四川盆地下古生界海相页岩微孔隙特征及形成机制研究

As unconventional gas resources, the main occurrences of shale gas are adsorbed gas and free gas accumulated in shale beds, accumulation has the characteristics of self generation，self storage and self sealing. Reserrvoir characteristics of shale gas is foundation of reservoir evaluation and development. The project intends to lower Paleozoic marine organic-rich shales Sichuan Basin as the research object, on the basis of the shale mineral composition, porosity and permeability, characteristics of organic matter, low-pressure isothermal adsorption-desorption method are employed to analyze quantitatively of shale pore size distribution and specific surface area , Then we will use scanning electron microscopy to characterize pores for studying type of nanoscale pores, pore structure and connectivity, but key to our success in imaging these nanopores is the use of Ar-ion-beam milling. At last, we will obtain shale samples of different thermal evolution stages through forwarding high-temperature and high-pressure thermal simulation experiment of cylindrical samples, and then study the correspondence between shale thermal evolution of organic matter, hydrocarbon generation and expulsion process and micropore. Comprehensive analysis of the relationship between shale parent material types, thermal evolution , diagenetic transformation, structural stress and micropore types, characteristics of nanoscale pores, furthermore, we will clarify formation mechanism, evolution rule and the main controlling factors of nanoscale pores, thus quantitative characterization of the micro-pore system, establish the dynamic model of the micropores, and look forward to providing reliable basis for the study of shale gas accumulation conditions and accumulation mechanism.

页岩气主要以吸附和游离状态赋存于泥页岩地层，具有集“生、储、盖”于一体的成藏特点，因此页岩储层储集特征研究是页岩气藏评价及开发的基础。本项目拟以四川盆地下古生界页岩为研究对象，在页岩矿物成分、有机质特征及孔渗分析的基础上，利用低压等温吸附/解吸附方法定量分析页岩的孔径分布和比表面积，采用氩离子抛光技术制备无损的页岩样品，再运用扫描电镜研究微孔隙的类型、孔隙结构及连通性；同时运用柱状页岩样品的高温高压正演热模拟来制备不同热演化阶段的模拟样品，进而研究页岩有机质热演化程度及生排烃过程与微孔隙之间的对应关系。综合分析页岩母质类型、热演化程度、成岩转化及构造应力与微孔隙类型及特征的相互关系，阐明页岩纳米级孔隙的成因机理及主控因素，进而定量表征页岩微孔隙体系的特征，建立微孔隙的动态模型，为我国页岩气的成藏条件和成藏机理研究提供可靠依据。

页岩气是目前国际油气勘探与研究领域的热点和前沿。近年来我国四川盆地下古生界页岩气勘探已经取得很大成果，但是页岩气赋存条件和成藏机理研究还存在很大争议，这使勘探开发方向很难确定，也造成较高的勘探开发成本。开展四川盆地下古生界富有机质页岩微孔隙特征研究，可以揭示页岩储集特征及孔隙演化过程，为预测储层有效厚度和计算含气量提供基本依据。本项目以四川盆地下志留系龙马溪和上奥陶系五峰组黑色页岩钻井和露头样品为研究对象，采用民和盆地侏罗系油页岩为热模拟分析替代样品，开展富有机质页岩的微孔隙类型及孔隙结构研究，分析页岩纳米级孔隙形成及演化过程，探讨微孔隙的形成机理及主控因素。得出以下主要结论：.（1）下古生界富有机质页岩微孔隙类型主要为有机质孔、黏土矿物晶间孔、矿物粒间孔和溶蚀孔，并且以有机质孔占主导地位；页岩主要孔径在0.35-20nm，孔隙度在2.6%-5.1%之间，比表面积在30.62 m2/g -56.18 m2/g之间。比表面积主要由孔径0-2nm的微孔贡献，其次为孔径2-5nm的中孔；孔体积主要由微孔组成，其中2-5nm的中孔贡献最大，其次为5-20nm的中孔。.（2）页岩孔隙发育规模主要由有机质含量控制，其孔隙度和比表面积随着TOC%的增加而显著增加，并且有机质主要控制孔径在20nm以下微孔和中孔。页岩孔隙度和比表面积随着黏土矿物含量增大而增大，随着碳酸盐含量增大而减少，黏土矿物主要控制孔径在2-5nm的中孔，说明这是黏土矿物晶间孔的孔径范围。.（3）热模拟实验表明，随着页岩热演化程度的增大，有机质孔和黏土矿物层间孔由不发育到逐步发育；孔隙度呈现出先增加后减小的演化规律，由原始地质样品的3.57%增加到350℃的26.09%，之后降低至370℃时的20%；平均孔径呈现出先减小后增加的趋势，热演化初期孔径减小是由于大量生油充填孔隙，导致测试孔径值偏小。