Microstructure characteristics of Wufeng-Longmaxi shale gas reservoirs with different depth, southeastern Sichuan Basin
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摘要: 采用薄片、X衍射、常规扫描电镜、高分辨率氩离子抛光扫描电镜及压汞、氮气吸附等方法，结合有机碳含量（TOC）与孔隙度的分析，对川东南不同深度上奥陶统五峰组-下志留统龙马溪组页岩储层的矿物组成、微观结构和微孔体积进行了研究。该区深层D1井和浅层J1井页岩储层硅质、黏土、碳酸盐等矿物除含量上略有差异，纵向变化规律相近，仅硅质含量大于40%的优质页岩层厚度不同。深、浅层五峰组-龙马溪组页岩储层微孔隙都以有机质内微孔为主，常见粒缘缝（隙）、微层（页）理缝，粒间微孔因有机质充填而不发育；底部优质页岩储层表现为结构各向同性，向上纵横向结构各向异性变强，单一微纹层厚度变薄，微层理缝密度增加；底部优质页岩层段TOC高，向上明显降低；底部储层孔隙度高，向上降低，表明有机孔隙的贡献大，但孔隙度降幅明显小于TOC，应与页岩中存在较多的无机孔隙有关。深、浅层五峰组-龙马溪组页岩纵向上矿物组成、微观结构、TOC、孔隙度随深度具有相似的变化规律。与浅层J1井相比，深层D1井底部优质储层具有微孔、介孔、大孔与总孔体积数量更大的特点，这与深层D1井储层存在较多的粒缘缝等无机孔有关。川东南五峰组-龙马溪组页岩储层在深埋条件下孔隙保持较好，具有良好的储集空间。Abstract: Thin section analysis, X-ray diffraction, conventional scanning electron microscopy (SEM), high-resolution Ar+ ion polishing SEM, mercury intrusion, nitrogen adsorption, total organic carbon (TOC) content and porosity analyses were used to determine the mineral composition, microstructure and micro-pore volume of shale reservoirs from different depths in the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in southeastern Sichuan Basin. There are slight differences in the content of siliceous, clay, carbonate and other minerals in shale reservoirs of well D1 in the deep layer and well J1 in the shallow layer, and the longitudinal changes are similar. Only high-quality shale layers with a silica content greater than 40% have different thicknesses. The shale reservoirs of the Wufeng-Longmaxi formations in the deep and shallow layers are dominated by micro-pores in organic matter disseminated in the shale matrix, and the microfractures around the boundaries of silt particles or organic grains and micro-foliation seams parallel to bedding plane are also common, while and intergranular pores are not developed. The high quality shale reservoir at the bottom is structurally isotropic, and the vertical and horizontal structural anisotropy becomes stronger upwards, with thinner micro-lamina and more foliation seams. The TOC content is high at the bottom, and diminishes upwards. The porosity at the bottom shows the same changing trend, indicating a large contribution of organic pores; however, the decrease in porosity is significantly smaller than that of TOC, which is inferred to be related to the existence of more inorganic pores in the shale. The mineral composition, microstructure, TOC and porosity of the Wufeng-Longmaxi shales in the deep and shallow layers have similar changes with depth. Compared with well J1 in the shallow layer, the high-quality reservoir in well D1 in the deep layer has a larger number of micro-pores, meso-pores, macro-pores and total pore volume, which might be explained by more inorganic pores such as grain boundary pores.The shale reservoirs of the Wufeng-Longmaxi formations in the southeastern Sichuan Basin maintain a good porosity under deep burial conditions and have good storage space.
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图 1 四川盆地及周缘下志留统底界埋深及深层D1井和浅层J1井位置
据参考文献[2]修改。
Figure 1. Bottom depth of Lower Silurian in Sichuan Basin and locations of deep well D1 and shallow well J1
图 2 川东南地区深层D1井(a)和浅层J1井(b)五峰组—龙马溪组矿物组成变化
Figure 2. Mineral composition of Wufeng-Longmaxi formations in deep well D1 (a) and shallow well J1 (b), southeastern Sichuan Basin
图 3 川东南地区深层D1井和浅层J1井五峰组—龙马溪组孔隙度、TOC垂向变化与对比
Figure 3. Porosity and TOC contents with depth of Wufeng-Longmaxi formations in deep well D1 (a) and shallow well J1 (b), southeastern Sichuan Basin
图 4 川东南地区深层D1井五峰组—龙马溪组页岩储层微层(页)理缝随深度分布
Figure 4. Micro fracture distribution with depth of Wufeng-Longmaxi shale in deep well D1, southeastern Sichuan Basin
图 5 川东南地区深、浅层五峰组—龙马溪组页岩储层微观结构薄片分析
页岩自深向浅，结构各向异性变强：a.D1井, 4 225 m；b.D1井, 4 218 m；c.D1井, 4 170 m；d.J1井，2 407 m；e.J1井，2 392.84 m；f.J1井，2 352 m
Figure 5. Thin-section micrographs of Wufeng-Longmaxi shale in deep and shallow layers, southeastern Sichuan Basin
图 6 川东南地区深、浅层五峰组—龙马溪组页岩不同深度储层微观结构扫描电镜分析
页岩自深向浅为均匀致密状、厚纹层和薄纹层：a.D1井，4 224 m；b.D1井，4 199 m；c.D1井，4 183 m；d.J1井, 2 411 m；e.J1井, 2 401 m；f.J1井, 2 335 m
Figure 6. SEM analyses of micro-structure of different depth of Wufeng-Longmaxi shale in deep and shallow layers, southeastern Sichuan Basin
图 7 川东南地区深、浅层五峰组—龙马溪组页岩储层氩离子抛光高分辨率扫描电镜微孔隙结构分析与对比
a.D1井，有机质内微孔隙，4 226 m；b.D1井，粒缘隙较发育，4 226 m；c.D1井，与笔石有关的微层理缝，4 220 m；d.J1井，有机质内较大微孔隙，2 407 m；e.J1井，粒缘隙发育，2 386 m；f.J1井，与笔石有关的微纹理缝，2 406 m
Figure 7. Argon ion polishing high resolution SEM analyses of micro-structure of Wufeng-Longmaxi shale in deep and shallow layers, southeastern Sichuan Basin
图 8 川东南地区深层D1井五峰组—龙马溪组页岩储层微层(页)理缝及粒缘隙扫描电镜分析
Figure 8. SEM analyses of micro-fractures and grain boundary fissures in Wufeng-Longmaxi shale in deep well D1, southeastern Sichuan Basin
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