柴达木盆地东缘上石炭统泥页岩孔隙结构及分形特征

我国海陆过渡相烃源岩分布范围广，具有成熟度适中及厚度大等特点，页岩气勘探前景良好。前人研究主要聚焦于页岩气成藏条件、模式及生烃潜力，而对储层孔隙结构影响因素及复杂程度的定量表征研究较少。为了探讨海陆过渡相泥页岩的孔隙结构和分形特征，以柴达木盆地东部上石炭统泥页岩为研究对象，对研究区15件泥页岩样品采用低温液氮吸附及扫描电镜进行孔隙结构、分形特征研究，并在此基础上探讨了泥页岩有机地球化学、矿物组成、孔隙结构参数与分形维数的关系，进而揭示泥页岩孔隙结构发育影响因素。结果表明：①泥页岩孔隙形态主要有2类,第一类为楔形—狭缝型和细颈瓶—墨水瓶状型,第二类为四周开放的平行板状孔,孔隙类型以粒间孔缝、溶蚀孔和有机质孔最为发育。②应用FHH分析模型计算出了泥页岩孔隙分形维数，以P/P₀=0.45为界，泥页岩存在2种不同吸附解吸机制，用D₁和D₂分别表示P/P₀<0.45和P/P₀>0.45范围内的孔隙分形维数。泥页岩孔隙具有明显的分形特征，D₁分形维数与黏土矿物和TOC含量呈负相关关系；D₂分形维数与黏土矿物和TOC含量呈正相关性。表明影响分形维数的主要因素为黏土矿物和有机质含量。D₁和D₂与孔隙结构参数的相关性都较好，但D₁趋势线比D₂拟合性更好，说明小孔对孔隙结构参数影响更为重要。③分形维数D₁和D₂之差△D=0.393，表明其结构复杂程度相差很大，双重分形特征明显。但分形维数D₂越大，孔隙结构趋于复杂，孔隙表面积更加粗糙，不利于气体的渗流。

Pore structure and fractal characteristics of the Upper Carboniferous shale,eastern Qaidam Basin

Source rocks of marine and continental transitional facies in China have a wide range of distribution, moderate maturity and large thickness, and good prospects for shale gas exploration. Previous studies mainly focused on shale gas accumulation conditions, patterns and hydrocarbon generation potential, however, few studies are focused on quantitative characterization of factors affecting shale pore structure and complexity. In order to inquire the pore structure and fractal characteristics of marine-continental transitional shale, we collected 15 shale samples from the Upper Carboniferous of eastern Qaidam Basin and performed low-temperature nitrogen adsorption and scanning electron microscopy analyses. The relationship between organic geochemical parameter, mineral composition, pore structure and fractal dimension of shales are discussed. These results indicate: ①There are two types of pore morphology. The first type is mainly wedge-slit type and thin-neck-ink bottle type. The second type is reflected in the open parallel plate pores. The pore types of shale reservoirs are mainly intergranular pores, dissolution pores and organic matter pores. ②FHH analysis model was used to calculate the fractal dimension of the pores of mud shale. With P/P₀=0.45 as the boundary, there are two different adsorption and desorption mechanisms in mud shale. D₁ and D₂ were used to represent the fractal dimension of the pores within the range of P/P₀<0.45 and P/P₀>0.45, respectively. The pores of shale samples have obvious fractal characteristics, and the content of clay minerals and TOC is negatively correlated with the fractal dimension of D₁, while the fractal dimension of D₂ is positively correlated with the content of clay minerals and TOC. The results showed that the main factors affecting the fractal dimension were the content of clay minerals and organic matter. D₁ and D₂ have a good correlation with pore structure parameters, but D₁ trend line has a better fit than D₂, indicating that small pores have a greater impact on pore structure parameters. ③Fractal dimension D₁ and D₂ difference △D=0.393, indicating that the complexity of the structure varies greatly, and the dual fractal characteristics are obvious. However, the larger the fractal dimension D₂ is, the more complex the pore structure tends to be and the rougher the pore surface area is, which is generally not conducive to the gas seepage.