The Pore Structure and Difference between Wufeng and Longmaxi Shales in Pengshui Area, Southeastern Sichuan
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摘要: 页岩孔隙结构及差异性是页岩含气性和产能评价的基础性问题.针对川东南彭水地区五峰组与龙马溪组页岩的孔隙结构已有若干研究成果,然而在页岩孔隙结构差异性和有机孔定量特征方面还缺乏研究.利用低温低压氮气吸附测定和氩离子抛光-场发射扫描电镜(FE-EM)技术,对页岩样品纳米孔隙进行了二维观察与统计以及分形特征计算,研究了3nm至几百nm页岩孔径范围的孔隙结构及其差异性.研究区五峰组-龙马溪组页岩有机孔十分发育;氮气吸附测定页岩孔隙形状包含开放型圆筒状、层状结构狭缝状和墨水瓶状等;扫描电镜观察有机孔形态主要有近圆形、椭圆形和多角形等.五峰组和龙马溪组页岩孔隙结构具有明显的差异性,主要体现在孔径大小、形态和数量上.氮气吸附测定表明,五峰组页岩孔隙比表面积和总孔容较龙马溪组大,微孔所占总孔的比例也较高;五峰组页岩孔径相对龙马溪组更细窄.扫描电镜二维图像观察与统计结果表明,五峰组有机孔径以小于3nm为主,形态以多角形为主;龙马溪组有机孔径以小于0nm为主,形态多呈近圆形和椭圆形.五峰组页岩的分形维值大于龙马溪组页岩,说明前者孔隙复杂程度较高.Abstract: Pore structure and its difference is a basic issue in shale gas content and capacity evaluation. A number of studies have been made on the pore structure of the shale in the Wufeng Formation and Longmaxi Formation in Pengshui area, southeastern Sichuan. However, there is a lack of research on shale pore structure difference and organic pore quantitative characteristics. Using low-temperature and low-pressure N2 adsorption and Ar ion milling field emission scanning electron microscopy (FE-SEM), for Wufeng and Longmaxi marine shales in Pengshui area of southeastern Sichuan, this paper investigated the pore structure of shale samples with the pore size ranging from 3nm to hundreds nm, and its difference between Wufeng and Longmaxi formations was also analyzed by observing and counting nano-pores in two-dimension, and computing fractal dimension. Results indicate that organic pores from Wufeng and Longmaxi shales in Pengshui area are very developed. Results of N2 adsorption show the shale samples contain open cylindrical-like pores, layered slit-like pores and ink-bottle pores. And observation of scanning electron microscopy reveals organic pore morphology mainly is nearly circular, elliptic and polygon. The obvious difference of pore structure between Wufeng and Longmaxi shales is mainly reflected on the pore size, shape and quantity. N2 adsorption results show that Wufeng shales have higher specific surface area and total pore volume than that of Longmaxi shales, and micropore within Wufeng shales accounts for higher proportion of total pore; pore size of Wufeng shales is narrower than that of Longmaxi shales. Observation and statistics of FE-SEM two-dimensional images indicate organic pores in Wufeng Formation are mainly pores with diameter less than 35nm and have irregular shapes. While Longmaxi organic pores are mainly less than 50nm in diameter and pore shapes mainly are nearly circular or elliptic. Based on the pore fractal dimension calculation of N2 adsorption and FE-SEM, Wufeng Formation has higher pore fractal dimension values than Longmaxi Formation, which indicates that the pore structure of Wufeng shales is more complex than that of Longmaxi shales.
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Key words:
- pore structure /
- fractal features /
- Wufeng Formation /
- Longmaxi Formation /
- marine shale /
- Pengshui area /
- petroleum geology
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图 1 研究区位置(a),川东南彭水地区构造位置(b)和构造剖面(c)
