Fractal analysis of pores and the pore structure of the Lower Cambrian Niutitang shale in northern Guizhou province: Investigations using NMR, SEM and image analyses

Abstract

Abstract The pore structure and fractal characteristics of the Lower Cambrian Niutitang formation shale in northern Guizhou were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) observations and nuclear magnetic resonance (NMR). The relationships between mineral compositions, total organic carbon (TOC), porosity, permeability and NMR fractal dimensions of shale samples were discussed, and the properties of organic pores and organic matter were quantitatively analyzed by pore extraction based on SEM images. The shale pore distribution is irregular and complex, with multifractal and anisotropic characteristics. Fractal characteristics of shale samples were studied by NMR and apply to analyze the lithology and physical properties. Db and Dm represent fractal characteristics of bound fluid and movable fluid pore respectively. The quartz content is the negative correlation with Db and positive correlation with Dm. The clay content is the negative correlation with Db and positive correlation with Dm, and the feldspar content is the positive correlation with Db and negative correlation with Dm, whereas TOC content has no correlation with Db but is the negative correlation with Dm. The relationship between feldspar content and fractal dimension depends on the chemical reaction process in the diagenetic stage. Db has no obvious relationship with porosity and permeability, and Dm has negative correlation with porosity and positive correlation with permeability. The fractal dimension Db represents the roughness and complexity of adsorption pores surface. The fractal dimensions Dm represents the roughness and complexity of seepage pores surface. The fractal analysis provides a novel idea for the study of shale reservoir properties and lithology.