四川盆地东南部龙马溪组页岩微-纳米孔隙结构特征及控制因素

以四川盆地东南部重庆地区下志留统龙马溪组页岩为研究对象,通过场发射扫描电镜、CO₂及N₂低温低压吸附实验,探讨海相页岩储层微-纳米孔孔隙结构特征及其控制因素。结果表明:龙马溪组页岩发育有机孔、粒间孔、粒内孔、晶间孔、溶蚀孔和微裂缝6种孔隙类型,其中有机孔、黏土矿物层间粒内孔最为发育,由于热演化程度高也发育大量的溶蚀孔隙;龙马溪组页岩BET比表面积介于3.5~18.1 m²/g,BJH总孔容介于0.00234~0.01338 cm³/g,DA微孔比表面积介于1.3~7.3 m²/g,DA微孔孔容介于0.00052~0.00273 cm³/g。页岩微孔比表面积占总比表面积的23.1%~80.2%,平均占比50.3%,微孔孔容占总孔容的12.1%~48.5%,平均占比32.3%,微孔提供比表面积的能力远大于中孔和宏孔,是页岩储层中甲烷吸附的主要场所;泥页岩孔径分布复杂,孔径分布曲线存在多个不同的峰值,在0~100 nm范围内主要呈现双峰或三峰特征,偶见四峰特征;有机碳含量与泥页岩微孔、中孔+宏孔及总孔的孔隙结构参数均呈现非常好的线性关系,表明TOC是泥页岩中微-纳米孔隙结构最重要的控制因素,将孔隙结构参数对TOC进行归一化处理后,总孔和中孔+宏孔孔隙结构参数与黏土矿物含量呈正线性关系,与脆性矿物含量呈负线性关系,表明黏土矿物和脆性矿物主要控制页岩的中孔和宏孔的发育。

Micro-nano pore structure characteristics and its control factors of shale in Longmaxi Formation,southeastern Sichuan Basin

Lower Silurian shale in Longmaxi Formation of Chongqing, southeastern Sichuan Basin is taken as the research object in this study. The micro-nano pore structure characteristics and its control factors of marine shale reservoirs are explored based on field-emission scanning electron microscope and low-temperature low-pressure N₂ and CO₂ adsorption experiments. Results show that six kinds of pore are developed in the shale of Longmaxi Formation, i.e., organic pores, intergranular pores, intraparticle pores, intercrystalline pores, dissolution pores and micro-fractures, of which the organic pores and interlayer intraparticle pores of clay mineral are most developed, and dissolution pores are also well developed because of high thermal evolution degree. The BET specific surface area of shale in Longmaxi Formation is within the range of 3.5-18.1 m²/g, and BJH total pore volume is 0.00234-0.01338 cm³/g. The DA micro-pore specific surface area is 1.3-7.3 m²/g, and DA micro-pore pore volume is 0.00052-0.00273 cm³/g. The micro-pore specific surface area in shale accounts for 23.1%-80.2% of total specific surface area, averaged at 50.3%. The pore volume of micro-pore in shale accounts for 12.1%-48.5% of total pore volume, averaged at 32.3%. Micro-pore has a rather greater capacity for providing specific surface area than meso-pore and macro-pore, thus forming the main methane adsorption sites in shale reservoirs. Mud shale presents the complex distribution of pore size, and multiple different peaks exist in the distribution curve, showing two or three peaks in the range of 0-100 nm as well as occasional four peaks. TOC has a very good linear relationship with the pore structure parameters of micro-pore, meso-pore, macro-pore and total pore in mud shale, indicating that TOC is the most important control factor of micro-nano pore structure in mud shale. After normalizing the pore structure parameters on TOC, the pore structure parameters of meso-pore, macro-pore and total pore have a positive linear relationship with clay mineral content, but a negative linear relationship with brittleness mineral content, showing that clay minerals and brittle minerals are the main factors for controlling the development of meso-pore and macro-pore in shale.