Fig. 1. Location of the study area (a), tectonic location (b) and profile (c) of Pengshui area in southeastern Sichuan
图 4 IUPAC吸附等温线与脱附迟滞分类
修改自Thommes et al.(2015);a.物理吸附等温线分类;b.脱附迟滞分类
Fig. 4. IUPAC classification of isotherms and hysteresis
图 6 PY1井页岩样品纳米孔缝类型
a.有机孔,孔隙长轴方向与有机质延伸方向基本一致,PY1井2 100.80 m;b.有机孔,孔隙长轴方向与有机质边缘基本平行,PY1井2 153.19 m;c.较大有机孔内有较小有机孔相连通,PY1井2 129.73 m;d.充填在矿物颗粒之间不规则有机质,发育大量椭圆状、扁长形有机孔,脆性矿物边缘发育粒间孔,内部有粒内孔,PY1井2 158.12 m;e.充填在矿物颗粒之间不规则有机质,有机质内发育大量有机孔,矿物颗粒内发育少量粒内孔,PY1井2 153.19 m;f.页岩中孔隙发育复杂,溶蚀孔发育, 黄铁矿与石英颗粒间存在粒间孔,碎屑颗粒与粘土矿物间存在贴粒缝,PY1井2 129.73 m;g.充填在草莓状黄铁矿晶体之间的有机质,发育大量不规则晶间-有机质复合孔,PY1井2 144.77 m;h.矿物结晶微裂缝,最宽处裂缝可达188.6 nm,PY1井2 100.80 m;i.构造微裂缝,PY1井2 129.73 m
Fig. 6. Pore types of shale samples from well PY1
图 8 五峰组-龙马溪组页岩有机孔隙形态对比
a.12万倍下龙马溪组典型有机孔扫描电镜图像,孔径相对较大,形态呈椭圆、近圆形;b.图a处理图,红色区域为有机孔;c.25万倍下龙马溪组典型有机孔扫描电镜图像;d.图c处理图,红色区域为有机孔;e.12万倍下五峰组典型有机孔扫描电镜图像,孔径相对较小,形态呈椭圆、扁长、菱角状;f.图e处理图,红色区域为有机孔;g.25万倍下五峰组典型有机孔扫描电镜图像;h.图g处理图,红色区域为有机孔;i.图a和图e为12万倍下有机孔圆度系数对比;j.图c和图g为25万倍下有机孔圆度系数对比
Fig. 8. Organic pore shapes comparison between Wufeng Formation and Longmaxi Formation
表 1 页岩样品地化参数和矿物组成特征
Table 1. Geochemical parameters and mineral composition of shale samples
样品编号 深度(m) 层位 岩相 TOC (%) 矿物组成(%) 粘土 石英 长石 碳酸盐 黄铁矿 PY1-1 2 100.80 龙马溪组 富泥/硅混合质页岩 0.82 29.4 36.6 16.9 15.2 1.9 PY1-2 2 129.73 龙马溪组 富泥/硅混合质页岩 1.27 37.9 36.4 14.7 7.6 3.4 PY1-3 2 138.08 龙马溪组 富泥/硅混合质页岩 2.34 25.9 40.8 12.3 15.9 5.1 PY1-4 2 144.77 龙马溪组 富泥硅质页岩 3.07 25.4 43.1 9.5 17.5 4.5 PY1-5 2 153.19 五峰组 富泥硅质页岩 3.27 25.9 55.8 9.6 4.3 4.4 PY1-6 2 158.12 五峰组 富泥/硅混合质页岩 3.92 41.8 39.6 10.5 5.3 2.8 表 2 页岩样品孔隙结构参数
Table 2. Pore structure parameters of shale samples
样品编号 深度(m) 层位 BET比表面积(m2/g) BJH总孔容(mL/g) t-plot微孔比表面积(m2/g) t-plot微孔孔容(mL/g) 微孔比表面比例(%) 微孔孔容比例(%) PY1-1 2 100.80 龙马溪组 8.229 9 0.009 862 2.859 0 0.001 217 34.74 12.34 PY1-2 2 129.73 龙马溪组 10.240 8 0.012 067 3.641 3 0.001 548 35.56 12.83 PY1-3 2 138.08 龙马溪组 15.936 1 0.016 718 6.417 6 0.002 744 40.27 16.41 PY1-4 2 144.77 龙马溪组 16.662 4 0.017 147 6.774 4 0.002 905 40.66 16.94 PY1-5 2 153.19 五峰组 17.648 8 0.017 850 7.213 0 0.003 095 40.87 17.34 PY1-6 2 158.12 五峰组 21.102 2 0.017 778 9.632 4 0.004 147 45.65 23.33 表 3 基于FHH模型的吸附孔孔隙分维值计算结果
Table 3. Calculation results of adsorption pore fractal dimension obtained from FHH model
样品编号 层位 吸附体积与孔径双对数关系1 相关系数R2 分维值D1 吸附体积与孔径双对数关系2 相关系数R2 分维值D2 吸附体积与孔径双对数关系3 相关系数R2 分维值D3 PY1-1 龙马溪组 y=1.245 3-0.081 0x 0.994 8 2.919 0 y=0.993 3-0.163 4x 0.997 3 2.836 6 y=0.974 8-0.307 3x 0.995 2 2.692 7 PY1-2 龙马溪组 y=1.504 7-0.069 2x 0.989 7 2.930 8 y=1.206 9-0.164 4x 0.997 3 2.835 6 y=1.191 5-0.306 2x 0.993 5 2.693 8 PY1-3 龙马溪组 y=1.957 0-0.043 4x 0.995 9 2.956 6 y=1.644 8-0.151 5x 0.992 5 2.848 5 y=1.627 0-0.290 1x 0.992 0 2.709 9 PY1-4 龙马溪组 y=2.040 5-0.031 9x 0.995 8 2.968 1 y=1.688 9-0.156 8x 0.988 6 2.843 2 y=1.669 8-0.283 7x 0.991 2 2.716 3 PY1-5 五峰组 y=2.098 0-0.027 0x 0.995 4 2.973 0 y=1.751 8-0.152 1x 0.983 5 2.847 9 y=1.727 2-0.282 7x 0.991 6 2.717 3 PY1-6 五峰组 y=2.097 7-0.029 1x 0.996 4 2.970 9 y=1.913 5-0.097 0x 0.978 1 2.903 0 y=1.891 8-0.256 0x 0.985 8 2.744 0 -
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