中上扬子地区龙马溪组页岩微孔结构演变与页岩气成藏效应

Taking the organic-rich shale & argillite in Longmaxi Formation of Upper-Middle Yangtze region as the example, this project concentrates on the relation between structure evolution of micropore and hydrocarbon accumulation effects in shale-gas reservoirs. By taking the step of field study - experiment - theoretical analysis, and investigating the samples by mercury porosimetry, low-temperature liquid nitrogen adsorption,SEM/FESEM, isothermal adsorption and other methods, we work out 8 factors: effective thickness, depth of burial, abundance of organic matter, maturity, gas saturation, brittleness, porosity，tectonically reworked intensity.Based on the key elements, and taking further research on depositional environment, evolution through diagenesis, sturcture of nano-pore in clay mineral and organic matter, evolution of source rock during thermal maturity, distribution of gas in reservoirs and the relationship of these three statements: free, absorbed, dissolved gas, evolution of structural history and thermal history as well as source - reservoirs reconstruction character, combining with inversion of pore structure evolution, this article focuses on how those factors involve in the developing of pore structure, as well as building the deposition - diagenesis, structural - thermal evolution model, and finding out the coupling relationship of occurrence of shale gas and pore structure evolution in source rock, furthermore their influence on shale gas accumulation. The final purpose is to reveal the mechanism of how pore structure evolution works on the forming - accumulation - enrichment of shale gas, so as to predict the positive area and arrange the strategy of exploration and exploitation.

围绕页岩微孔结构演变与页岩气成藏效应这一核心科学问题，以中上扬子地区龙马溪组富有机质黑色泥页岩为对象、以野外调查-实验测试-理论分析为思路，借助高压压汞、低温液氮吸附、扫描电镜/场发射扫描电镜和等温吸附等先进测试技术，以页岩有效厚度、埋藏深度、有机质丰度、成熟度、含气饱和度、岩石脆度、孔隙度、构造改造强度等成藏"八度"要素研究为基础，通过研究沉积环境与成岩作用演化过程、粘土矿物与有机质微（纳米级）孔隙结构特征、有机质热成熟生烃演化、气体赋存空间与游离、吸附和溶解等"三态"方式，及构造-热史演化与源-储改造特征等内容，结合孔隙结构演化过程反演，重点研究其与孔隙结构演变的关系，构建沉积-成岩、构造-热演化和页岩气赋存与孔隙结构演变间的耦合关系及对页岩气成藏效应的影响，揭示研究区龙马溪组孔隙结构演变与页岩气生成-赋存-富集等核心成藏机理，为页岩气有利区带科学预测和勘探开发战略部署提供重要依据。

本次研究针对中上扬子区龙马溪组页岩，通过野外地质调查、地化分析、古生物识别和储层物性测试，结合室内数值模拟，对研究区龙马溪组微孔结构演变及其对页岩气成藏的控制机理展开系统研究。研究主要取得如下成果：（1）在系统阐明研究区沉积背景的基础上，通过氩离子抛光扫描电镜定性-半定量界定了有机质孔、粒内孔及粒间孔的形貌与孔径特征，并结合孔隙形貌及结构特征，对页岩储集空间类型进一步分类。同时借助高压压汞、低温液氮和二氧化碳吸附联合表征，对页岩微孔结构进行精细定量全尺度表征，揭示了微孔为比表面积的主要贡献者；（2）从生物输入与源岩特征角度揭示笔石生物的发育程度、输入与保存为源岩TOC的主要贡献者。龙马溪组底部发育大量笔石化石，且笔石体有机质微孔结构发育，具有较高面孔率，连通性较高，开放性好，且笔石体叠层分布形成的微观层间间隙连通性较好，对页岩气赋存富集均有积极贡献；（3）通过页岩物质成分分析结合储层微孔结构发育特征，阐明页岩微孔缝结构发育的物质成分控制规律，并结合不同成因类型孔隙影响因素，阐释了页岩在地质历史埋藏过程中随成岩热演化条件改变，不同孔隙演化模式。进一步结合高压甲烷吸附实验结果，揭示微孔对吸附态页岩气起到控制作用；（4）基于页岩储层微孔结构物理测试参数，依据分子势能理论，利用蒙特卡洛法模拟不同孔径下页岩吸附态与游离态气体的耦合关系，进一步阐明吸附态页岩气主要赋存与微孔之间；（5）利用野外实测地质剖面、钻井资料，结合PetroMod盆地演化模拟软件，分析得到渝东南地区和涪陵地区龙马溪组暗色页岩进行沉积埋藏史、生烃演化史。在次基础上，利用核磁共振实验对目的层饱水样品进行测试，研究构造改造过程中不同温压条件对孔隙结构演化的控制作用。同时，从能量动态平衡角度阐明页岩成藏能量动态平衡过程对页岩气富集的影响机制，揭示后期构造抬升是的储层弹性能随地层压力降低而迅速减小，从页岩系统能量平衡角度分析可为筛选有利区提供可靠依据。