渝东南下志留统龙马溪组不同岩相页岩的孔隙结构与分形特征

下志留统龙马溪组是中国重要的页岩气勘探开发层位.其中页岩气主要产自下部含黏土硅质海相页岩.为探究渝东南地区各井段勘探效果差异性,压裂段产气不均等问题,以YC-6井为例,通过矿物组分将页岩样品划分为含硅黏土质页岩和含黏土硅质页岩,结合有机碳含量(total organic carbon,TOC)测试,镜质体反射率测试,X射线衍射分析(X-ray diffraction,XRD),低压氮气吸附及高压压汞(mercury intrusion capillary pressure,MICP)实验,分析了YC-6井龙马溪组两种不同岩相页岩的孔隙结构特征.并基于Frenkel-Halsey-Hill(FHH)模型计算了不同岩相页岩的孔隙分形维数.结果表明:YC-6井龙马溪组整体含气量低.下部含黏土硅质页岩平均TOC为4.31％,高于上部含硅黏土质页岩(1.13％),同时总孔体积和比表面积也高于后者.不同岩相页岩的分形维数D2均大于D1,表明孔隙结构复杂程度大于孔隙表面.分形维数与总孔体积和比表面积均呈正相关关系,表明具有大比表面积,大孔容的孔隙结构更复杂.TOC和矿物组分含量是页岩孔隙分形维数的重要影响因素.TOC与分形维数呈正相关关系,有机质含量越高,在热演化过程中将生成更多微小的孔隙,从而使孔隙结构更加复杂;黏土矿物含量越多,分形维数越低,黏土矿物在成岩过程中受到压实作用,使得黏土矿物颗粒排列更为紧密,孔隙更加规则,均质性更强,分形维数更低;海相页岩中的石英多为生物成因,其自身的不规则性将使孔隙结构复杂,分形维数变大.

Pore structure and fractal characteristics of different lithofacies shales of the Lower Silurian Longmaxi Formation in southeast Chongqing

The Lower Silurian Longmaxi Formation is an important shale gas exploration and development stratum in China. The shale gas is mainly produced from clay-bearing siliceous marine shale in the lower part. In this paper, shale samples are divided into silicate-bearing clayey shale and clay-bearing siliceous shale by mineral composition, combined with organic carbon content(TOC) test, vitrinite reflectance(Ro) test, and X-ray diffraction analysis(XRD), Low-pressure nitrogen adsorption and high-pressure mercury injection(MIP) experiments, analyzed the pore structure characteristics of two different lithofacies shales in the Longmaxi Formation of YC-6. Based on the Frenkel-Halsey-Hill(FHH) model, the pore fractal dimensions of shales in different lithofacies were calculated. The results show that the gas content of YC-6 is low, the average TOC of the clay-bearing siliceous shale in the lower part is 4.31%, which is higher than that in the upper silicate-bearing clayey shale (1.13%). At the same time, the total pore volume and specific surface area are also higher than the latter. The fractal dimension D2 of shale in different lithofacies is greater than D1, indicating that the complexity of the pore structure is greater than the pore surface. The fractal dimension has a positive correlation with the total pore volume and specific surface area, indicating that the pore structure with large specific surface area and large pore volume is more complicated. TOC and mineral content are important factors affecting the fractal dimension of shale pores. TOC is positively related to the fractal dimension. The higher the content of organic matter, the more pores will be generated during the thermal evolution process, which will make the pore structure more complex. The more the clay mineral content, the lower the fractal dimension will be. The reason is clay minerals subjected to compaction during the diagenesis, the clay mineral particles are arranged more closely, the pores are more regular and the homogeneity is stronger, so the fractal dimension is lower; Most of the quartz in marine shale is of biogenesis. Its irregularity will complicate the pore structure and increase the fractal dimension.