下扬子地区二叠系页岩有机质孔隙发育特征及其影响因素

利用高分辨率成像技术实现不同类型页岩孔隙结构的定性描述和定量表征是页岩储层评价研究的重要课题之一。为此,选取下扬子地区二叠系大隆组和孤峰组2套页岩作为研究对象,首先采用氩离子抛光—扫描电镜技术对其有机质孔隙结构进行了系统的观察,再结合三维重构数据和Image J软件对有机质孔隙结构参数进行了统计与分析,最后探讨了影响有机质孔隙发育的主要因素。结果表明:(1)该区页岩有机质孔隙可划分为有机质粒内孔、有机质边缘收缩缝、有机质内部微裂隙3种类型,不同的有机质其孔径、孔隙数量、面孔率、有机质孔隙度等孔隙结构参数存在着较大的差异;(2)不同的有机质孔隙类型是随着有机质热演化作用推进阶段的不同而形成的,伴随着有机质生烃演化过程的进行,有机质粒内孔持续发育,并逐渐形成有机质边缘收缩缝,高演化条件下在有机质局部(特别是镜质体组分)还会产生有机质内部微裂隙;(3)该区页岩有机质孔隙发育非均质性的内因主要受控于有机碳含量、有机质热演化程度、有机质显微组分以及黏土矿物含量等因素,而外因则主要受控于异常压力。

Development characteristics and influential factors of organic pores in the Permian shale in the Lower Yangtze Region

One of the important subjects for shale reservoir evaluation and research is to describe qualitatively and characterize quantitativelypore structures by using the high-resolution imaging technology. Two shale reservoirs of Permian Dalong and Gufeng Fms in the Lower Yangtze Region were taken in this study. Firstly, the structures of organic pores were systematically observed by using argon-ion polishing SEM technology. Then, a statistical analysis was conducted on the structural parameters of organic pores by using the ImageJ software based on the 3D reconstruction data. And finally, the main factors influencing the development of organic pores were discussed. It is indicated that the organic shale pores in this area can be divided into three types, i.e., organic matter intragranular pores, contraction joints on the edge of organic matter, and micro fissures inside organic matter; different organic matters vary greatly in pore structure parameters,such as pore diameter, pore quantity, plane porosity and organic matter porosity. Besides, different types of organic pores are formed in different stages as the thermal evolution of organic matter advances; in the process of hydrocarbon generation and evaluation of organic matter, organic matter intragranular pores are developed continuously and contraction joints on the edge of organic matter are formed gradually; and in the condition of high evolution, micro fissures are regionally generated inside organic matter (especially vitrinite).Finally, in this area, the development heterogeneity of organic pores in shale is internally controlled by TOC, Ro, organic macerals and clay mineral content, and externally by abnormal pressure.