Analyses of the characteristics and main controlling factors of shale pores in the Niutitang Formation in the Guizhou region

Abstract

The shale of the Lower Cambrian Niutitang Formation in the Guizhou region has undergone complex diagenesis and has developed different types of pore textures, which affect the occurrence status of shale. In the present study, we applied scanning electron microscopy (SEM) and an isotherm analysis of low-temperature nitrogen gas adsorption to shale core samples drawn from the Niutitang Formation in the Guizhou region to quantify the shale pore development characteristics. In addition, we conducted a shale geochemical analysis in light of the main controlling factors for pore development. The results indicate that the shale pores and fractures of the Niutitang Formation can be divided into three types: organic pores, inorganic pores, and micro fractures. The organic pores are mainly distributed in the organic matter between inorganic mineral particles, with small pore diameters, which are characterized by inkpot, elliptic, and beaded shapes. The inorganic pores are mainly composed of narrow slit intragranular pores and intergranular pores. The micro fractures develop parallel plates with four-side openings and splint plates. The pores of the shale are mainly mesopores (53%), followed by micropores and macropores, with pore diameter distributions ranging mainly from 1 to 50 nm. The specific surface area is mainly provided by nanoscale pores with average diameters of less than 4 nm. Therefore, the smaller pore makes a greater contribution to the specific surface area, while the specific surface area increases with an increase in the total pore volume. The study further indicates that organic carbon content is the most important internal factor for shale pore development, especially in terms of the control of volume and the specific surface area of micropores. Moreover, quartz content has a certain effect on shale pores; the pore volume and specific surface area increase with increasing quartz content, but the control effect is not obvious. The effect of clay minerals on shale pores is negligible. The type of organic matter is also an important factor in controlling the developmental difference of shale pores, and a high degree of thermal evolution is not conducive to organic pore development. It was therefore concluded that intergranular pores and microfractures, mainly mesopores, are the main reservoir space and migration channels of Niutitang shale in the study area. The organic carbon content, mineral components, organic matter type, and degree of thermal evolution jointly control the development of shale pores, among which the organic carbon content is the most important influencing factor.