陆相页岩中粉砂质纹层成因及其对页岩含气性的影响

Lacustrine shale has great potential for shale oil/gas exploration. Abundant silty laminae can be observed in organic rich lacustrine shales in Yanchang Formation in the Ordos Basin, China, which is the “sweet spots” for shale gas exploration. However, studies on this specific shale are still inadequate. Specifically, the stratigrafic distribution of the silty laminae, the mechanisms of the silty laminae formation and the effects of the silty laminae on occurrence state and shale gas content of lacustrine shale remain unclear. Therefore, understanding these vital issues can promote lacustrine shale gas exploration in China. Detailed lithology approaches including FIB-SEM, QEMScan, XRF, et al. are proposed to determine the characteristics of the silty laminae in shale from Yanchang Formation. Elements and isotope geochemistry and organic chemistry methods are conducted to discuss the formation process of silty laminae. Based on the evaluation model, the adsorption gas, the free gas and the dissolved gas content can be determined. The effects of silty laminae on shale gas occurrence and content are discussed. This project can provide lacustrine shale oil and gas exploration and development with basic theory support and be of great significance for shale evaluation, sweet spots prediction in China.

我国陆相页岩广泛分布，油气勘探潜力巨大。陆相页岩中粉砂质纹层发育，对页岩气富集和高效开发具有关键控制作用。但是，陆相粉砂质纹层页岩的非均质性强，纹层成分复杂，对其发育规律、成因机理及其对页岩气赋存状态和含气量的影响不明确，制约了陆相页岩气的高效开发，成为亟待解决的关键问题。本项目拟以鄂尔多斯盆地延长组粉砂质纹层页岩为研究对象，利用聚焦离子束扫描电镜（FIB-SEM）、X-射线衍射（XRD）、X-射线荧光（XRF）等先进测试技术，针对“陆相页岩中粉砂质纹层成因及其对页岩含气性的影响”这一关键科学问题开展研究，查明页岩中粉砂质纹层发育特征及规律，阐明粉砂质纹层的成因，通过创建不同赋存状态页岩气含气量评价模型，定量评价粉砂质纹层层偶页岩气赋存状态，厘定控制页岩含气性的关键地质要素，揭示粉砂质纹层对页岩赋存状态和含气量的影响机制，为陆相页岩气的高效开发提供理论支持。

我国鄂尔多斯盆地南部延长组（长7段）陆相页岩中粉砂质纹层发育，对页岩气富集和高效开发具有关键控制作用。本项目以延长组长7段粉砂质纹层页岩为研究对象，利用微量元素、聚焦离子束扫描电镜（FIB-SEM）、微区矿物定量分析（QEMScan）、X-射线荧光（XRF）等测试技术，针对“陆相页岩中粉砂质纹层成因及其对页岩含气性的影响”这一关键科学问题开展研究。结果表明：（1）研究区延长组长7段页岩中粉砂质纹层发育，单层厚度多为0.5～8mm，粉砂质纹层相对块状页岩层更富集石英和长石矿物，而TOC和粘土矿物含量相对较低。纵向上粉砂质纹层的非均质性特征明显，平面上粉砂质纹层分布广泛；（2）长7段页岩岩性组合分布与沉积环境有关，微量元素结果揭示长7段页岩总体为淡水—半咸水的静水贫氧-厌氧沉积，粉砂质纹层页岩段沉积于潮湿、还原条件下的淡水湖泊分层水体环境；（3）FE-SEM图像分析表明粉砂质纹层页岩和块状页岩中的孔隙类型存在差异，前者以石英和长石粒间孔为主，同时发育石英晶间孔和粘土矿物粒间孔，后者主要发育粘土矿物相关粒间孔和层间孔、有机质孔等；（4）全孔径孔隙结构表征表明，与块状页岩相比粉砂质纹层页岩的总孔体积更大，总比表面积稍小，孔径分布更宽，宏孔比例更大。粉砂质纹层抗压实能力更强，溶蚀作用强烈，是粉砂质纹层页岩储集物性更好的关键因素；（5）基于高压甲烷等温吸附实验，建立了考虑粉砂质纹层发育的页岩赋存状态含气量预测模型，评价了长7段页岩不同赋存状态页岩气含量，其中页岩吸附气含量1.61～4.11 m3/t，游离气含量0.26～2.16 m3/t，溶解气含量0.21～0.51 m3/t；（6）页岩中的粉砂质纹层储层物性更好，有利于游离气富集，是影响粉砂质纹层中页岩气赋存状态和含气量的重要因素。粉砂质纹层页岩层段有利于陆相页岩气富集，可作为鄂尔多斯盆地南部陆相页岩气开发的优选层段。