鄂尔多斯盆地三叠系陆相页岩微孔隙特征与天然气赋存方式研究

Shale gas, bearing in dark shale stratas, has high commercial mining value with the characteristics of self source-reservoir-caping combination during the processes of hydrocarbon accumulation. Thick-bedded dark continental shales widely occurr in continental sedimentary basins in China since Mesozoic- Cenozoic and have the basic geological conditions and great potential to form shale gas reservoirs. Shale gas may be a new target for an important replacement field of energy. Ordos basin is the second largest continental sedimentary basin in China. The source rocks mainly developed in Ordos basin is Chang 7 member of Triassic Yanchang formation. This set of source rocks developed when Ordos basin entered a heyday in Triassic. Its high in abundance of organic matter (1.0%～5.28%), good in kerogen type (sapropelic and near sapropelic) and low in organic matter maturity (Ro: 0.6%～1.2%). The source rock bed is thick with its thickness between 30-160m and has favorable conditions for hydrocarbon accumulation and shows great exploration potential. Based on study of Triassic continental oil shales from Ordos basin in this project, we simulate the formation process of hydrocarbon in continental shales and prepare shale samples in defferent stages of thermal evoluton, using thermal simulation method at high temperature and high pressure in certen geological conditions. The samples will be studied using the methods of SEM observations, Nitrogen adsorption isotherm at low pressure and specific surface area analysis, aiming at determing the micro-pore types, pore structures, pore connectivity and finding out the formation mechanisms of miro-pores and main controlling factors of their adsorbability. We also study the types of organic matter, abundance of organic matter, thermal evolution degree, inorganic mineral compositons in shales and their effects on natural gas adsorption. Most importantly, we study the mutual relationships among thermal evolution of organic matter, miro-pores and natural gas adsorption and establish dynamic model of the free air content in the liquid and the maximum amount of absorbed gas. Finally, we establish theoretical calculation model for amount of absorbed gas in shales and provide theoretical basis for predicting and evaluating shale gas resources in Ordos basin.

我国陆相沉积盆地广布，发育有厚度较大的暗色泥页岩，具有形成页岩气的基本地质条件和巨大潜力。本项目拟以鄂尔多斯盆地三叠系陆相页岩为研究对象，利用高温高压短时间热模拟方法研究地质条件下陆相页岩的烃类形成过程，制备出不同热演化阶段的页岩样品，采用扫描电镜、低压氮气等温吸附、比表面积分析等实验方法，查明页岩的微孔隙类型、孔隙结构及连通性，阐明页岩微孔隙的形成机理及吸附性的主控因素，探究页岩有机质类型、丰度、热演化程度和无机矿物组成等因素对吸附性的影响，揭示页岩有机质热演化－微孔隙－吸附性三者之间的相互关系，建立游离气量和最大吸附气量的动态模型。在此基础上，根据质量平衡原理确定游离气量和吸附气量，建立页岩含气量理论计算模型，为该盆地陆相页岩气资源分布预测和评价提供理论依据。

页岩气是目前国际油气勘探与研究领域的热点和前沿。开展鄂尔多斯盆地三叠系陆相页岩微孔隙特征与天然气赋存方式的研究，不仅可以揭示有机质在不同热演化阶段页岩微孔隙结构、吸附性特征及演化过程，而且可以准确进行页岩含气性评价和含气量估算，为该盆地页岩气资源分布预测和评价提供依据。.项目以鄂尔多斯盆地延长组7段黑色页岩岩芯样品孔隙基本特征为基础，选用低熟、富含有机质的页岩，利用高温-高压-加水-短时间热模拟方法，通过分析各样品的有机地化参数、矿物组成、孔隙结构以及甲烷吸附量来研究地质条件下富有机质页岩在烃类形成过程中孔隙的演化规律，明确页岩孔隙的形成机理及热演化过程中地质因素演变对页岩甲烷吸附能力的影响。同时，对盆地长7段陆相页岩含气潜力进行了分析，得出以下主要结论：.（1）延长组7段低熟页岩孔隙类型主要有原生残留孔隙、次生溶蚀孔隙、黏土矿物粒间孔和黄铁矿晶间孔。在低熟-成熟演化过程中，样品孔隙度呈现出先增加后减小的演化规律，从最初的3.8%增加到17.53%后又逐渐降低到8.15%，孔径峰值从20-100nm变为2-10nm，尔后又升至20-200nm，页岩孔隙度的增加主要是有机孔的贡献。有机质孔大量形成之后，由于地层压力的增大，岩样孔隙度减小幅度达5.68%，且主要是孔径>50nm的孔隙体积减少，说明有机质成熟阶段压实减孔作用显著；.（2）针对盆地长7段页岩，根据不同热模拟样品模拟埋藏深度的实际温度及压力，进行了热模拟样品高温高压条件下的等温吸附实验，进而分析了地质埋深对页岩甲烷吸附能力的影响。研究发现微孔是影响页岩甲烷吸附能力的最重要因素，与页岩甲烷吸附能力呈正相关；其次是有机质丰度和比表面积，页岩热演化过程中有机质丰度与其甲烷吸附能力呈负相关。页岩埋深与其甲烷吸附能力呈负相关，且埋藏越深吸附能力下降越快。鄂尔多斯盆地延长组7段页岩甲烷吸附能力多在1mg/g以上，最大可达3mg/g以上，甲烷吸附能力较强的页岩分布在定边―吴起地区及庆阳―正宁地区，大致与盆地沉积中心相叠合。