously negative, and carbon isotope reversals of shale gas in over mature stage. The difference between the occurrence state of shale gas and conventional natural gas may be an important reason for the abnormal geochemical characteristics of shale gas. The carbon isotope fractionation in gas adsorption process is a factor that cannot be ignored. Based on the literature research and prior period study, we found that the carbon isotope fractionation in the gas adsorption process is significant and the control factors are very complex. The adsorption fractionation mechanism and influencing factors have not been studied clearly at present. This project intends to study the mechanism and influencing factors of carbon isotope fractionation in the process of gas adsorption by using the self-made gas adsorption/desorption instrument, taking into account the properties of shale, the characteristics of gas generated in different stages and experimental conditions, and the "penetration curve method" principle and theoretical calculation are used to study the mechanism and influencing factors of carbon isotope fractionation in gas adsorption process. Moreover, the research results are applied to the study of the shale gas geochemical anomalies combining with the geological background of shale gas in Ordos basin and Sichuan basin, which provides new evidence for the study of the origin of carbon isotope negative and isotope reversals of shale gas, it is expected that the connotation and geological significance of the gas geochemical parameters of shale gas could be revealed.

碳同位素组成是天然气地球化学研究的关键性参数，与常规天然气相比，部分页岩气的甲烷碳同位素组成明显偏轻，高过成熟阶段经常出现页岩气碳同位素倒转等异常现象。页岩气与常规天然气赋存状态的差异可能是造成页岩气地球化学特征异常的重要原因，其中气体吸附过程中的碳同位素分馏是一个不容忽视的因素。研究发现气体吸附过程中碳同位素分馏显著且控制因素非常复杂，具体的吸附分馏机理和影响因素目前并未研究清楚。本项目拟采用课题组自制的气体吸附/解吸仪，综合考虑页岩的各项性质、不同阶段生成气体的特征以及实验条件等因素，利用“穿透曲线法”原理及理论计算来研究气体吸附过程中碳同位素分馏机理及影响因素。结合我国鄂尔多斯盆地和四川盆地页岩气的地质背景将其应用于页岩气气体地球化学特征异常的研究中，为部分页岩气甲烷碳同位素偏轻和高过成熟阶段页岩气碳同位素倒转成因研究提供新的证据，揭示页岩气气体地球化学参数的内涵和地质意义。

部分页岩气与煤层气的地球化学特征存在一些异常，甲烷碳同位素组成偏轻是异常之一，气体吸附/解吸过程中的分馏是一个重要的因素。该项目采用不同组成的混合气体和各类吸附剂（包括延长组页岩、龙马溪组页岩、石英砂、碳酸钙、不同目数的活性炭、高岭土、蒙脱土、伊利土、各种规格的分子筛、高阶煤、褐煤等）研究了气体吸附/解吸过程中组分和碳同位素分馏机理。研究结果表明，矿物组成、孔隙结构与大小、TOC、颗粒粗细等特征均对气体组分和碳同位素分馏有重要影响，其中岩石孔隙结构特征可能是主控因素。在此基础上，重点围绕鄂尔多斯盆地延长组陆相页岩气进行重点解剖，首先证明了延长组页岩气甲烷碳同位素组成在地下原位状态时就是偏轻的，同时应用活性炭和5A分子筛的气体吸附分馏实验结果，认为延长组页岩气甲烷碳同位素组成偏轻是由于有机质和粘土矿物吸附保留了生物气和早期热成因气所致，偏轻程度与吸附作用息息相关。另外，还解释了龙马溪组页岩气和牛蹄塘组页岩气烷烃碳同位素组成异常的原因。此外，在本项目的资助下，研究了鄂尔多斯盆地延长组陆相页岩可溶有机质中饱和烃和NSO极性化合物在轻微-中等生物降解过程中的变化及对相应生物标志化合物参数的影响。