Pore structure of the graptolite-derived OM in the Longmaxi Shale, southeastern Upper Yangtze Region, China
Pore structure of the graptolite-derived OM in the Longmaxi Shale, southeastern Upper Yangtze Region, China
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DOI：
10.1016/j.marpetgeo.2016.01.009
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发表时间：
2016-04
影响因子：
4.2
通讯作者：
Yong Ma;N. Zhong;Lijun Cheng;Z. Pan;Na Dai;Zhang Yu;Liu Yang
中科院分区：
文献类型：
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作者：
Yong Ma;N. Zhong;Lijun Cheng;Z. Pan;Na Dai;Zhang Yu;Liu Yang
上扬子地区东南部下志留统龙马溪页岩为黑色海相富有机质页岩，富含笔石，是中国页岩气勘探和生产的主要目标。笔石是龙马溪页岩的重要有机组成部分，通常只保存在页岩的周皮中。然而，笔石周皮的孔隙结构及其对储气的贡献尚未得到研究。采用光学显微镜对笔石进行鉴定和“标记”，并用扫描电镜对龙姆溪页岩样品进行孔隙观察。结果表明，龙马溪笔石周生孔隙具有各向异性，与孔隙的精细结构密切相关。在与层理平行的层理剖面上，皮层带中皮层原纤维之间微米级的裂缝和纺锤状孔隙发育较多，而在垂直层理剖面上则较少。此外，页岩中还离散分布着大量富含纳米孔隙的腐泥碎屑。虽然从SEM图像分析来看，笔石周皮为低孔隙度，但沿笔石周壁的微裂缝和细长孔隙仍可能使笔石成为一个相互连接的系统。这些笔石有机质与页岩中离散的多孔腐殖碎屑一起，可能在页岩中形成一个相互联系的有机孔隙系统。笔石周皮与腐泥岩碎屑孔隙发育的差异也为有机孔隙非均质性研究提供了新的思路。岩石中有机质的组成、精细结构和取向可能是控制有机质孔隙发育的重要因素。光学显微镜下的有机质类型识别与扫描电镜下的孔隙观察相结合，可能是研究有机质孔隙发育的有效方法，特别是在含有机质较多的页岩中。
The Lower Silurian Longmaxi Shale in the southeastern Upper Yangtze Region, which has been the main target for shale gas exploration and production in China, is black marine organic-rich shale and rich in graptolites. Graptolites, usually only periderms preserved in shales, are important organic component of the Longmaxi Shale. However, the pore structure of graptolite periderms and its contribution to gas storage has not yet been studied before. A combination of optical microscopy for identification and “mark” of graptolite and scanning electron microscope (SEM) for pore observations were conducted for the Longamxi Shale samples. Results show that pores are anisotropic developed in the Longmaxi graptolite periderms and greatly associated with their fine structure. Micrometer-sized fractures and spindle-shaped pores between cortical fibrils in the cortical bandage are greatly developed at section parallel to the bedding, while they are rare at section perpendicular to the bedding. Besides, numerous sapropel detritus rich in nanometer-sized pores are discretely distributed in the shale. Though graptolite periderms are low porosity from SEM image analysis, microfractures and elongated pores along the graptolite periderm wall may still make the graptolite an interconnected system. Together with the discrete porous sapropel detritus in shale, these graptolite-derived Organic Matter (OM) may form an interconnected organic pore system in the shale. The difference of pore development observed in graptolite periderms and sapropel detritus also give us new insight for the organic pore heterogeneity study. The OM composition, their fine structure and orientation in the rock may be important factors controlling OM pore development. The combination of identifying OM type under optical microscopy and pores observation under SEM for may be an effective method to study the OM pore development especially in shale that contain more OM.