页岩解析气碳同位素分馏特征及其指示意义研究

In shale gas exploration and development process, desorption experiment of shale core is the most direct, quick and effective method to obtain the shale gas content. But in the process of exploration and scientific research only the gas content was focused, and the carbon isotope fractionation of desorption gas which was an important geochemical phenomena was ignored usually in the past .On the basis of literature research and preliminary study, we found that desorption gas carbon isotope fractionation is mainly composed of parse diffusion, percolation effect and desorption process. This project is based on the desorption experiment to the upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation shale cores of drilling well in fuling areas, and collect desorption gas, core samples continuously , and analysis the relevant gas and core samples , to clarify the relationship between the characteristics of carbon isotope fractionation and shale gas content, occurrence state, physical condition, mineral composition, organic matter type, etc. Research results could provide theoretical support to study shale gas formation and occurrence mechanism, it could be used to identify the shale gas reservoir.

页岩气勘探开发过程中，利用页岩气现场解析仪对页岩岩芯进行现场解析是获得页岩含气量最为直接、快速和有效的方法。但在以往的勘探和科研过程中主要关注页岩含气量，忽视了页岩解析气碳同位素分馏这一重要的地球化学现象。在文献调研和预研究的基础上，我们发现页岩解析气碳同位素分馏主要由解析过程中的扩散、渗流作用和解吸过程导致。本项目拟通过对涪陵焦石坝地区在钻取芯井上奥陶统五峰组-下志留统龙马溪组不同埋深的页岩岩芯样品进行现场解析、连续采集解析气样和岩芯样品，并对解析气和岩芯样品进行相关分析测试，厘清碳同位素分馏特征与页岩含气量、赋存状态、物性条件、矿物组成、有机质类型等的关系。研究成果可为研究页岩气成因和赋存机理提供理论支撑，可为识别页岩气优质产层、选择压裂层位提供参考。

围绕“页岩解析气碳同位素分馏机理”这一主题，按照申请书设计的研究内容，利用“页岩气现场解吸仪”在钻井现场对目的层页岩岩心进行解吸，快速、准确地获得页岩的现场含气量数据，并且研究解析气组分和同位素随解吸时间的规律，结合井口气、岩心样品等进行了系统的地球化学研究，取得以下认识：1）龙马溪组富有机质页岩含气量与TOC正相关，大量有机质微孔、介孔是页岩气的储集空间。2）解析气烷烃气组分和同位素变化规律表明，一阶解析气以游离气为主，同位素分馏较小，是解吸过程中的“质量分馏效应”导致；二阶解吸以吸附气为主，同位素分馏较大，是“吸附-解吸”和“质量分馏效应”导致；同时，解析气同位素分馏越大，相应样品层段的含气量越大。3）焦石坝地区井口气和解析气烷烃气碳同位素序列均出现“完全倒转”现象，可能是同源不同期具有不同碳同位素组成的页岩气复合的结果，随解吸时间增加会出现碳同位素倒转程度增大的现象。4）通过实测解析气量和同位素数据，求取解析气碳同位素分馏系数，建立了解析气同位素分馏的数学模型，预测结果与实测结果吻合，可以在此基础上建立能够预测页岩气产量的同位素动力学模型。稳定同位素等地球化学方法的引入，为勘探过程中识别页岩气产层、判断页岩气保存条件等提供重要参考，对页岩气的勘探有理论和现实意义。