四川盆地五峰组—龙马溪组重点井含气页岩孔隙类型与孔径分布

doi:10.13745/j.esf.sf.2020.5.23

摘要/Abstract

摘要：

针对四川盆地五峰组—龙马溪组过成熟海相页岩,开展普通薄片观察、SEM成像、X-衍射全岩组分和黏土矿物分析、TOC含量测定、N₂-CO₂联合吸附实验和RG测定。结果表明,海相页岩发育有机孔、无机孔和微裂缝。有机孔多呈气孔状或海绵孔状,大、小混杂;无机孔多呈三角状、棱角状或长方形状;微裂缝多呈条带状,能有效沟通有机孔和无机孔。过成熟海相中,有机孔数量个数>97%,无机孔和微裂缝数量<3%。孔隙以孔径<10 nm的微孔和介孔为主,孔隙数量和体积占比均大于80%,孔径>10 nm的孔隙数量和体积不足20%。在大于10 nm的介孔和宏孔中,有机孔数量超过97%,面孔率超过50%,无机孔和微裂缝数量不足3%,面孔率低于50%。介孔孔径以5~400 nm为主,孔径<20 nm的数量占比>70%。有机孔面孔率随着孔径增大而增大,孔径为100~400 nm的孔隙面孔率占比普遍超过50%。无机孔主要为石英晶间孔和碳酸盐溶蚀孔及少量长石溶蚀孔,孔径为100~400 nm的孔隙面孔率贡献最大,多数页岩均接近100%。随着TOC含量增高,有机孔孔径减小,其面孔率则先增大后减小,TOC为5.5%时常出现拐点。龙马溪和龙一段页岩孔隙由下至上,面孔率逐渐降低。平面上,龙一_1¹小层不同地区面孔率大小及孔隙组成也存在差异,泸州地区面孔率最大,渝西地区最低。渝西地区无机孔含量最高,长宁地区无机孔含量最低。石英晶间孔隙度与石英含量呈正相关,溶蚀孔隙度与碳酸盐矿物含量呈正相关,这可能与石英抗压实能力强、碳酸盐矿物易于溶蚀有关。

关键词: 海相页岩, 孔隙组成, 孔径分布, 过成熟, 龙马溪组, 五峰组, 四川盆地

Abstract:

On the overmature Wufeng-Lungmachi marine shale in the Sichuan Basin, China, we carried out a detail investigation through standard thin section observation, argon ion polishing sheet production and SEM imaging, X-ray diffraction of whole rock component, clay mineral analysis, TOC content determination, N₂-CO₂ adsorption experiment, and RG measurement. The results show that the gas-bearing shale contains organic and inorgainc pores and micro-fractures. The organic pores are bubbly or sponge shaped, have various sizes, and are mostly evenly distributed in organic matters. The inorganic pores are mainly triangular, angular or rectangular shaped and can be divided into quartz intercrystalline and dissolved pores. The micro-fractures are mostly strip-shaped and connected to the organic and inorganic pores to form a network. In the overmature marine shale samples, organic pores exceeded 97% and inorganic pores and micro-fractures were less than 3% in quantity. The overmature shale is composed of more than 80% micro and mesopores, with pore size <10 nm. For meso and macropores with pore size above 10 nm, the organic pores were more than 97% in quantity and more than 50% in plane porosity. In comparison, the inorganic pores and micro-fractures were less than 3% in quantity and less than 50% in plane porosity. The pore size of mesopores ranged from 5 to 400 nm and those with pore sizes below 20 nm exceeded 70% in quantity. Of these, the plane porosity of organic pores increased with increasing pore size and reached the maximum value with pore size between 100-400 nm. The inorganic pores are composed mainly of intercrystalline pores and dissolved pores and had the highest plane porosity (nearly 100%) with pore size between 100-400 nm. The organic pore porosity increased with increasing TOC content, reaching the 5.5% maximum value and then decreasing progressively. The plane porosity of Layer 1 of sub-Member 1 was the highest, decreasing gradually from bottom to top in the Lungmachi Formation. In addition, plane porosity varied for different areas and the Luzhou and Yuxi areas area had the highest and lowest values, respectively. In the Wufeng-Lungmachi shale, the intercrystalline porosity was positively correlated with the siliceous mineral content due to material hardness and organic origin. Besides, the dissolution porosity was positively correlated with the carbonate mineral content as a result of dissolution under burial and compaction. This pattern is possibly relating to reduced pore size caused by collapse under burial and limited resolution of FE-SEM imaging.

Key words: marine shale, pore type, pore size distribution, overmature, Lungmachi Formation, Wufeng Formation, Sichuan Basin

中图分类号:

TE122
P534.4

施振生, 武瑾, 董大忠, 孙莎莎, 郭长敏, 李贵中. 四川盆地五峰组—龙马溪组重点井含气页岩孔隙类型与孔径分布[J]. 地学前缘, 2021, 28(1): 249-260.

SHI Zhensheng, WU Jin, DONG Dazhong, SUN Shasha, GUO Changmin, LI Guizhong. Pore types and pore size distribution of the typical Wufeng-Lungmachi shale wells in the Sichuan Basin, China[J]. Earth Science Frontiers, 2021, 28(1): 249-260.

图/表 12

图1 四川盆地五峰组—龙马溪组展布及资料点分布

Fig.1 Distributions of the Wufeng-Lungmachi shale and data collection sites in the Sichuan Basin

图2 四川盆地巫溪2井五峰组—龙马溪组取样位置

Fig.2 Sampling positions in the Wufeng-Lungmachi Formation of the Wuxi-2 well in the Sichuan Basin

图3 四川盆地巫溪2井五峰组—龙马溪组龙一段矿物组分、TOC及孔隙孔径特征及分布

Fig.3 Mineral composition, TOC content, and pore size of the Wufeng-Lungmachi shale from the Wuxi-2 well in the Sichuan Basin

图4 四川盆地巫溪2井五峰组—龙马溪组常见矿物组分照片 a—斑点状黄铁矿,反射光,1 584 m;b—草莓状黄铁矿集合体,黄铁矿单晶为八面体状,大小约2 μm,1 620 m,扫描电镜照片;c—草莓状黄铁矿集合体,集合体大小4~8 μm,1 571 m,扫描电镜照片;d—草莓状黄铁矿集合体,夹有机质和纳米级孔隙,1 618.5 m,背散射成像照片;e—硅质海绵骨针,反射光,1 603 m;f—放射虫,顺层分布,1630 m,单偏光。

Fig.4 Photos showing common minerals in the Wufeng-Lungmachi shale from the Wuxi-2 well in the Sichuan Basin

图5 四川盆地巫溪2井五峰组—龙马溪组龙一段孔隙类型及特征 a—有机孔,1 625 m;b—有机孔,1 630.5 m;c—赋存于草莓状黄铁矿晶体之间的有机孔,1 620.5 m;d—石英晶间孔,1 625 m;e—溶蚀孔隙,1 625 m;f—裂缝孔隙,1 570 m。

Fig.5 Pore types and features of the Wufeng-Lungmachi shale from the Wuxi-2 well in the Sichuan Basin

图6 四川盆地巫溪2井五峰组—龙马溪组不同类型孔隙数量(a)和面孔率(b)及不同孔径孔隙数量(c)和面孔率(d)

Fig.6 Number percentage (a) and plane porosity (b) for different pore types, and number percentage (c) and porosity (d) for different pore sizes for the Wufeng-Lungmachi Wuxi-2 well

图7 四川盆地巫溪2井五峰组—龙马溪组龙一段不同类型孔隙不同孔径数量分布 a—有机孔;b—石英晶间孔;c—溶蚀孔隙;d—微裂缝。

Fig.7 Number percentage (%) of various pore sizes for different pore types

图8 四川盆地巫溪2井五峰组—龙马溪组龙一段不同类型孔隙不同孔径面孔率分布 a—有机孔;b—石英晶间孔;c—溶蚀孔隙;d—微裂缝。

Fig.8 Plane porosity percentage (%) for various pore types and sizes

图9 四川盆地巫溪2井五峰组—龙马溪组龙一段不同层段(a)及不同地区龙一 1 1(b)孔隙组成特征

Fig.9 Pore types and percentage of each pore type for different intervals (a) and areas (b)

图10 四川盆地巫溪2井五峰组—龙马溪组矿物含量与面孔率

Fig.10 Mineral content and plane porosity of the Wufeng-Lungmachi shale in the Wuxi-2 well in the Sichuan Basin

图11 四川盆地五峰组—龙马溪组龙一段TOC与面孔率

Fig.11 TOC content and plane porosity of the Wufeng-Lungmachi shale in the Sichuan Basin

图12 四川盆地巫溪2井五峰组—龙马溪组龙一段有机孔隙特征 a—龙一2亚段,w(TOC)=3.0%;b—龙一 1 1小层,w(TOC)=5.3%;c—五峰组,w(TOC)=6.5%。

Fig.12 Organic pore character of the Wufeng-Lungmachi shale in Wuxi-2 Well in the Sichuan Basin

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